Talk:Paper - On the development and morphology of the mandibular and hyoid muscles of mammals

From Embryology

THE l\1AND]'BULAR AND HYOID MUSCLES Oll MA)L\lALS. 573

Edgeworth FH. Paper - On the development and morphology of the mandibular and hyoid muscles of mammals. (1914) Quarterly J. of Microscopical Science 573-645.

By I-‘. ll. Etlgcworlh, 1VI.D., Professor of Medicine, University of Bristol.

With Plates 38-45 and 9 Text-figures.

IN the course of a paper on the morphology of the muscles of the head in Vertebrates published in this Journal two years ago, I gave a short description of those of the mandibular and hyoid segments in the rabbit. There is great difficulty in distinguishing muscle-Anlageu from surrounding mesoblast in the early stages of developing Mammals, and, not quite satisfied with some of the statements made, I have re-investigated the phenomena. The inquiry has been much facilitated by Prof. J. P. Hill, who very kindly lent me sections of Dasynrus viverrinus—an animal which is born with two mnsticatory muscles only and an iucudomeckelian joint, and in which the development of typical mammalian muscles and of a squamoso-mandibular joint takes place after birth. These changes can be easily followed, and enable one to interpret the more obscure phenomena. occurring in Mammals with a longer intra-uterine development. Other Mammals have also been investigated, viz. pig, rabbit, Phoca vitnlina, Halichoerus grypns, Bradypus marinoratus, Dasypus uovemcinctus, Manis pentadactyla, Didelphys aurita, Echidna aculeata, Ornithorhyuchus.

The first investigator of the development of’ the masticatory muscles was Renter, who stated that in the pig their Anlage is first visible in embryos of 16 mm. Nacken-Steissliinge in the form of an inverted Y, the two limbs of which lie on either side of the lower jaw. The temporal muscle develops from the upper limb, the masseter from the lower external limb, and the two pterygoids from the lower internal limb. He did not mention the tensor tympani or the tensor veli pala.tini. Eschweiler described the development of the tensor tympani and tensor veli palatini in the pig. He found the first indication of the muscles in embryos of 1575 mm. Scheitel-Steissl'zi.uge, when the ina,sticatory musculature appears in the “Blastenisiiule ” which contains Me-clsel’s cartilage as its nucleus. The Anhigen of the tensor tympani and the malleus portion of Mecl<el’s cartilage form at first an indivisible mass. The differentiation of the muscle-Anl-age to muscle occurs in embryos of 20'5 mm. It forms the aboral end of the “Blastemsiiule,”Which, orally,forms the masticatory muscles. On the medial side of the Anlage of the masticatory muscles develops the Anlage of the tensor veli pnlatini; its n.bora.l end is gradually lost in connective tissue, which is continuous, aborally, with the Anlege of the tensor tympani (i. e. the two muscles do not overlap). The Anlage of the tensor tympani shifts aborally, whilst its development into muscle takes place in the reverse direction. In later development the oral end of the tensor veli palatini spreads into the velum, mid the tensor tympani gains an attachment to the labyrinth c-.i.psule.

Lewis (1910) gnve an account of the development of the masticatory muscles in man, which is very similar to that given by Renter in the pig. The chief point in which he differed is that “in a. 14 mm. embryo the Min. tensor tympani and tensor veli palatini are to be recognised and are connected with the ptyerygoid mass from which they probably arise?”

In the following account of the development of the mandibular and hyoid muscles, those of the mandibkllar segment are first described and subsequently those of the hyoid segment. On account of the relationship of the anterior digastric to the posterior digastric, it is convenient to consider the depressor mamdibulze anterior and anterior digastric (which are proliferated from the intermsmdibularis) with the hyoid_ muscles.

In stage A (just born) of Dasyurns (figs. 1 and 2), there is a. cartilaginous a.la orbitalis, orbito-parietal commissure, and parietal-plntte,continuous with one another. The ala, temporalis forms a. veutro-lateral process of the presphenoid region of the chondrocranium ; its free extremity turns forward. There is no line of demzwcatioii between the processns alaris and ala. tempora.lis. The incus is a. precnrtilaginons mass. Me-ckel’s cartilage is a. continuous cztrtiluginous structure, and has not divideclinto mstllens and portion ‘in front. The Anlage of the mandible is a group of cells dorsal to and extending down a. little distance on the outside of Meckel’s cartilage. In this Aulage, dorso-lnternl to Meckel’s c2trti1a.ge, ossification is just visible in the region of attacliment of the lzttera.l muscle; anteriorly it is more marked and extends forward almost to the front end of the cm-tilnge. Two nia.stica.tory muscles are present, medial nnd laternl. The medial muscle arises from the aln. temporalis ; its most anterior fibres pass downwards and outwards to the inner surface of Meckel’s cartilage, whilst the succeeding ones pass more and more obliquely backwards to the bar; the hindmost fibres are attached as far backwa.i-ds as the mnlleus portion; the fibres form one continuous sheet. The latera.l muscle—the front end of which is anterior to that of the medial nJuscle—a.i-ises from the orbito-parietal cominissnre, passes dowmvnrds, and is attached to the upper border of the Anlage of the mandible. Both muscles consist of cross-striated muscle-fibres. The third division of the fifth cra.nia.l nerve passes downwards between the two muscles, giving oi? the ramus medialis to the medial muscle and the minus la.teralis to the lateral muscle. The intermamdibnlai-is forms a ventrzilly curved transverse sheet between Meckel’s cartilages; it has a, median raphé, and is attached laterally to the inner surface of the cartilage. The mylohyoid nerve passes down on its outer surface.

In stage C (figs. 6 to 11) the medial muscle has separated into anterior and posterior portions. The former is the internal pterygoid, the latter the common Anlage of the tensor veli palatini and tensor tympani. The internal pterygoid retains its origin from the ala temporalis, whilst the upper end of the Anlage of the tensor veli palatini and tensor tympani has grown inwards beneath the ala temporalis and is attached to the (mammalian) pterygoid bone, which has now developed behind and in continuity with the palatine bone. The coronoid process of the mandible is beginning to form, and the insertion of the lateral muscle has spread down a little on its outer side. Cells proliferated downwards and backwards from the anterior of these muscle-fibres on the outside of the mandible form the beginning of the masseter muscle (fig. 6).

In stage D (figs. 14 to 18) a cartilaginous bar—the processus ascendens alae temp0ralis—~has appeared, extending from the upper lateral edge of the ala teinporalis to the orbiteparietal cominissure; it is a cartilaginous thickening of the anterior edge of the membrane obturatoria. covering in the spheno-parietal foramen. The Anlage of the tyuipanic hone has appeared as a straight rod external to the binder part of l\Iecl<el’s cartilage; its anterior end is overlapped by the posterior end of the mandible. The origin of the lateral muscle—hithe1-to confined to the orbito—parietal commissure— has now additionally spread downwards, so that the muscle arises from orbito-parietal commissure, the membrana obturatoria and the processus ascendens. The coronoid process of the mandible has extended further upwards and the inner fibres of the lateral muscle are attached to its medial side. The Anlagen of the squamous and malar "bones are formed, and the masseter in part arises from them. The anterior digastric is being proliferated from the hinder part of the intermandibularis.

In stage E (figs. 20 and 21) a condylar process has formed as a. slight elevation of the hinder end of the upper edge of the mandible; and the external pterygoid muscle, of which there is no trace in stage D, is diiferentiated from the lower posterior edge of the lateral muscle. It consists of muscle THE MANDIBULAR AND HYOID MUSCLES or MAMMALS. 577

cells which are much smaller in size than those of the rest of the lateral muscle, and is probably proliferated from it, and not formed by change of direction of already existing musclefihres. The external pterygoid muscle takes origin from the lower end of the ascending process of the ala temporalis and passes outwards to the condylar process of the mandible. The lower anterior fibres of the lateral muscle, which new forms the temporal, arise from the ascending process in front of the origin of the external pterygoid.

In stage F (figs. 22 and 23) there is a downward and backward growth from about the middle of the tympauicbone. The gouiale bone‘ is also formed; it lies dorsal-median to the tympanio bone, between it and Meckel’s cartilage; its anterior end extends slightly further forwards than does the tympanic, and its posterior end further back, nndei-lapping the malleus portion. 'l‘he origin of the temporal muscle has extended backwards so that it additionally arises from the pa.rieta.lplatte. The Common Anlage of the tensor veli palatini and tensor tympani is beginning to separate into those muscles, and the insertion of the latter has shifted down the side of the malleus portion of Meckel’s cartilage to its ma.nubria.l process. The tensor veli palatini is inserted on the inner side of Meckel’s cartilage and does not reach the tympanic bone lying on the other side of Meckel’s cartilage. fig. 39, taken from a 10 mm. specimen of Didelphys aurita, shows H. little more advanced stage of the same condition.

In stage H (figs. 24~27) the parietal bone is formed outside the orbito-parietal oommissure and parietal-platte, and the temporal arises from it-.

The alisphenoid bone is formed on the outer and upper sides of the ala temporalis, and upwards round the processus ascendens, which is also involved in the ossification. The bone extends in front of and behind the process. The lower anterior fibres of the temporal, the external pterygoid, and the upper head of‘ the internal pterygoid correspondingly arise from this bone, and the upper fibres of the temporal also from the parietal bone. The external pterygoid has two heads, separated from one another by the bnccal nerve. ’l‘he lower portion of the temporal is beginning to be separated olf as the zygornatico-mandibnlaris. The internal pterygoid has become partially separated into two portions, one hea.d—the original one—arising from the alisplienoid hone, tl1e other from the palate bone; the latter head, in stage J, additionally arises from the pterygoicl bone. The proximal portion of the tensor tympani, i. e. that attached to the pterygnid bone, has begun to disappear, mid the proximal end of the persisting distal portion lies alongside the distal end of the tensor veli palatini which has lost its atmclnnent to Mec-.kel’s cartilage. This process continues, so that by st:ig'e J (fig. 28) the adjacent ends of the two muscles join end to end, separated only by a tendinous intersection. In stages 0 to F the proximal end of the tensor veli pnlntini is, as stated above, attached to the ptei-y_<_(oid bone; in stage H (Hg. 26) it has additionally grown inwards below the pterygoid bone (in which a nodule of ourtilage—-the han1ulus——has appeared) into the soft palate ; and in stage J a. well-marked transverse aponeurosis connects together the inner ends of the muscles of the two sides. A similar process occurs in Didelpliys aurita.


  • I use Ga.upp’s nomenclature. Pal merhns recently homologised it with the supra-engulare, though without any discussion of Gaupp’s views.


In stage H there is :2. cap of dense tissue just above the dorsal end of the condylar process of the lower jaw, and into this the squamous bone is beginning to spread. In stage J the extension of the squamous inwards is more marked, and a. split—the joint cavity—is developed in the dense tissue. Cartilage is developed in the condylar process in stage J (fig. 28). The external pterygoid is inserted into the condylar process and its neck in both stages;

In a 8'5 mm. embryo of Ornith0rli_ynchus (figs. 29—33) the Anlagen of the incus, Meckel’s cartilage, ale temporalis, and prucessns alaris are present in H. precartilaginous condition. Aggregated inesoblast cells connect together the ala temporalis and processns alaris. The Anlage of the mandible is present, dorso-lateral to that of Mecln;-l’s cartil-a.ge. The mastic-atory muscles consist of long striated muscle-cells, in two groups—a medial and lateral. The anterior fibres of the medial muscle descend vertically from just below the ala. temporalis towards Meckel’s cm'tila.ge, those next behind pass downwards and baokxvamds, and the succeeding ones more and more l.)E!.Cl{\VELl‘dS; the hiudmost reach as far back as the hind end of Meckel’s can-tila.ge; there is no break in the muscle mass. The lateral muscle consists of fibres which, anteriorly, pass towards the upper edge and outer side of the anlage of the mandible; the upper edge of the innermost fibres is close to the nln. tempornlis, whilst that of the more externa.l fibres is outside the 1n.teral surface of the Grasserinn ganglion. The innscle-fibres which dorsally lie behind the hind end of the Aulage of the mandible pass downxvm-d and forwards t0WEl.1‘dS its outer surface, and the lower end of this portion of the muscle is not continuous with that of the miterior portion,dorsally, the two portions of the muscle are continuous. '

The third division of the fifth nerve passes downwards between the medial and lateral muscles, giving ofi the lateral and medial rami to the muscles. The buccal nerve penetrates the inner fibres of the la.tera.1 muscle.

The intermandibnlaris forms a. ventrally curved sheet, zit:-aclied on each side to the inner surface of Mecl<e1’s cni-tilages. It does not at this stage extend further buck. The depressor mandibulae nnterioris being p1~olife1'nted transversely outwards from the under surface of the interma.n— dibularis.

In a 25 mm. specimen of Echicl na. ( = Stage 50 of Seinon), the mnsticatory muscles consist of pterygo—t_ympa.uicus, tensor tyinpani, temporad, externzil pterygoid, zygotna.tico-m;uidibnlnris, masseter, and detrnhens nizmdibulae (figs. 34-36). The pterygo-tympa.nicus arises from the (M-aimnaliun) pterygoid bone and is inserted into Meckel’s cartilage. The tensor tympani arises from the (Mauiiiialiaii) pterygoid bone and is inserted into the xnulleiis portion of Meckel’s cartilage. The rnmns medialis of the in-.i.ndibu|a.r division of the fifth nerve iunervates these two muscles. The deti-a.heus nm.ndi—


masticatory muscles consist of long striated muscle-cells, in two groups—a medial and lateral. The anterior fibres of the medial muscle descend vertically from just below the ala. temporalis towards Meckel’s cm'tila.ge, those next behind pass downwards and baokxvamds, and the succeeding ones more and more l.)E!.Cl{\VELl‘dS; the hiudmost reach as far back as the hind end of Meckel’s can-tila.ge; there is no break in the muscle mass. The lateral muscle consists of fibres which, anteriorly, pass towards the upper edge and outer side of the anlage of the mandible; the upper edge of the innermost fibres is close to the nln. tempornlis, whilst that of the more externa.l fibres is outside the 1n.teral surface of the Grasserinn ganglion. The innscle-fibres which dorsally lie behind the hind end of the Aulage of the mandible pass downxvm-d and forwards t0WEl.1‘dS its outer surface, and the lower end of this portion of the muscle is not continuous with that of the miterior portion,dorsally, the two portions of the muscle are continuous. '

The third division of the fifth nerve passes downwards between the medial and lateral muscles, giving ofi the lateral and medial rami to the muscles. The buccal nerve penetrates the inner fibres of the la.tera.1 muscle.

The intermandibnlaris forms a. ventrally curved sheet, zit:-aclied on each side to the inner surface of Mecl<e1’s cni-tilages. It does not at this stage extend further buck. The depressor mandibulae nnterioris being p1~olife1'nted transversely outwards from the under surface of the interma.n— dibularis.

In a 25 mm. specimen of Echicl na. ( = Stage 50 of Seinon), the mnsticatory muscles consist of pterygo—t_ympa.uicus, tensor tyinpani, temporad, externzil pterygoid, zygotna.tico-m;uidibnlnris, masseter, and detrnhens nizmdibulae (figs. 34-36). The pterygo-tympa.nicus arises from the (M-aimnaliun) pterygoid bone and is inserted into Meckel’s cartilage. The tensor tympani arises from the (Mauiiiialiaii) pterygoid bone and is inserted into the xnulleiis portion of Meckel’s cartilage. The rnmns medialis of the in-.i.ndibu|a.r division of the fifth nerve iunervates these two muscles. The deti-a.heus mandibulae arises from the outer end of the inturned crista. pm-otica and is inserted into the outer surface of the mandible.

The intermandibulai-is (figs. 34-37) has spread behind the jaw as far back as the stylohyale, its anterior fibres———attached to the inner surface of Meckel’s cartilage in stage 47—spread up towards the outer edge of the (Monotreme) pterygoid bone ; the fibres next behind these are attached dorsally to the tympanic bone ; the most posterior fibres are not attached to the stylohya]. The depressor mandibnlaa anterior is oblique in position, its inner end is more posterior than its outer, and is attached to dense connective tissue in front of the hyoid ,the fibres pass outwards and forxvurds and are attached tojtlie inner snrfa.ce of the mandible.

In a. 4:} mm. embryo of the rabbit the Anlagen of the masti THE MANDIBULAR. AND HYOID MUSCLES OE‘ MAMMALS. 531

catory muscles and the iutermaudihulamis are continuous with one another and lie internal to the third division of the fifth nerve; the upper limit of the cell column lies below the Gaseerian ganglion.

In :t 5} mm. embryo (fig. 45) the Anlage of the masticatory muscles spreads laterally in front of the third division of the fifth nerve, so that the nerve passes down behind the Anlageg whilst the primitive position of nerve and muscle Anlage is preserved lower clown in the region of the interma.ndibnla.ris.

In a. 7 mm. embryo the Anlegen of the masticatory muscles and interma.ndihula.ris separate from one another. The first branch of the third division of the fifth nerve to be given oif is the bucoal, which runs forward above the Anlage of the ma.stica.tory muscles. The mmi xnedialis and letemlis are formed in an 8 mm. embryo, passing respectively inwards and outwards into the masticatoi-y Aulage. The ramus posterior is formed in a 9 mm. embryo.

In a 13 mm. rabbit embryo (figs. 65~—67) the Ga.sseria.n ganglion is a relatively huge mass above the Anlagen of the masticatory muscles. Below the ganglion is the Anlage of the ala. teniporalis; the proeessus alaris is not yet formed.

fiG. 67.


Meckel’s cartilage is a. slightly curved rod. The Anlage of the mandible, in which ossification has not yet begun, lies on the outside of Meckel’s cartilage; its anterior edge does not extend quite so far forwards as that of the muscle-Aulagen ; pusteriorly its upper part ends abruptly except below, where there is a. slight bzickivard process. The auriculo-temporal nerve runs outwards and dowvmvamls behind the abrupt posterior edge and over the slight bacl(\vard projection of the mandible. The Anlage of the masticatory muscles has not yet divided into medial and lateral portions, these being continuous in front of the mandibular division of the fifth nerve. The medial portion extends downwards from just below the ale. temporalis inside Mecke1’s cartilage towards the mandible; there is an inward bulge from its upper part —the Anlage of the tensor veli palatini, and a long posterior projection—the Anlage of the tensor tympani, extending backwards to the mallens 5 the main mass of the medial portion is the Anlage of the internal pt-erygoid. The main mass of the lateral portion (Anlage of tempora.l) has an upward projection on the outer side of the Gasser-iaii ganglion, its lower end embraces the upper part of the mandible. There is a. notch on its posterior border——the first indication of the separation of 3. zygonmtico-mandibnlaris and masseter portion from the temporal, and an inward projection—the Anlage of the external pterygoid, from the hinder part of its inner aspect, towards the a.la. temporalis.

In a.16n1m. embryo (figs. 46-48 and 68, 69) the 11.12. orbitalis and oi-bito-parietal commissure, the latter with a. free posterior extremity, are present. The parietal-platte and processus a.la.ris are not formed. The ale. itemporztlis has no processus ascendens. The Anlage of the mandible has coronoid and condylar processes. The Anlage of the masticatory muscles has divided into medial and lateral parts, each of which is imperfectly separated into muscles, which consist of groups of spindle-shaped cells. The medial part consists of the following: the internal pterygoid, which arises from the ala temporalis and is inserted into the mandible, it shows the beginning of a. division into two parts. The tensor veli palatini lies just internal to the upper end of the internal

fiG. 69.


An.z)/go. mass. mi‘

pterygoid with a. free anterior end; it passes backwards above the hinder margin of the internal pterygoid and has a free posterior end. The tensor tympani lies immediately above and in contact with the hinder part of the tensor veli palatini, with a free anterior end; it passes backwards and inwards to the side of the malleus. The lateral part of the Anloge of the masticatory muscles consists of the following : the temporal, which has an oblique slightly concave inner edge; its upper ext1‘emit_y extends up outside the Gasserian ganglion, but has no dorsal attachment; it is inserted into the coronoid process. The external pterygoid arises from the ala teinporalis behind the anterior fibres of the temporal and is inserted into the condyla-r process of the Anlage of the mandible. The zygomatico-mandibularis-masseter is partially marked of from the external fork of the temporal by a. groove passing upwards from the posterior edge.

In a 23 mm. embryo (figs. 49-53) the muscles have become quite separate from one another. The internal pterygoid now arises by two heads, one from the under-sun-fa.ce of the 9.13. temporalis, the other from the palatine portion of the common Anlage of the palate and pterygoid bones; they unite and are inserted into the inaudible. The anterior end of the tensor veli pa.la.tini winds round the hamulus into the soft palate; the muscle extends bztclnvm-cls medial to the internal pterygoid and is inserted partly to the tympanic bone, partly to the proximal end of the tensor tympani. The tensor tympani now lies wholly behind the internal pterygoid and tensor veli palatini owing to disa.ppeara.nce of its anterior portion; it arises pamtly from the tendon of the tensor veli palatini, partly from the outer surface of the auditory capsule. The external pterygoid arises partly from the external surface of the ela. temporalis and partly from the palatal portion of the common Anlage of the palate and pterygoid bones. The temporal arises by two hea.ds,f1-om the lamina.

fiG. 71.


ascendens else tempo:-alis and from the Anlage of the pa,1~ieta,l bone. The masseter and zygomatico-mandibularis, not yet sepamted, ave continuous dorsally with the temporal.

In a. 33 mm. embryo (fig. 54) the zygomatico-ma.ndibula.ris and massetev have become separated. The palatine and pterygoid bones have ossiiied, anti one head of the external pterygoid arises from the palate bone.

figs. 70 and 71 represent a. model made from sections passing through the Aulztge of the ma.stica.tory muscles in 2:. 19 mm. embryonic pig; the stage of development is a. little less advanced than a. 13 mm. rabbit embryo. Meckel’s cartilage is a. slightly curved rod of pre-cartilage; its hinder portion and the Anlage of the mandible, though present, had not sufiiciently clean‘ outlines to be modelled. The Anlage of the a.la. temporalis is present ; those of the ala orbitalis, orbiteparietal commissure, parietal-platte, and processus a.la.ris, are not yet formed. The Aulage of the masticatory muscles forms a. continuous mass, approximately A-shaped, with an apex just beneath the ala tempomlis, and having an upward projection on the outer side of the Gasserian ganglion. The outer limb of the mass, with the upward projection, is the Anlage of the temporal, external pterygoid and masseter muscles. The inner limb is the Anlage of the internal pterygoid, tensor veli palatini, and tensor tympani; there is an inward bulge of its upper pin-I——the future tensor veli palatini, ancl a backxvard projecting part—the future tensor tyuipani ; its lower end is separated from Meckel’s cartilage by the chorda tympani.

The third division of the fifth nerve gives off the buccal nerve (which penetrates the upper part of the outer limb of the mass), then passes clown behind the apex of the mass, giving oil? the raini medialis and lateralis, and then forwards in the gap between the two limbs, dividing into auricnlo-te1nporal, mylohyoid, inferior dental, and lingual branches.

figs. 59-63 and 72,73 are made from a. 21 mm. pig embryo. The stage of development of the masticatory muscles is slightly more advaiiced than that of a 16 mm. rabbit embryo. The ala orbitnlis is continuous posteriorly with the orbito-parietal coinniissure, which does not reach the pan-ietal-platte. The ala temporalis is just beginning to choudrify. The processus n.la.ris is not yet formed. The Anlage of the mandible, in which ossification has not begun, lies outside Meckel’s cartilage and shows slightly marked coronoid and condylar processes. The Anlage of the masticatory muscles has divided into medial and la.tera.l portions. The former consists of internal pterygoid, tensor veli palatini and tensor tynipani—which are sepa.ra.te structures, though very close together. Theinternal pterygoid arises from the ala temporalis and is inserted into the mandible, the spindle-shaped cells not having any attachment to Meckel’s cartilage. The tensor veli palatini, which is not attached to any skeletal structure, lies internal to the npperencl of the internal pterygoid, with its muscle-cells at right angles to those of the internal pterygoid. The tensor tympani lies behind the tensor veli palatini, with st free anterior extremity; it is inserted, posteriorly, int-0 the non-chondrified malleus portion of Meckel’s cartilage. The lateral portion of the Anlage of the masticaitory muscles consists of temporal external pterygoid, zygomatico-mandibular-is, and masseter muscles, which are only partially separated from one another. The temporal has no upper attachment, its lower anterior fibres are in front of the external pterygoid, those above extend upwards outside the Gasserian ganglion; it is inserted into the coronoid process of the mandible. The external pterygoicl arises from the ala tenlporalis and is inserted into the eondylar process of the mnndilile.

The Anlage of the zygoinatico—1nanclibnlai-is and masseter is continuous with the temporal and is inserted into the Anlage of the mandible. The united lingual and inferior dental giving oil? the raini medialis and lateralis, and then forwards in the gap between the two limbs, dividing into auricnlo-te1nporal, mylohyoid, inferior dental, and lingual branches.

figs. 59-63 and 72,73 are made from a. 21 mm. pig embryo. The stage of development of the masticatory muscles is slightly more advaiiced than that of a 16 mm. rabbit embryo. The ala orbitnlis is continuous posteriorly with the orbito-parietal coinniissure, which does not reach the pan-ietal-platte. The ala temporalis is just beginning to choudrify. The processus n.la.ris is not yet formed. The Anlage of the mandible, in which ossification has not begun, lies outside Meckel’s cartilage and shows slightly marked coronoid and condylar processes. The Anlage of the masticatory muscles has divided into medial and la.tera.l portions. The former consists of internal pterygoid, tensor veli palatini and tensor tynipani—which are sepa.ra.te structures, though very close together. Theinternal pterygoid arises from the ala temporalis and is inserted into the mandible, the spindle-shaped cells not having any attachment to Meckel’s cartilage. The tensor veli palatini, which is not attached to any skeletal structure, lies internal to the npperencl of the internal pterygoid, with its muscle-cells at right angles to those of the internal pterygoid. The tensor tympani lies behind the tensor veli palatini, with st free anterior extremity; it is inserted, posteriorly, int-0 the non-chondrified malleus portion of Meckel’s cartilage. The lateral portion of the Anlage of the masticaitory muscles consists of temporal external pterygoid, zygomatico-mandibular-is, and masseter muscles, which are only partially separated from one another. The temporal has no upper attachment, its lower anterior fibres are in front of the external pterygoid, those above extend upwards outside the Gasserian ganglion; it is inserted into the coronoid process of the mandible. The external pterygoicl arises from the ala tenlporalis and is inserted into the eondylar process of the mnndilile.

The Anlage of the zygoinatico—1nanclibnlai-is and masseter is continuous with the temporal and is inserted into the Anlage of the mandible. The united lingual and inferior dental nerves pass between the internal and external pterygoids, then separate; the lingual is joined by the chorda tympani medial to the internal pterygoid and passes to the inferior maxillary ganglion ; the inferior dental passes forward on the medial surface of the Anlage of the niandible. The niylohyoid branch of the inferior dental passes downwards between Meckel’s cartilage and the Anlage of the inaudible and then forwards on the lateral surface of the interniandibularis muscle. The auriculo-temporal passes outwards between Meckel’s car— tilage and the Anlage of the mandible.

In a 24 mm. pig embryo the condyloid and coronoid processes of the mandible are better marked, and ossification has begun. The upper end of the temporal has grown further upwards outside the membranous cranium, but is not attached to the orbito-parietal commissure. The Anlage of the parietal bone is not yet formed. The Anlage of the zygomatico-mam dibulai-is and masseter arises partly from the temporal, partly from the Anlage of the zygomatic arch.

In a 32 mm. embryo (Hg. 64), the orbito—pa1-ietal commissure is complete; it is covered by the Aulage of the parietal bone. The alisphenoicl bone is not yet formed. The temporal arises from the parietal bone and the lamina ascendens also temporalis which is now formed. The Anlage of the zygomatico-mandibularis and masseter is beginning to separate into those two muscles. The internal pterygoid remains single, arising from the under surface of the ala temporalis. The proximal end of the tensor veli palatini has spread into the soft palate round the hamulus ; its distal end is connected with the tensor tympani by a. tendinous intersection.

The above-described phenomena show that the Anlage of the masticatory muscles in Mammals divides into medial and lateral portions, and that each develops into certain muscles. These may be taken separately. .

Medial Muscle.——The primitive form of the medial muscle is present in 8'5 mm. embryos of Ornithorhynchus and stages A and B of Dasyurus, taking origin from the ala temporalis and inserted into the whole length of Meckel’s THE MANDIBULAR AND EYOID MUSOLES or MAMMALS. 591

cartilage. This is not present in the rabbit and pig, being passed over in the Anlage stage.

The medial muscle or muscle-Anlage in Dasyurus, rabbit and pig, separates into three—froin before loackward, the internal pterygoid, tensor veli palatini, and tensor tympani. The internal pterygoid is the first to separate, the division into tensor veli palatini and tensor tyuipaui occurring later. The internal pterygoid of Dasyurus first forms a muscle passing from the ala teinporalis to Mecl<el’s cartilzige. On the occurrence of ossification it passes from the alisphenoicl bone to the mandible and also gains an additional origin, from the palate and pterygoid bones. This is also the case in Didelphys aurita. In the rabbit and pig, the muscle at first arises from the aln temporalis and is inserted into the mandible—the stage of insertion into Me-ckel’s cartilage being passed over. On ossification the muscle arises from the alisphenoid bone, with—in the case of the rabbit an additional head from the palate bone.

In adult forms of Ornithorhynch us and Echid na there exists a muscle which was stated by Meckel and by Toldt to be homologous with the internal pterygoid of other inammuls. Schnlman stated that it is a true meinber—the caput anterius ——of the temporalis group, as shown by its connection with the rest of the muscle and by its innervation by the N. ternporalis profundus. This latter ststtcment is confirmed by the evidence of embryos of Echldna in Stages 47 and 50 (of Semen); in these there is no muscle arising from the ala Leinporalis or (Mammalian) pterygoid bone and inserted into Mecl<el’s cartilage or mandible. An internal pterygoid is thus absent.

An 85 mm. embryo of Ornithorhyncli us (fig. 30) shows that the anterior fibres of the median nmsticatory muscle descend vertically to M.eckel’s cartilage, i. e . fibres are present homologous with those which form the internal pterygoid in Dasyurus. It may be concluded that this muscle tttrophies in Monotremes, though whether its development in O rn ithoihynchus advances a. stage further than that just mentioned before it atrophies could not be determined owing to want of ma.teria.l.

The pterygo-tympanicus s. tensor veli palatini is the middle one of the three into which the primitive medial muscle or muscle-Anlage separates.

Its development could not be followed in Ornitho rhynchus. In Echidua.—stage 50, the earliest availablethe pterygo-tyrnpanicus arises from the (Mammalian) pterygoid bone and is inserted into Meckel’s cartilage. The muscle subsequently disappears, being absent in the adult (_SchLilma.11n).

In Dasyurus the pterygo-tynipanicus arises from the pterygoid bone and is inserted into Meckel’s cartilage, there being no stage in which, as a separate muscle, it arises from the ala temporalis. In later stages the distal end of the muscle loses its attachnient to i\[eckel’s cartilage and becomes connected with the persisting distal end of the tensor tympani. Insertion into the tympanic bone does not occur. At the same time the proximal end of the muscle—hitherto attached to the pterygoid bone———ad(litionall_y grows inwards in the soft palate and forms an aponeurosis with the muscle of the opposite side. The pterygo-tympanicns thus becomes the tensor veli palatini. The same series of events occurs in Diclelphys anrita.

In the rabbit the pterygo-tympanicns s. tensor veli palatini, when first visible as 1-1, separate muscle, lies on the medial side of the internal pterygoid. Its proximal end does not gain any attachment to the ala temporalis or pterygoid bone, but, later, grows inward in the soft palate round the hamulus. Its distal end becomes attached to the tympanic bone, but there is no antecedent stage comparable to that of Echidna. and Dasyurus in which it is inserted into Meckel’s cartilage. It also becomes attached to the persisting distal end of the tensor tympani. In the pig the process of development is abbreviated. The muscle, as in the rabbit, is first visible on the medial side of the internal pterygoid; its proximal end does not gain any attaclnnent to either the ala tempora.lis or

THE MANDlBULAlt AND HYOID MUSCLES OF l\lAi\Ii\lALS. 593

pterygoid bone, but grows round the hamnlus into the soft palate; its distal end does not gain any insertion to either Meckel’s cartilage or the tympanic bone, but becomes attached to the tensor tympani by tendon. In Dasyurus, Didelphy s, rabbit and pig, the development of the hamulus is synchronous with the ingrowth of the muscle into the soft palate.

On comparison of the descriptions of the pterygo-tympanicus given by Kostanecki, Schulman and Lubosch, those of the tensor veli palatini given by Kostanecki, and the embryological phenomena described above, it may be concluded that the muscle is derived from one taking origin from the ala ternporalis and inserted into Meckel’s cartilage. This stage is not present in any of the animals investigated, but its existence, in phylogeny, may be inferred from the phenomena in early stages of Ornithorhynchus and Dasyurus, i . e. an undivided medial muscle passing from the ala temporalis to Meckel’s cartilage, and after separation of this into parts, the insertion of the pterygo-tympanicusfin Echidna and Dasyurns, into Meckel’s cartilage.

On the occurrence of ossification the muscle extended from the (l\Iam1nalia.u) pterygoid bone to the tympanic bone. In Dasyurus this stage is present as regards the origin of the muscle but insertion into the tympanic bone is passed over; in the rabbit it is present as 1‘ega.rds the insertion but passed over as reg‘-.irds the origin; in the pig neither attachment occurs.

The Anlage of the tympanic bone in Dasynrus is formed as a straight rod ventro-lateral to the hinder part of Meckel’s cartilage, with its anterior part overlapped by the Anlage of the inaudible; subsequently an outgrowth downwards and backwards occurs, originating just behind the hind edge of the mandible. This method of formation is in harmony with the theory of van Kampen that the tympanic bone was primarily a. covering bone for the hinder part of Mecl:el's cartilage, and only snl)seqnently—on development of a squamoso-inandibular jaw joint—becaine it part of the wall of the tympanic cavity, He suggested that it was derived from the supra.-angulare or angulare of the lower jaw, which are present in Amphibia and Reptiles. This opinion was adopted by Graupp, who homologised the tympanic bone with the angulare, and pointed out that the presence of a. pterygo-tympanicus in some Mammals was an additional fact in favour of van Kampen’s theory.

It is probable, then, that the n1uscle—on the occurrence of 0ssificatiou—arose from the (Mammalian) pterygoid bone, and was inserted into the meckelian (perhaps the only) portion of a tympanic bone which formed a covering bone for the old jaw with an incudo-meckelian joint. On the development of the new squamoso-manclibular joint, the muscle, being inserted behind this point, lost its significance as regards any action on the jaw, and as far as present information goes, this condition is not preserved in any adult Mammal.

No young enough Edentate embryos have been investigated to ascertain whether, in ontogenetic development, the muscle is inserted into the meckeliau portion of the tympanic bone, but an indication of at least its phylogenetic existence is given by the temporary insertion of the muscle in Dasynrus iuto Meckel’s cartilage opposite this portion of the bone. In late embryos of Dasyurns novemcincta and Bradypus marmoratus the muscle is inserted into the later formed lower limb of the bone.

Insertion solely into the tympanic bone is preserved to the adult condition in Bradypus tridactylus; the muscle generally spreads to neighbouring structures. Thus Schulinan described its insertion in Ornithorhynohus to the junction of the os sphenoideum and 0s petrosuin, in Cholcepus to the ligameutum accessorium mediale and walls of the fissura squainosopetro-tyrnpanica, in Manis to the “'I‘rommelh<')'hle,” in Tamandua to the bone round the cleft through which the chords. tympani passes; a.nd Kostanecki described the insertion in Dasyurus sexcinctus to the bulla. tympauica and 0s sphenoides.

The origin of the pterygo-tympanicus from the pterygoid bone is, similarly, rarely preserved; in Edentates only, as far as present information goes, in Tamandua (Schulinan),


Tolypentes (Lubosch), Bradyp us trid a cty l 11 s (Kostanecki) Manis pen tadactyla. (F. H. E.), whereas in Choltepus and Manis Ja.va.nica (Schulman), Dasypus sexcinctus (Kostanecki), Dasypus novenicincta and B1-adypus ma,rmor:1tns (F.H.E.) the muscle spreads into the softpalate, forming a tensor veli palatini.‘ This secondary ingrowth remains in connection with the original pterygo-tympanicus, and may or may not have fibres taking origin from the pterygoid bone.

The condition present in the latter four Edentates hardly diEers—u.s far as regards the proximal portion of the muscle —from that present in higher Mammals where the whole muscle, i. e. pterygo-tympanicus + its ingrowth into the soft palate, has generally been called the tensor veli palatini.

Schulman was of opinion that the relations of the pterygospiuosus s. tynipanicus of Ornithorhynchus indicated the existence of a. movable (Monotreme) pterygoid in Proniamnialia. - In Echidna, however, the pterygo-tympanicus arises from the (Mammalian) pterygoid in stage 50 (though subsequently a.trophying)—a.n occurrence that suggests that the origin of the muscle in the adult Ornithorhynchns from the (Monotreme) pterygoid is a secondury occurrence. Further, adoption of van Kan1pen’s theory that the tyuipanic bone was, phylogenetically, a covering bone for a jaw with an incudo-meckelian joint, and so Et movable structure, makes the theory improbable.

Lubosch doubted the homology of the pterygoid bone in the Zenarthra, and Pholidota. with that of other Mammals, and suggested that it might be homologous with the Echidna pterygoid, or possibly the result of fusion of n. Mammalian with an Echidna pterygoid. The early stages are not yet known, but in Dasypus novemciucta (embryo 30 mm.) and Bradypns marmoratus (embryo 30 mm.) the relations of the pterygo-tympanicus muscle to that bone were identical with those in Dasyui-us, and the bone, which showed no evidence of fusion, lay ventro-median and close to the ale temporalis, and was apparently homologous with the ordinary mammalian pterygoid.

‘ Schulman was the first to recognise the genetic relationship between the pterygo-tympzmieus and the tensor veli palatini.



The attachment of the (original) proximal1 end of the tensor veli palatini to the pterygoid bone and hamulus is preserved in Man, Primates, Pinnipedia, Iiisectivora, Perisso~ dactyla, Artiodaotyla, Marsupials; or it may altogether fail as in Cheiroptera, Ca.rnivora, and Rodents (Kostanecki). The embryological phenomena. in the rabbit and pig show that the latter occurrence may, at any rate in some cases, be due to its non-development.

The distal end of the tensor veli palatini in the rabbit becomes attached to the tympanic bone, whereas in Dasym-us, Didelphys and pig this ancient attachment is never gained. In the rabbit it persists up to the stage of 33 mm. (the oldest investigated), but judging from the account given by Kostanecki it is not present in any adult Mammal. It has yet to be determined in what orders other than Rodents a temporary attachment to the tympanic bone is gained. A secondary attachment to other adjacent bones is gained in some orders, e. g. to the bulla ossea in Marsupials and Rodents.

One almost constant feature of the distal end of the tensor veli palatini is attachment to the tensor tympani. Thus Kostanecki states that “ Der Zusammenhang zwischen dem Tensor veli und Tensor tympani ist bei vielen S‘a'.ugethieren erhalten, wo aber ein solcher verloren gegangen ist, weisen doch manche Punkte ant die friiher bestehenden Beziehnngen hin, die so sogar nnr im Ansohlnss daran erklaren lassen.” The developmental phenomena in Dasynrns, pig and rabbit, show that this connection with the tensor tympani is intimately related to the atrophy or want of development of the proximal portion of the latter muscle.

‘ The nomenclature adopted he1'e is in reference to the original form and attacliments of the pterygo-tympanicns, from which the tensor veli palatini is derived, and is the reverse of that used by Kostanecki. The tensor veli palatini is a.n interesting instance of transference of “origin” from the proximal end to the distal end of a muscle.

THE MANDIBULAR AND HYOID MUSCLES or MAMMALS. 597

The distal end of the tensor veli palntini also gains attachment to the walls of the Eustachian tube, with which it is from the Very first in close relationship.

As regards innervation, Schulman determined that the pterygo-spinosus of Ornitliorhynclius is innervated by a branch of the ramus medialis, s. ventralis, of the mandibular division of the fifth. Lubosch did not describe the innervation of the pterygo-tympanicus of Edentates; but in Echidna, Dasypus noveincincta, Brudypus mD.1'1'l10I‘a.i7uS, and Manis pentadactyla, I found the same innerva.tion as in Ornithorhynchus.

The tensor veli palatini has long been recognised as being innervated by the ra.mus medialis of the mandibular division of the fifth. I have recently confirmed this by showing that in Macacns cynomolgus the muscle degenerates after section of the fifth nerve proximal to the Gasserian ganglion.

Tensor Ty in pani .—'l‘he tensor tympani in early stages of Dasyurns passes from the pterygoid bone to the side of the body of the malleus portion of Meel<el’s cartilage. Later on the proximal portion of the muscle disappears, and the proximal end of the persisting distal portion becomes attached to the distal end of the tensor veli palatini and the cartilaginous wall of the auditory capsule. It may be inferred that a stage has been passed over in which the tensor tyinpani—as a separate muscle—arose from the ala temporalis. Such a. stage is present in a 30 mm., i.e. relatively late embryo of Dasypus novenicincta.

In Eehidna, in stage 50, the tensor tympani passes from the (Mammalian) pterygoid bone to the malleus. The proximal portion of the muscle subsequently disappears, the proximal end of the persistingdistal portion being attached in the adult to the patrons bone (Eschweiler).

In the rabbit the muscle never has any origin from either the ala temporalis or the pterygoidbone, though a subsequently atrophying, proximal portion of the muscle, overlapping the tensor veli palatiui, is formed. The proximal end of the persisting distal portion becomes attached to the tensor veli palatini by tendon.

In the pig such a proximal portion is not formed; only the persisting distal portion, behind the Anlage of the tensor veli pa-latini, is developed, as found by Eschweiler. All trace is thus lost, even in developmental stages, of the ancient origin from the pterygoid bone.

It may be concluded, from the descriptions given by Zuckerkandl, Kostanecki. Schulman, and Eschweiler, that the tensor tympani does not arise from the (Mam malinn) pterygoid bone in any adult mammal. This may be due to loss of such attachment during development, e. g. in Echidua, Dasyurus, Bradypus marm oratus, orto its non-formation, e . g. rabbit and pig.

Disappearance of the proximal portion of the muscle either by ontogenetic atrophy or by non—fo1-mation is a marked characteristic of the muscle. In the former case it is apparently brought about by the bulging wall of the pars cochlearis of the auditory capsule interrupting the line of action of the muscle.

The proximal end of the persisting, or solely developed, distal portion of the muscle gains secondary attachments, and especially to the pars oochlearis of the auditory capsule and Eustachian tube. Of these attachments that to the Eustachian tube is in close association with the secondary insertion of the tensor veli palatiui, as described above.

Eschweiler was of the opinion that these various attachments of the tensor tympani are derived from the condition found in Ornithorhynchus, where the muscle in the adult consists of two parts, both inserted into the malleus—a “Rachenbauch” which takes origin from “der Muskulatur hervor w elche am hiutern lateraleu Choanenwand entspringt,” and a “Felsenbeinbauch ” springing from the labyrinth wall.

'l‘he development of the muscle in Ornithorhychus is not yet known, but comparison with Echidua, Dasyurus, pig and rabbit leads to the conclusion that in all probability the “ Raeheubauch” of Ornithorhynchus is a secondary formation.


Temporal.—In Dasyurus the first origin of the lateral niasticatory muscle (from which the temporal, masseter, zygomatico-mandibularis and external ptery goid are subsequently formed) is from the orbito-parietal commissure; the dorsal attachment of the muscle then spreads backwards to the parietal-platte and downwards to the membrana obturatoria covering in the spheno-parietal foramen, with its front edge to the lamina ascendens of the 21.12. temporalis. On the occurrence of ossification the temporal muscle, which has been formed owing to the differentiation of the lateral group muscles, arises from the parietal and alisphenoid bones, the former being developed over the orbito-pa.rietal_ cominissure and parietal-platte, the latter over the ala tempcralis, lamina ascendens and the membrana Obl2l1I'Ett01‘lEL.

In the rabbit and pig, with a longer intra.-uterine development, the Aulage of the lateral masticatory muscle separates into the muscles it forms before attachment takes place. The temporal becomes attached to the lamina asceudens alée temporalis, and spreads upwards to the territory of the whimparietal commissure and parietal-platte, but these latter structures are by that time covered by the Aulage of the parietal bone from which the muscle gains an origin. On the subsequent ossification of the alisphenoid bone the lower fibres arise from that bone.

The lamina ascendens alae temporalis is complete in Dasyurus and Didelphys aurita; it has a free upper end in rabbit, pig, and Dasypus. The orbito-parietal commissure is complete in all four animals.

The lower end of the lateral masticatory muscle in the just born Dasyurus is at first attached to the upper edge of Meckel’s cartilage via”: a mass of cells in which the Anlage of the mandible is just visible. Ossification subsequently extends upwards forming a. coronoid process and downwards on the lateral side of Meckel’s cartilage; in relation with these changes the insertion of the muscle becomes forked and laps the coronoid process on both sides, though to a greater extent laterally than medially.


In the rabbit and pig there is no stage in which the teiupora-l muscle is inserted solely into the upper edge of the Anlage of the mandible; its cells become spindle-shaped before it spreads downwards, with a. forked extremity, on either side of the Anlage of the mandible.

The formation of the coronoid process of the mandible takes place in Dasyurus after ossification has begun, in the rabbit and pig in the Anlage stage. In all three animals the process is formed by :1 dorsal extension into an already formed muscle or muscle~Anlage.

The stages of Ornithorhyuehns and Echidna available did not permit of observations on the development of the individual muscles formed from the lateral masticatory muscle.

\‘Iasseter an cl Zygom atico-mandil)ula.ris.—Inmany

Mammals a zygomatico-uiandibularis is differentiated between the temporal and masseter muscles. Allen, who first described the muscle, in map, considered it to be a separated deep portion of the masseter, whilst Toldt held that it is derived from the temporal. Toldt stated that “ er stammt von der illuskelfasergruppe her, welche die oberfliichlichste Schichte der urspriinglicheu Anlage des Schlafenmuskels darstellt uud von jener Mesodermschichte kommt welc-he sich bleibeucl zur Fascia temporalis gestaltet. Da sich in derselben Mesodermschichte spiiter das Joclibein, beziehungsweise der gauze Jochbogen entwickelt, so tritt ein bestimmter Auteil der bezeichneteu Fasergruppe mit der medialen Fliiche des Jochbogens in Beziehung, d. ll. er nimmt Von dieser bis an ihreu uutereu Rand herab bleibend seiuen Urspruug. Er steht clemgemiiss au seinen Ursprungmit dem Schlafenuniiskel in ununterbrochenen Zusammenhang, hat aber durch die erworbene Beziehung zum Jochbogen eineu besonderen Character erlangt.”

In Dasyurus, as detailed above, the masseter muscle is proliferated, in stage C, downwards and backwards from the anterior ventral edge of the lateral muscle on the outside of the inaudible; and from that time onward can be distinguished in transverse section by the direction of its fibres. The zygomatico-ma.ndibularis is formed subsequently, in stage H, by a sepa.ra.tion of the ventral fibres of the ternpora.l muscle, clorso-posterior to the uiasseter muscle. The fibres of the zygon1a.tico-n1a.ndibula.ris have the same direction as those of the tBuJp01'£‘tl, and up to stage J—the latest iuvestigated—the sepa.ra.tion was not complete.

In the rabbit and pig the process is at little different. The Anlage of the la.tera.l nmsticntory muscle is inserted by a. forked lower edge into the mandible. A groove appears in the posterior edge of the external fork, and spreading upwa.rds and forwa.rds separates the common Anlage of the nmsseter and zygoumtico-nia.ndibula.ris from the temporal. This Anlztge divides into mnsseter and zygomatico-maiidibulamis, the latter being out 0E from the upper posterior p:1.rl: of the common mass. In view of the many modifications associated with the prolonged iutrii.-uterine development, it is probable that the phenomena/ occurring in Dasyurus more closely represent the phylogenetic history of the muscles than do those of the rabbit and pig. Both inasseter and zygonmtico-nmudibulan-is have their first insertion into the Anlege of the nnmdible. Neither has any temporary insertion into Meckel’s cartilage.

The detrahens mandibulee is present only i11 Monotremes. No trace of it was seen in» the developmental stages of Dasyurus, pig and rabbit. It is innervated by the fifth nerve (Westling, fiirbringer, Gaupp, Schulman). Schulnian regai-cled it as belonging to the dorsal (in this paper regarded as “ lateral ”) group of inasticatoi-y muscles as—lil-re the temporal, external pterygoid, masseter, and zygomutico-ma,ndibularis—he found it to be supplied by the dorsal s. lateral division of the ina.nclihnlm~ nerve.

Owing to want of ma.teria.l its development could not he completely followed in either Ornithorhynchns or Echidnzi, but the stages described above permit of an approxiniate account. The muscle can be identified with the hinder fibres of the liiteral niusticutoi-y muscle in 8'5 mm. embryos of Ornitl1orhynchus-—those fibres which pass downwards and foi-wa.rds towards the outer surface of the hind end of the Anlage of the mandible. In stage 47 of Echidna. the muscle has separated off and passes from the root of the crista parotica downwards and forwards to the outer surface of the mandible. This condition is also present in stage 50. In the adult Echidna. this primary origin is lost, and the muscle arises from the ventral surface of the squamosal bone (Schulman).

These phenomena support the view of Schulman that the detrahens mandibulac is one of the lateral group of masticatory muscles. They negative the view of Toldt that it is split off from an already differentiated masseter, and suggest that the muscle is, pliylogenetically, more ancient than either the masseter, zygomatico-mandibularis or external pterygoid.

The external pterygoid muscle is developed from the temporal in Dasyurus, pig and rabbit. Its origin in Ornithorhynchus and Echidna was not determined owing to a want of the necessary stages. The process could be most clearly seen in Dasyurus, where the cells forming the muscle are at first small and non-striated, and probably proliferated from, and not formed by change of direction of, the longer cross-striated muscle cells of the temporal. In the rabbit and pig the formation of the external pterygoid takes place in an Anlage consisting of oval cells, and a.ppea.red to be due to separation of a portion of the temporal Anlage.

The primary origin of the external pterygoid in Echidna is from the membrana obtnratoria above the aln temporalis; in Dasyurus, and Didelphys aurita. it isfrom the lower end of thelamina ascendens alee temporalis; in the rabbit and pig from the ale. temporalis. On the formation of the alisplienoid bone outside the ale, and its lamina ascendens, the muscle——in Dasyurus, Didelphys, rabbit and pig——arises from this bone; and in the rabbit it gains an additional origin from the palate bone. In the adult Echidna (Schulman) the muscle arises from the plenum infratemporale of the great wing of the sphenoid and from the alt temporalis palatini.

Origin of the muscle from the alisphenoid bone is common : thus Lubosch describes the origin of the muscle, in Bradypus from the pterygoid bone, in Dasypus and Tolypentcs by two heads from the alisphenoid, in Tatusia by two heads, one from the alisphenoid, the other from the alisphenoid and palate bones, in Tamandna. by two heads, one from the parietal, the other from the palate.

The first insertion of the external pterygoid is into the condylar process of either the ossified mandible (Dasyurns) or its Anlage (rabbit and pig). There is no transitory insertion into Meckel’s cartilage. The development of a condylar process is synchronous with the development of the external pterygoid. These phenomena support the view of Gaupp that the condylar process is primarily a “ Muskelfortsatz.”

An external pterygoid muscle is present in most Mammals. Two exceptions have been recorded and are of considerable interest. Leche was of opinion that the internal and external pterygoid muscles “ auf einen gemeinsamen Ursprung zuriickgefuhrt werden kiinnen,” but did not record any instances other than that of Phoca, in which one muscle only was present, which might represent a common muscle mass from which in other Mammals internal and external pterygoids were differentiated.

Humphry had described in Phooa. communis one pterygoid muscle only “ arising from the outer side and edge of the slightly developed pterygoid part of the sphenoid and passing to the inner side of the angular part of the jaw.” He did not mention its innervation nor did he describe the temporal muscle.

Ifind that in Halichoeru s gryp us (greyseal) the masseter, zygoma.tico—ma.ndibnlaris, temporal, external pterygoid, internal pterygoid, tensor veli pa-latini and tensor tympani muscles are present. The temporal is inserted into the coronoid process, the external pterygoid into the condyloid process and interarticnlar meniscus, and the internal pterygoid into the inner side of the angular part of the jaw. The ramus lateralis of the mandibular division of the trigeminal nerve supplies the masseter, zygomatico-mandibularis, temporal (by three twigs), and the external pterygoid; the ramus medialis supplies the internal pterygoid, tensor veli palatini, and tensor tympani.


In Phoca communis s. vitulina (common seal) there is one pterygoid muscle only, as described by Humphry; it agrees in form, origin, insertion and innervation from the ramus medialis, with the internal pterygoid of Halichosrns. The “ temporal” is inserted into the interarticular meniscus, the neck of the condyloicl process and the coronoid process (fig. 44) ; it is innervated by four twigs from the ramns lateralis.

It is clear that the “temporal” of Phoca represents the temporal + external pterygoid of Halichoerus, and the condition is probably due to non-separation of these two muscles .—a. persistence of an embryonic stage.

The matter, however, is coniplionted by the statement of Toldt that in Phoca vitnlina both pterygoid muscles exista statementwhich does not agree with the findings of Humphry and myself in Phooa. vitulina, though it does agree with what I found in Halichuarus grypus.

In Manis tetra.da.ctyla, according to Lubosoli, there is only one pterygoid muscle arising from the palate bone and passing down internal to the N. mandibularis. No separate external pterygoicl was found. Lnbosch was of the opinion that “ er mit dem inneren Flfigelniuskel verschmolzen ist, und ich loeziehe die Qccipitalsten Fasern seiner Ansatzes, die in der figur an den Condylus tretend gezeichnet sind, aur" ihn.” I found a similar condition in Manis pentadactyla.

The above-described observations show that the Anlage of the niastioatory muscles in Mammals divides into medial and lateral portions. From the medial portion or muscle are developed the internal pterygoid, pterygo—tympanicns or tensor veli palatini, and the tensor tympani. From the lateral portion or muscle are developed the temporal, masseter, zygomatico-mandibularis, and external pterygoid.

These phenomena of development are in harmony With, and offer an explanation of, the method of innervation by the mandibular division of the fifth nerve. The muscles developed from the medial portion are innervated by the ramus medialis, those developed from the lateral portion by the ramns lateralis.


The division of the muscles and nerve-branches into medial and lateral groups is further evidenced, as I have recently shown, by the differing paths of the nerve-fibres—both motor and sensoi-y—into the rami. The fibres of the ramus lateralis have a simple direct path, whilst the fibres of the ramus inedialis are for a space split into two parts by the mums posterior.‘ Both ra.mi contain motor fibres from the motor root, and muscle-sensory fibres from the Gasserian ganglion.

The division is also corrobated by the grouping of the cells in the motor nucleus of the trigeminal nerve. Willems found three groups of cells in this nucleus” in the rabbit—a dorsal, a ventro-median, and a ventro-la.teral. The occurrence of chromolytic changes after avulsion of individual motor branches showed that the external pterygoid, the temporal and sphenoidal,3 and the mssseter, are innervated by the dorsal group, the internal pterygoid by the ventro-lateral group, the mylohyoid and the anterior di gastric by the vent:-omedinn group. No chromolytic changes were found after avulsion of the tensor tympani, and no operations were performed on the tensor veli palatini.

Willems was of the opinion that “ces groupemeuts cellnlaires répondent relativement bien it des functions differeutes.” The internal pterygoid, however, is more closely associated in function with the temporal and masseter than is the external pterygoid. The grouping agrees with the division of their Anlage into lateral and medial masticalory muscles or muscle-Anlagen during development, and is a. morphological one—probabl y datiu g from 9. period antecedent.

1 I showed this to be the case in Man, Macacus, dog, and rabbit. I can now also add Dasyurus, Ornithoi-hynchus. and Echidna.

9 Willems applied the term “masticatory nucleus” to the Whole of the motor nucleus of the trigeminus. In this paper the term “ masticatory” is used—as is usual in English text-books—to denote those muscles innervated by the fifth cranial nerve which are situated dorsal to the lower jaw, i. e. to the exclusion of the mylohyoid and anterior digastric, with a. similar restriction in the case of the parts of the motor nucleus. 3 The part of the temporal innervated by the anterior deep temporal nerve .



to the difierentiation of the individual muscles which characterise Mammals. Similarly, the veutro-median group probably dates from a period antecedent to the differentiation 0E a depressor mandihulae anterior s. anterior digastric from the intermandibularis.

The buccal nerve is generally described as a branch of the ram us lateralis, alongside of which it runs for a short distance in the adult. It would be preferable, however, in View of its earlier development and different function, to describe it as a separate branch of the mandibular division of the fifth nerve, and to restrict the term “ ramus lateralis ” to the muscular branch.

The inter mandib ularis in early stages of Ornithorhynchus, Dasyurus, rabbit and pig forms a ventrally curved muscular sheet, with a median rap h e, and attached laterally to the inner surface of Meckel’s cartilage. At a late stage, in Dasyuriis, pig and rabbit, it becomes attached to the mandible. In Ornithorhynchus (Schulman) the muscle is attached laterally to the lower jaw, ligamentum pter_ygomandibulare, os pterygoideum and annulus tympauicus, the last-mentioned portion forming a separate muscle, the tympanico-hyoideus. In Echidna, stage 47, the muscle has already extended backward, but its anterior part is still attached to Meckel’s cartilage ,- in stage 50 this attachment is lost and the fibres extend up towards the outer edge of the (Monotreme) pterygoid bone; the fibres behind are attached to the tympanicbone. In the adult (Schulman) the muscle is attached to a fascial sheet at the level of the foramen rotundum, the hinder half of the outer edge of the as palatiuum, the os pterygoideum, the ligamentumpterygo-mandibulare, the tympanicum and the stylohyal. The above mentioned embryological phenomena support his view that the absence of attachment of the muscle to the jaw is a, secondary occurrence.


On the Horn ologies between the Masticetory Muscles of Mammals and Non-ma.mma1s.—The embryological evidence detailed above suggests that Mammals have originated from forms characterised by two masticatory muscles only: H medial arising from the ala. temporalis and inserted into Meckel’s cartilage and innervated by the ramus medialis of the mandibular division of the fifth nerve, and a. lateral arising from the orhito—parietal commissure and inserted into a rudimentary mandible and innervated by the ramus lateralis.


In Suuropsida‘ the Anlage of the masticatory muscles divides into upper and lower portions, the former inserted into the palatine process of the quadrate, the latter extending from the palatine process of the quadrate to Meckel’s cartilage. The upper portion forms the spheno-pterygo-qnadratus or a homologue of this muscle, or—as in Chelonia. and Croc0dilia— atrophies. The lower portion of the Anlage of the masticatory muscles separates into innerand outer parts, the inner developing into the pterygoid muscle or muscles, and the outer into the capiti—mandibularis. The dorsal end of the capiti-mandibularis, at first attached to the palato-quadrate, grows upwards and gains a dorsal attachment to the skull.


Thus, whilst in Mammals the Anlage of the masticatory muscles does not divide into upper and lower portions, in Sauropsida it does so. This cardinal difference is not taken into account by fiirbringer,Kostanecki, Gaupp, and Cords, so that the various homologies which they regard as existing between individual masticatory muscles in these two great Vertebrate groups are questionable.


In Amphihia. the Anlage of the niasticatory muscle does not divide into upper and lower portions, above and below the palatine process of the quadrate. The whole dorso-ventral strip separates into a medial and a lateral muscle.


The initial stages of development of the masticatory muscles in Mammals are thus exactly comparable to those of

' Only the barest outline of the phenomena is stated here. In the ‘Quin-terly Journal of Microscopical Science,’ vol. 51, 1907, I gave a short account, in all groups of the Sauropsida, with an analysis of the somewhat bewildering array of names given to the muscles by various observers.


Ampliibia, and a distinct homology exists between the medial and lateral masticatory muscles in the two Vertebrate groups. In Mammals these two muscles undergo considerable changes as detailed above, so that no homology exists between an_\_' one lllamlhalian Lnasticatory muscle and any one Amphibian muscle.

The intermandibularis is homologous with the similarly named muscle in other Vertebrate groups, Elasmobranchs, Teleostomi, Amphibia and Sauropsida. Its development in mammals, just as in other Vertebrates, lends no support to the theory of Ruge that it is primarily a hyoid muscle.

As will be shown later (pp. 579, 613, 616), the depressor mandibularis anterior of Ornithorhynchus and the anterior digzistric of Dasyurus, rabbit and pig, are formed by proliferation from the ventral surface of the intermandibularis. There are no homologies in other Vertebrate groups, so that the muscle must have arisen within the Manimalian phylum.

On the Changes in the Jaw Muscles accompanying the Development of the Squamoso-mandibular Joint in Mani in a.ls.——Dasyurus and Echidna are born at an ea.1‘ly stage with a functional incudo-meckelinn joint, and the change of jaw-joint occurs during the pouch stage of existence.

In the earlier stages of Dasyurus the prominent feature is the great development of the lingual muscles——the genioglossus, hyo-glos sus, stylo—g1ossus, and transverse lingual nJuscle—fibres; this and the concavity of the anterior part of the dorsal surface of the tongue are intimately associated with the intrabuccal position of the maternal teat. Then follows the development of two new mnscles——-the transversely directed anterior digastric, which soon, however, becomes more oblique in position, with its medial. end attached to the transverse aponeurosis of the hyoid ventral constrictor, and the external pterygoid. The adjacent ends of the genio-hyoid, sternuhyoid and omo-hyoid lose their attachment to the first branchial cartilage and become united by tendon. These muscle phenomena. precede and apparently initiate the development of the squamoso-mandibular joint. In Phascolarctos, however, the genio-hyoid retains its attachment to the basibranchial, and in Didelphys aurita the hyoglossus is attached to the first hranchial bar. In Ecliidna} where there is no mammary teat, there is, in stage 50, a complicated system of protrztctors and retractors of the tongue, the former consisting of genieglossus, genio-glossus posticus externus and internns, the latter of stei-no-glossus and lztryngo-glossus (figs. 34-37). The sterno—glossus passes through a loop formed by a- M. annulus. The transverse lingual fibres are not specially developed. The external pterygoid and depressor mandibular: anterior are developed. The posterior end of the genio-hyoid is attached both to the first braucliial cornu and to the thyroid cartilage. A sterno-thyroid is attached anteriorly to the posterior edge of the thyroid ca.1‘tila.ge. The omo-hyoid, arising from the medial surface of the scapula, is in part inserted into the thyroid cartilage and in part joins the genio-hyoid.


On comparison of the above phenomena it would appear that there are two common characteristics of the jaw-muscles in Marsupials and Monotremes which may be supposed to have played a part in the phylogenetic change of an incud0meckelian to a squamoso-uiandibular joint: (1) The development of an external pterygoid. (2) The development of a (transverse) depressor mandibulte anterior. This muscle in Dasyurus quickly becomes oblique and forms the a.ut.erior iligastric. The condition in Monotremes, however, negatives any idea. that a. digastric had any share in the phylogenetic development of a squanioso-mandibnlm joint. In Echidua, and Marsupials various secondary connections are developed between the elements of the hypo-bronchial spinal muscles which are situated behind the first branchial bar (i. e. sternuhyoid, omo-hyoid, and thyro-hyoid) and those in front (i.e. geuio-hyoid, stylo-glossus, and hyoglossus), whereby a series of long retractors of the front end of Meckel’s cartilage and of the tongue are formed. Though such retractors may possibly play a part in the formation of the new jaw-joint in the individual, yet the absence of any uniformity in their development suggests that they did not do so in its phylogenetic development.


‘ This account difiers from that given by Fawkes in (1) the more extended posterior insertion of the genie-hyoid, (2) the existence of a, sterno-thyroid, (3) the existence of an omo-hyoid—confi1-niing the statement of Leche. The M. annulus was not yet separated into M. annulus inferior and M. annulus intinius.



It has generally been supposed that the formation of the new squanioso-malidibnlar joint was associated with the disappearance of a depressor ruandibulee. As is stated later (p. 630), there is no ontogenetic evidence of the disappearance of such a muscle, and comparison of the early stages of the hyoid muscles of Amphibia and Sanropsida with those of Mammals shows that the levator hy0idei—fron1 which in Amphibia. and Sanropsida the depressor mandibulae is derived ——preSe1'ves in Ma.mma.ls, and especially in Monotremes, its p1‘ima.1‘y condition as a muscle aflixed to the hyoid bar.

The above considerations support the theory of fi.'11‘b1‘inger that the change of jaw-joint was associated with the development of the mammary function: “Dieses Abweichen der Mammalia. von dem fortschreitenden Eutwickelungsgange, wie ihn Ainphibien und Sauropsiden einschlagen, legt den Gedanken nahe, class hei ihnen in jugendlichen Entwickelungsstadien, in den Benteljungenkindheit, ein den Stingethieren eigentiimlicher étusserer Anstoss einti-at welcher zu dieser Lockernng des Unterkiefers Vera.nlassung' gab. Ich neige dazu, diesen in der saugenden Tleitigkeit der Benteljungen zu erblicken. \/Viebei den Anurenlarven nur der vordere Teil des zu einiger Selbstandigkeit gestalteten Unterkiefers zum Anheften nnd Kauen verwendet wurde, so dient auch bei den mammalen Benteljungen nur der vordere Absclmitt der Mandibnla der Saugfunction, welche zusammeu mitder Ansbildnngder Milchdrijseneine neue Erregungsschaft gegeniiber alleu Nicht-Saugetiereu ist.”

Some of the muscle-phenomena, appear to be secondary to the change ofjaw-joint. The pterygo-tympanicus of Dasyurus and higher Mammals becomes transformed into the tensor veli palatini, though in Ornithorhynchus and some Edentates its ancient insertion into the tympanic bone is still preserved. The proximal end of the tensor tympani disappears and the muscle gains a new origin. The disappearance, in the ontogeny of Dasyurus, pig and rabbitof the detrahens mandibulae may also be related to the change of jaw-joint. Two slightly different views as to this muscle have been put forward. Toldt held that the muscle “ bei der Nenbildung des Kiefergelenkes, als eiu infolge des Schwindens des M. depressor mandibulae notwendig gewordener Factor fiir die Kieferbewegung, von dem M. masseter abgespalten liat”; whilst Graupp was of the opinion that it “sich zugleich mit den) Masseter selbst ans einer gcmeinsamen Muskelmasse der

Reptilien (dem sogennanten M. capiti-mandibularis) bei der Neubildung des Kiefergelenkes herandifierenzierte.” According to both theories the muscle was developed with the new squamoso-mandibular joint. The above-recorded observations, however, suggest a converse explanation——that it is an old muscle possessed by Mammalian ancestors with an incudomeckeli-an jaw-joint, which has dropped out, except in Monotremes, owing to its being inserted behind the new joint. The phylogenetic history would thus present an interesting contrast to that of the pterygo-tympanicus and tensor tympani.

The theory above advocated differs in some particulars from that advanced by Graupp. In his treatise on “Die Reichertsche Theorie ” it is assumed that the substitution of an incudo-meckeliau jaw-joint by asquamoso-mandibular one was accompanied by a change in the masticatory muscles— from a Reptilian to a Mammalian type. The developmental phenomena occurring in the muscles, however, suggest that this substitution occurred subsequent to changes in the masticatory muscles from what may be called a pre-amphibian type, i. e. one characterised by median and lateral masticatory muscles and a levator hyoidei, to a Mammalian one.

The amount of such change is very doubtful. Possibly Mammalian ancestors with an incudo-meckelian jaw-joint possessed internal pterygoid, a muscle representing the pterygo-tynipanicus and tensor tympani (derived from the medial muscle) ; temporal and detrahens mandibnlze (derived from the lateral muscle) ; but not external pterygoid, masseter, zygoma.I.ico-mandibularis, or depressor niandibulae anterior.

Muscles of THE HYOID SEGMENT.

In stage A of the Dasyurns (fig. 3) the hyoid apparatus consists of hyoid and first hr-anchial bars and a basibranchial. There is no basihyal. The ventral ends of the hyoid and first bi-aiicliial bars are continuous with the ba.si-branchial. The hyoid bar forms a. continuous structure—-—of stapes, inter-hyale, and stylo-hyale; the latero-hyale extends upwards from the interhyale to the auditory capsule. There is as yet no crista parotica. The only chondrified parts of the hyoid bar are the extreme lower end and the middle part; to the latter are attached the stylo-pharyng-ens on the inside and the upper end of the posterior digastric on the outside. The posterior end of the stylo-glossus is attached to the tendinous part of the hyoid bar just below the middle cartilaginous part. The brrmchio-hyoid passes from the first branchial to the hyoid bar. The adjacent ends of the genio-hyoid, omo-hyoid and sternehyoid are attached to the first bran chial bar. The thyro-hyoid is not yet developed. There are two hyoid muscles——the stapedins and posterior dig-astric. The former arises from the outer chondrified wall of the upper part of the auditory capsule and passes downwards and forwards to be inserted into the back of the upper part of the stylo-hyale. The posterior digastric takes origin from the middle chondrified portion of the hyoid bar, and passes downward and forwards outside the bar to a. transverse tendon connecting the muscle to its fellow just behind and slightly underlapping the interniandibularis. No trace of the anterior digastric or of the Anlage of the sphincter profnndus and platysma is visible. The N. faciztlis passes backward on the external surface of the stapedius and then downwards on the external surface of the posterior digastric.


In stage 0 (figs. 7—l0 and 12) the stapes1 has chondrified. The crista. parotica. is beginning to form, as a. slight downward extension from the chondrified portion of the outer wall of the a,udito|'y capsule. It is deepest in front, where the latex-o-hyale is attached, and g-ra.clna.lly lessens behind. The stapedius muscle, taking origin from the lower edge of the crista parotica, passes forwards and downwan-ds to the hyoid. bar below the interhyule. The upper end of the posterior digasti-ic——attn.ched solely to the hyoid bar in stages A and B—h-as now an additional origin, by a long tendon passing upwards behind the hyoid bar to the paroccipital process.

In stage D (fig. 19) chondrification has extended from the middle portion of the hyoid bar into the latero-hyale, the pars inter-hyalis remaining precartilaginous. The origin of the stapedius muscle has shifted still further inwards, themnscle arising from the floor of the fossn. stztpedii; it is inserted into the hyoid bar below the inter-hyale.

In stage F the inter-hy-ale has disappeared, and the cartilnginous lntero-hyale and style-hyale, but for a small nodule, are replaced by a ligament which, ventrally, is continuous with the cartilaginous hyoid cornn of the hyoid apparatus. The insertion of the stapedius has shifted inwards to the stapes.

In stage H the attachment of the posterior digastric to the hyoid bar is lost; its attachment to the parocoipital process is tendinous up to stage J——the latest investigated—but in the adult it has a. fleshy origin.

The condition of the stapes, stapedius and posterior digastric muscles of 10 mm. specimens o‘fDide1phys aurita is similar to that of the Dasyurus in stage H.

The anterior digastric is proliferated from the hinder part of the intermandibularis in stage D (fig. 18). The fibres are given off from the ventral surface of the posterior portion of the muscle and are directed transversely outwards, and have no lateral attachment. In stage E (fig. 21) the muscle has separated from the intermandibularis ; its ventral (inner) end has become attached to the ventral aponeurosis of the posterior digastric.


‘ A stzipedial artery passes through the stapes in stages A-0; it disappears subsequently.



In stage F (fig. 22) the muscle is more obliquely situated, the muscle-fibres passing from its posterior attachment to the posterior digastric forwards and outwards to the ventral edge of th_e mandible. A digastric muscle is thus formed.

In an 8'5 mm. embryo of Ornithorhynchus (figs. 30, 31, 33), the hyoid bar, in a. pre-cartilaginous condition, is a continuous structure passing from the stapedial portion above to near the middle line below. The upper, stapedial end abuts against the auditory capsule, which consists of aggregated mesoblast cells. There is no crista parotica and no latero-hyale. The hyoid muscles consist of a levator hyoidei‘ and a hyoid ventral constrictor s. styloideus, which are not quite separated from one another. The levator hyoidei has no dorsal attachment; itis inserted into the liyoid bar. The N. facialis passes hackward lateral to the muscle. The hyoid‘ ventral constrictor arises from the hyoid bar and passes down to a ventral aponeurosis connecting together the muscles of the two sides.

The hyoid muscles of Echidna, in stage 50 of Semon, consist of a. levator hyoidei and hyoid ventral constrictor s. styloideus (fig. 37). The levator hyoidei arises from the outer end of the intnrned crista. parotica and is inserted into the upper end of the stylo-hyal cartilage. The hyoid ventral constrictor arises from the stylo—hya.l cartilage just below the insertion of the levator hyoidei and passes downwards to a. median ventral aponeurosis.

The following is a summary of Futan1ura’s observations (1907) on the development of the hyoid muscles in the pig. In a, 8'4 mm. embryo their Anlage consists of an aggregation of myogenic cells which is continuous ventrally with its fellow across the middle line. It is penetrated by the seventh nerve.

‘ The introduction of this term is explained later; the muscle is homologous with the stapedins of non-Monotreme Mammals, but never gains any insertion into the stapes.


In a 11 mm. embryo the Anlage has split into ‘Eh superficiztl and a deep layer—-the former is the Anlage of the platysnia, the latter that of the stapedius, digest:-icus, and stylo-hyoid. In a 15 mm. embryo the antero-posterior fibres of the stapedius form the most dorsal portion of the Anlage; immediately below these is the upper end of the digastricus, i. e. this muscle has not yet extended up to the mastoid process. The digastricus is in two parts, of which the posterior (iiiiierirated by the.N. facialis) is inserted into Reichert’s cartilage; the anterior passes round the cartilage and is inserted in 1\Ieckel’s cartilage. A small outgrowth from the posterior part is the Aulage of the stylo-hyoid. In a. 21 mm. embryo the upper end of the digastricus has spread up to the mastoid process, and the jugulo—hyoideus has separated from its upper part and is inserted into the hyoid bar. The stylo-hyoid is fully separated.

Kztllius (1909) described the development of some of the liyoid muscles in his account of the tongue and associated structures of the pig. In stage 12 (= Keibe.-l‘s stage 22.) “von dem proximaleu Eude des Knorpelanlage des zweiten Schundbogen zieht ein schuialer Straug Von wenig diEerenrzierteu Muskelblastem nach der Mittelliuie zu in die G-egeud des oralen Endes der Copula. des Branohialskeletes.” There is no trace of an anterior digastric. In stage 17 (= between twenty-seventh and twenty-eighth stages of Keibel) the anterior digasti-ic forms :1. well “ abgrenzbaren Muskelbundel welcher ofteubar mit der ersten Portion (Myloliyoideus) von der urspriinglich einheitlichen Trigeminns muskulatur abgezweigt hat ”,' its inner end is continuous by a tendon with the posterior digastric, the lowerpart of wliichis almost surrounded by the style-hyoid. A little higher up these two muscles diverge, the stylo-hyoid being affixed to the hyoid cztrtilage, the posterior di gastric to the base of the skull. Both posterior -rligastric and stylo-hyoid receive branches from the N. facialis. (The earliest stages of development of these muscles were not described.) In stage 27 (embryo 92 mm. in greatest length) .a. jugulo-hyoideus is formed probably from a small part of the posterior digastric. The‘ stylo—h_yoid has lost relation with the posterior digastric; it passes downwards, ending partially in a fascia. lying at the posterior margin of the mylo-hyoid, partially by insertion into the hyoid.


Eschweiler (1911), who did not refer to the papers of Futamura and Kallius, gave an account of the development of the stapedins in the pig. He stated that the Anlage of the muscle is first visible in embryos of 13 mm. “ Scheitel-Steisslétnge,” and has no “Abstatnmnng von einer andern grossen Muskelgruppe.” On the first development of Reichert’s cartilage the muscle enters into relationship with it; it subsequently loses '

this and gains an attachment to the stapes. " Der Mnsculns Stapedius ist ein ecl1terAbkt')mn1ling des Hyoidbogens nnd tritt erst sekundétr mit den] Shapes, der dein periotischen Blastem enstammt, in Verbindung.” The first basal attachment of the muscle is to the wall of the fossa stapedii s. Autrum petrosum laterale, and a certain rotation of the muscle takes place “in der VVeise dass die Achse seiner Pyi-amide mit ihrem basale Ende nach hinten (nboralwarts) verschoben wird.”

A re-investigation of the process of development in the pig showed the following: In a 13 min. embryo there was no-' trace of an anterior digastric muscle. In a 14 mm. embryo it is being proliferated from the ventral surface of the hinder part of the intermandibularis, but is not separated from it, whilst the muscle Anlagen in the hyoid segment show the same structure as in the 15 mm. stage. In a 15 mm. embryo. (figs. 55, 56) the anterior digastric is a. little separated from the intermandibularis; it extends backwards and upwards on the lateral surface of the hyoid ventral constrictor portion of the muscle Anlagen in the hyoid segment, to about its middle. In the hyoid segment the hyoid bar consists of a continuous stapes, inter—hya.Ie, and stylo—hyale (figs. 55-57) in a precartilaginous condition, and the latero—hyale extends upwards from the junction of the inter-hyale and style-liyale towards the auditory capsule, which shows no trace of a fossa stapedii or crista parotica. The muscle-Anlagen (figs. 56-58), not yet separated into stapedius and hyoid ventral constrictor, extend as a continuous strip from behind thelatero-hyale downwards and forwards lateral to the stylo-hyale, and then anterior to the stylo—hyale to the middle li11e just behind the posterior edge of the intertnandibularis.

In a 17 mm. embryo the anterior digastric is quite separated from the intermandibularis. The stapedius portion of the hyoid muscle-Anlage is partially separated from the hyoid ventral constrictor, and some of its fibres are insertedinto the back of the upper end of the stylo-hyale.

In a 19 mm. embryo the crista parotica. is formed and the upper end of the latero-hyale is continuous with it. The stapedius muscle is now quite separated from the hyoid ventral constrictor ; it arises from the floor of the fossa stapedii and is inserted into the inter-hyale. The hyoid ventral constrictor has no dorsal attachment, its upper end lies just below the stapedius, and it extends downwards to a ventral aponeurosis. The anterior digastric extends laterally to Meckel’s cartilage, whilst posteriorly it extends backwards and upwards on the lateral surface of the hyoid Ventral constrictor, as in the 15 mm. stage.

In a 21 mm. embryo (figs. 61-63) the inter-hyale has disappeared and the stapedius is inserted into the stapes. The hyoid ventral constrictor has extended dorsally and its upper end is attached to the pai-occipital process, and it has divided into dorsal and ventral portions. The dorsal portion is partially separated into jngulo-hyoideus and posterior digastric. The jugulo-hyoideus is inserted into the stylo—hyal cartilage. The lower end of the posterior digastric is continuous with the posterior end of the anterior digastric. The anterior end of the anterior digastric is attached laterally to the Anlage of the mandible. The dorsal end of the ventral portion of the hyoid ventral constrictor is attached to the stylo-hyale ; from this point it passes downwards to the transverse ventral apoueurosis.

In a. 24 mm. embryo the only change is a slightly greater separationof thejugulo-hyoideus and posterior digastric. In a. 32 mm. embryo (fig. 64) the ventral end of the lower part of the original hyoid constrictor has gained an insertion to the external surface of the first branchial cornn, though keeping its transverse aponeurosis to the muscle of the opposite side—-a stylo-hyoid muscle is thus formed.

The developmental phenomena in the rabbit are, for the most part, similar to those in the pig. The anterior digastric is proliferated from the intermandibularis, and grows backwards medial (not lateral, as in the pig) to the hyoicl ventral constrictor. The latter divides into dorsal and ventral portions, the dorsal of which separates into posterior digastric and jugulo-branchialis, whilst the ventral forms the stylo-hyoid. The jugulo-branchialis becomes inserted into the first branchial cornu; and the stylo—hyoid into the basibranchial, losing its ventral transverse aponeurosis. The posterior digastric becomes tendinons.

On the Primary Form of the Posterior Digastric in Man.imals.—Bijvoet’s conclusion, from an examination of the adult condition of the stylo-hyoid and posterior digastric in many classes of Mammals, was that, primitively, “ sich vom Schiidel zum Znngenbein, oder loesser zu einer Bindegewebslager, das sich ventral am Hyoirl heftet, cine Muskelmasse erstreckt, die vom NI facialis innerviert wird.” " Den hier beschriebenen Zustand begegnen wir bei Echidna und Ornitliorhynclius. Der l\I. styloideus repritseutiert die einheitliche Muskelmasse, welche vom Schédel entspringt nnd die ventrohyoideale Bindegewebstnasse sich festheftet.”

The developmental phenomena in Ornithorhynchus, Echidna, Dasyurus, pig, and rabbit show that a still more primitive condition of the muscle is one in which it is attached dorsally to the hyoid bar and passes ventrally to a transverse aponenrosis, forming a hyoid ventral constrictor or interhyoidens. This condition in Dasyurus, pig and rabbit is succeeded by one in which the muscle loses its dorsal attachment to the hyoid bar, and, extending dorsally, gains a new one to the skull—to either the paroccipital or mastoid process. The more primitive condition, however, persists in Echidna, in which animal Toldt described the M. styloideus‘ as arising from the stylo-hyale close to its cranial end.“ This I can confirm (vide fig. 38) ; it arises from the upper end of the stylo-hyale and from the latero-liyale, lapping round the insertion of the levator hyoidei?

Comparison of this adult condition with stages 47 and 50 (fig. 37) shows that the origin of the muscle has spread slightly up the hyoid bar. The muscle also arises from the style-hynle in Manis peutadactyla (fig. 43).

It is doubtful whether this primitive condition exists in any Mammals other than these two. It is true that Kohlbrugge described the posterior belly of the digastricus as arising from the styloid process in the Marsupials, Cuscus orientalis and niaculatus, Paradoxux-us hermaphrodita, and Macropus brunii. But in Cuscus u1a.cula.tus, Bijvoet described and figured the posterior digastric as arising from the paroccipital process—an occurrence which suggests that in the specimen examined by Kohlbrugge the condition was one of division of a. posterior digastric s. hyoid ventral constrictor into dorsal and ventral portions (vide pp. 617, 618), of which only the ventral wa.s described. The same explanation possibly applies to the other Marsupials described by Kohlbrugge.

A condition interniedia.te between that of Echidna and the usual one of attachment to the skull is present in Ornithorhynchus, where Toldt described the muscle as arising from the stylo-hyztle and hinder wall of the cartilaginous external auditory meatus."



1 I employ the nolnenclature of Schiilinzui. Toldt names the muscle the “ stylohyoideus,”——a terminology suggesting a homology with the “ stylohyoidens” of the pig and rabbit, which is developed from the ventral part only of the hyoid ventral constrictor (vide pp. 617, 618).

3 Bijvoet described the muscle as arising “uumittelbarhinter dem iiusseren Geli61'gu.nge.”

3 No representative of a possible upper portion of the muscle was present.

4 Bijvoet described it as arising " V011 der Schiidelbasis und empfétngt mccessorisclie Urspriinge vom iiusseren Gehoi-gaiigesf‘ Schulmau did not state its origin.


The hyoid ventral constrictor or styloideus muscle, unconnected with the anterior digastric to form a digastricus, passes downwards to a. ventral aponeni-osis connecting it \vith its fellow below the basibranchial and ventral ends of the hyoid bars. This condition is present in stages A—D of Dasyurus, in 14 mm. embryos of the pig, and is preserved to the adult condition in Echidna and Oruithorhynchus (Schulman), and in Manis. It is also present in some of those Edentates, i. e. Dasypus villosus, Tolypentes tricinctus, and Tatusia novenicincta (Toldt), in which a.sterno-niandibularis exists. The latter muscle (vide p. (325) is formed by the union of the anterior digastric with the sterno-hyoid, and the transverse aponeurosis of the hyoid ventral constrictor s. posterior (llg'21Sl2l'lC lies dorsal to the longitudinal muscle.

In some Edentates, e. g. Bradypus tridactylus, the anterior end of the stei-no"-hyoid is in part attached to the posterior edge of the transverse aponeurosis of the hyoid constrictor s. posterior cligastric (Bijvoet).

In some Edentates the hyoid. ventral constrictor s. styloideus s. posterior digastric divides into dorsal and ventral portions, the former taking origin from the skull and inserted into the stylo-hyal, the latter arising therefrom and passing to :1. ventral a.poueui-osis.

Thus in Bradypns tridzictylus (Bijvoét) the posterior digastric arises from the mastoid process and passes to 2:. ventral aponeurosis (fig. 40) ‘; whilst in Bradypus marmoratus (fig. 4]) the muscle is divided into dorsal and ventral portions. The stapedius muscle is present. A similar condition was found in a. 30 mm. embryo, though the separation into dorsal and ventral portions was not quite complete.

Toldt described the posterior digastric in Dasypus villosus as arising‘ from the mastoid; its tendon broadens and unites with the posterior border of the mylo-hyoid. In Dasypus noveincincta. (embryo 30 mm.) the posterior digastrio is partly divided into dorsal and ventral portions, inserted into and arising from the stylo-hyale (fig. 42). A stapedius of usual origin and insertion is present.


  • This figure of Bijvoet is reproduced for comparison with one of Brudypus marnioratus.



Mztckintosli described the posterior digastric of Cholocpus as arisingfrom the stylo-hyz1le,but did not mention the existence of any muscle arising from the mastoid and inserted into the bar.‘ Such a. muscle, however, is pictured in a. figure of Schulman under the title of mastoideo-hyoideus.

Toldt stated that the - posterior digastric was absent in '[‘m11a1idLm, but described and pictured a. “ stylo-hyoidens,” innervated by the N. fa.cia.lis, taking origin from the os tympamicum and inserted into the cranial end of the hyoid cornn, and also described a. portion of the inte1'ma.nclil:>ulzLi‘is as arising from the hyoid cornu. These two muscles are in all probability the dorsal and ventral portions of the posterior digztstric. This identification is rendered all the more certain by his statement that there is an anastoinosis between the mylohyoid branch of the fifth and the seventh nerves.

Owen described three musclesin Myrni ecophaga jubatu —a “ stylohyoideus” passing from the petro-hyoid to the stylo—hyale, a, “cera.to-hyoidens” passing from the ceratohyale to 8. commissnral tendon with a. slip to the sterneinandibulai-is muscle, and a. “constrictor salivaris” passing from the cerato-liyale downwards over the salivary reservoir to the commissurztl tendon and blending with the back of the interma.ndibnlaris. It is probable from his description and figures that his “ stylo-hyoideus ” and “ cereto—hyoideus ” are the dorsal and ventral portions of a divided posterior digestric. Leche stated that the constrictor sa.liva.i-is is innervated by the N. mylo -hyoide us and is to be regarded as a. differentiated part of the intermandibularis.

The condition of the hyoid ventral constrictor is thus variable in Edentates, even in closely related species. Thus it is undivided in Bradypus trida.ctylus, divided into upper und lower parts in Bradypus ,ni:a.rmo1-atus; undivided in Dasypus villosus, divided in Dasypus novenicinctzt. And in at least one Marsupial—Cuscns ma,cula.tns—the condition is va,ria.ble within the “same species (vide p. 619).


A sterno-mandihularis may occur with either condition, e. g. with an undivided hyoid ventral constrictor in Tolypeutes tricinotus and Dasypus villosus, with a. divided one in D asypu s n ove In cincta, Myrmecopliaga. and Tamandua. In the pig, as described above, the hyoid ventral constrictor divides into dorsal and ventral portions. The dorsal portion separates into two muscles—the jugulo-hyoideus and the posterior digastric. The jugulo~hyoideus is inserted into the stylehynl. The ventral end of the posterior digastric unites with the posterior end of the anterior digastric and forms the digastric muscle. The ventral portion of the hyoid ventral constrictor becomes the stylo-hyoid; it arises from the stylo-hyal and passes to a. ventral aponeurosis uniting it with its fellow, and also gains an insertion to the side of the first branchial cartilage.

In the rabbit the sequence of events is, but for t\vo differ» ences, the same. The jugulo-hyoideus gains an insertion to the first hranchial cartilage, becoming the jugulo-branchialis. The ventral portion of the const1‘icth1‘ becomes the stylo-hyoid and is inserted into the basibranchial ; the ventral aponeurosis disappears.

In both animals the posterior digastric becomes tendinous and has no nerve—supply, but in the embryo a branch of the N. facialis nerve passes to it.

On the Formation of the Anterior Digastric Muscle. — Consideration of the adult form and innervation of the anterior digastric muscle led Gegenbaur, Rage, Furbringer, Schulman and Bijvoet to the opinion that it is derived from the intermandibularis s. mylo-hyoid, and the observations of Kallius, though he did not observe the earliest stages of the process, tended to confirm this view. Chaine stated that the digastric muscle is due to longitudinal division of a. muscle extending from the sternum to the inaudible into inner and outer portions, the inner forming thegenirrhyoid and sterno-hyoid, the outer the digastricus.

Futamura stated that in man and pig the anterior digastric is due to a forward extension to the jaw of the muscle blastema.



which gives rise to the stapedius, stylo-hyoid, and posterior digastric. This forward growth receives a secondary innervation from the fifth nerve. These statements \vere-adversely criticised by Bijvoet.

The above-recorded observations show that the depressor mandibulae anterior of Ornithorhynchus, and its homologue, the anterior digastric of Dasyurus, pig and rabbit, are formed by proliferation from the ventral surface of the intermandibularis.

On the Formation of the Digastric Muscle.—Schulman,follo\ved by Bijvoet, came to the conclusion that the primitive condition of the anterior digastric muscle is indicated by the depressor mandibulze anterior of Echidna. and Ornithorhynchus} viz. a sheet of approximately tmnsvei-se fibres below the intermandibularis, attached laterally to the lower jaw; that this transverse sheet became more longitudinal in direction, and that its hind end———the original inner end— gained an attachment to the transverse aponeurosis of the posterior digastric muscle. A condition similar to that of Bradypns tridactylus would result, and from this all the varied forms of a. digastricus verus and digastricus spurius can be easily derived.

Toldt’s theory of the phylogenetic development of the digastricus muscle difiers from that of Schulman and Bijvoet in some particulars, and is as follows. The anterior digastric “besitzt urspriinglich, gleich dem M. mylohyoideus, den Character und die Bedeutnng eines Eingeweidemnskels; er ist die aussere Langsfaserschichte des Mundh6l11enboclens, angelegt der Querfaserschichte desselben, dem M. 1nylohyoideus.” This longitudinal muscle unites with a hinder longitudinal muscle innervated by the twelfth, to form a continuous longitudinal umscle—the sternownandibula-ris, extending from the sternum to the lowerjaw. This condition is present in Myrmecopha.ga., T-a.tnsia., Da.sy pus vi llosus. In Tolypeutes tricinctus a. tendinons inscription is present between the anterior and posterior portions, and is attached to thehyoid. A sterno-hyoidensfliesdorsal to the posterior portion. The posterior digestric ends in a. transverse aponeurosis in intimate connection with the posterior inargin of the intermaaidibularis. In Bradyp us trid no t ylns there isa. tendinons inscription; the anterior portion forms an anterior digastric and the posterior portion a true sterno-hyoideus in that whilst its median portion is attaclied to the inscription its lateral portion is attached to the first bri1nchia.l bar. The posterior dignstric is attached to the lateral end of the inscription. A digastric muscle is thus formed. This condition is one intermediate between those described above and that found in higher Mammals.


' Schulnran, “fiihrt die Vereinigung des M. depressor mandibulu.‘ anterior mit vom N. facialis versorgteu M. depressor mandibulaa posterior oder richtiger einem hyoideuleu Oomponenten des M. constrictor zur Bildung eines wirklichen M. cligustrions."



The following observations bear on this question. In embryos of Amphibia. and Sé1.uropsida., and also in the rabbit and pig, the hypobranchial spinal n1usc1es~formed by downgrowths from two or more anterior body n1yotomes—forni a. longitudinal cell column, the anterior end of which, growing forward dorsal to the inter-hyoideuss. hyoid ventral constrictor and interina.ndibulztris to the front end of Meckel’s cartilage, divides into two at the level of the second or first brnnchia.l cartilage, the part in front forming the genie-hyoid, the part behind the sterno-hyoid. In the rabbit and pig this primitive sterno—liyoid develops into sterno—hyoid, sterno-thyroid, thyro-hyoid, omo-hyoid.‘

It would follow that any connection of the anterior end of the sterno-hyoid with 21 muscle lying ventral to the intermandibularis s. mylohyoid is a secondary phenomenon.

Further, in n30 min. embryo of Dasypns novemcinctus the following condition was found : The sterno-m-andibnlaris had a tendinons intersection at the level of the hind edge of the intermandibularis. The sterno-thyroid was attaclied to the basibranchial and thyroid cartilage by two separate slips. A thyro-hyoid extended from the thyroid to the first branchial bar. The geuio-hyoid and hyo-glossus arose from the basibranchial and first branchial bar and extended forward to the anterior end of the lower jaw and tongue respectively. A branchio-hyoid s. cerato-hyoid muscle, between the first branchial and hyoid bars, was also present. The posterior cligastric consisted of upper and lower parts, the upper extending from the mastoid to the hyoid bar, the lower from the hyoid bar to an aponeurosis iinmediately behind and continuous with the intermandibularis. A stylo-glossus arose from the hyoid bar.


  • Details of this development will be given in -.11-ater paper.



In this specimen, then, there was evidence of fusion between a sterno—hyoid and anterior digastric to form a sterno-mandibnlaris. In the other Edentates possessing a sternemandibularis Toldt does not state whether the muscle was investigated microscopically.

These phenomena tend to show that the connection of the sterno-hyoid to the transverse aponenrosis of the posterior dig-astric is a secondary phenomenon and related to its nonattachmentito the first branchial bar, and suggest that its fusion with the anterior digastric to form a sterno-niandibularis is also related to the same occurrence.

The theory of Toldt, again, quite fails to account for the fact that in Dasyurus the anterior digastric, when first formed, grows out transversely just as in Ornithorhynchus, and only subsequently takes up a. longitudinal direction.

The theory of Toldt is thus open to many objections, and may probably be rejected in favour of that of Schulman and Bijvoet.

The above observations, however, show that there are two main varieties of a rligastric muscle in mammals.

(1) The anterior digastric becomes connected with the ventral end of the hyoid ventral constrictor s. stylo-hyoideus; no stylo-hyoid muscle is present. This occurs in Marsupials and those Edentates in which a. digastric is formed. There are two sub-varieties: (a) the hyoid ventral constrictor remains undivided in most Marsupials and some Edentates, e.g. Bradypus tridactylus; (b) the hyoid ventral constrictor divides into dorsal and ventral portions, and the anterior digastric becomes connected with the lower end of the ventral portion, e. g. Bradypus marmoratus, Cholespus, and probably the Marsupials examined by Kohlbrugge (vide p. 619).

(2) The hyoid ventral constrictor divides into dorsal and ventral portions, and the anterior digastric becomes connected with the lower end of the dorsal portion; the ventral portion forms, as a rule, the stylo-hyoid muscle. This condition is present in all Entheria. Phylogenetically, it is probably derived from sub-variety (b) above, rather than from subvariety (3.). Such a digastric muscle may be of many different forms—classified by Bijvoet into “ digastricus verus” and “cligastricns spm-ins.”

The posterior digastric of this latter class of Mammals is partially or wholly—according to whether a. mastoideo- s. jugulo—hyoideus s. jugulo-branchialis is or is not developedhomologons with the dorsal portion of the posterior digastric of these Eclentates, in which division into dorsal and ventral portions takes place. The mastoideo- s. jngulo-hyoicleus preserves the ancient insertion of the muscle. The stylo-hyoideus is homologous with the ventral portion. It may pass to the middle line and join its fellow, thus preserving the primitive condition, as in Cynocephalus (Bijvoet) ; or, keeping the transverse aponeurosis it may gain an additional insertion to the branchial cornu, as in the pig ; more commonly, however, the aponeurosis disappears and the muscle is inserted into the basibranohial or its branchial con-nu, as in the rabbit. The origin of the muscle often extends upwards on the stylohya.l and may even reach the skull, so that it is overlapped by the posterior digastric, and the primitive positions of the two muscles are obscured.

According to Bijvoet a stylo-hyoid muscle is absent in Er-inaczeus among Insectivora, the Cheiroptera, and some Mustelidae, but he also states that in some Erinaczeus some bundles of the posterior digastric stream to a thin membrane which passes inwards to the middle line ventral to the hinder part of the mylo-hyoid, whilst in Cheiroptera. and Mustelidze

some of the hinder bundles of the mylo-hyoid arise from the hyoid. A representative of the. ventral part of the hyoid

constrictor is thus present. though not forming H: stylo—l1yoid muscle.

In view of these phenomena. of comp-.11-a.tive anatomy stud"

embryology it is probztble that the accounts given by Ronviére and Futztmnra. of the development of the stylo—hyoid of n.1a.n—that it is an outgrowth of the posterior diga.stric—are in need of revision. On the Reported Instances of a. Monogastric Dignstric Muscle Inserted into the Lower Jaw.Kohlbrngge found this condition present in one specimen of Hystrix with a. fifth nerve innervation, and found a. t\vobellied condition in another specimen. The former was explained by Schulnian as one in which 21.11 ztnterior digastric had extended backwards. .

In Ma.nis javanica Kohlbrugge described the digastric as a. simple muscle extending from the “liinteren Schiidelseite mum Kiefer\vin1:el,” whilst in Mnnis macrura. \'Vindle and Parsons found the digastric “inserted into the lower jaw as far as ha.lfwzty to the symphysis.” In Manis pentadact-yla. (fig. 43) there is a. muscle, with the insertion described by Windle and Persons, arising partly from the mastoid and partly by fibres which are continuous with the platysma. in the neck, i. e. an anriculo-mandibularis. The posterior digastric is present—in the form of a. hyoid ventral constrictor, arising from the stylo—hya.l and innervated by the N. facistlis; its etnterior edge, ventrally, is continuous with the posterior edge of the intermsmdibnlaris. The anterior digest:-ic muscle is absent.

In Tatusia and Dasypus a monogastric muscle, a.tta.ched below to the mandible, was described by Macalister, but laterinvestigations by Toldt show that a posterior digastric passing to a. Ventrztl aponenrosis is present, so that the first described muscle is an auriculo-mandibularis.

In Orycteropns, Humphry described the digztstric as extending " from mastoid process to angle and lower margin of jaw.” Subsequently, Chaine stated that " au lieu (in muscle que signale Humphry j’ai trouvé une formation mitendinense mi-muscnlaire qui parassait en tenir lieu,” probably, therefore, an auriculo-mandibularis; and he also depicted, though not describing, an additional muscle named digastric.

It is not known whether a true posterior digastric exists in Cyclothurus and Chla.my_(lopho1-us. I11 tl1e latter a small muscle passing from the bnlla tympani to the mandible was found by Macalister, but not by Hyrtl.

The case of Tamanclna is considered above (p. 621).

The existence of a monogastric muscle homologous with the posterior cligastric and inserted into the lower jaw is thus very doubtful. (the instances which have been described are probably cases of an auriculo~mandibula.ris—whicl1 is derived from the platysma (vide pp. 632, 634).

On the Primary Form of the Stapedius Muscle in Mam :11 al s .——The stupedius muscle in early post-embryonic stages of Dasyurus arises from the outer surface of the auditory capsule and is inserted into the upper part of the stylohyale, forming a levator hyoidei. Its origin subsequently shifts to the lower edge of the developing crista parotica, and then to the floor of the fossa stapedii, whilst its insertion shifts to the stapes. The initial stage present in Dasyurus is, in part, passed over in the rabbit and pig, for in them the first dorsal attachment of the muscle is to the floor of the fossa stapeclii; on the other hand, its first ventral attachment is to the upper end of the style-hyale, subsequently shifting to the inter-hyale and then to the stapes as in Dasyurus. The shifting of insertion from the inter-hyale to the stapes takes place with the disappearance of the inter-hyale in all three animals. The primary form of the stapedius as a levator hyoidei is present in 8'5 mm. embryos of Ornithorhynchus, and in stage 4-7 of Echidna, and in them persists into adult life. It is the muscle which was termed “ mastoideo-hyoideus ” by Schulman and Bijvoet.


This identification affords an explanation of the statements of Huxley, Eschweiler and fiirbringer, that a stapedius muscle, i. e. one of the usual mammalian type, is absent in Monotremes, and also of the method of innervation. In nonMonotreme Mammals the nerve to the stapedius is given oil“ from the N. facialis proximal to the chorda tympani. Now in Echidna—according to Scluilmau-—the mastoideo-hyoideus is innervated by several fine twigs from the N. facialis, some of which are given off proximal to, and some with, the chorda tympani.

The relations of the N. facialis to the levator hyoidei s. st-apedius in Echidna are different from those in Ornithorhynchus, Dasyurus, pig and rabbit. In the latter four animals the nerve passes backwards lateral to the muscle and then downwards. Earlier stages of Ornithorhynchus and Dasyurus were not available, but in the rabbit and pig this condition is preceded by one in which the nerve passes outwards dorsal to the Anlage of the stapedius and hyoid ventral constrictor and then down wards. The levator hyoidei s. stapedius thus extends upwards on the inner, medial, side of the nerve to gain a dorsal attachment to the outer surface of auditory capsule or to the floor of the fossa stapedii.

In stage 47 of Echidua the nerve passes backwards in the sulcus facialis dorsal to, and separated by the incurring crista parotica from the levator hyoidei which arises from it. In stage 50 (figs. 36 and 37) the origin of the levator hyoidei has extended outwards to the base of the crista parotica, and in the adult (fig. 38) to the outer surface of the auditory capsule. The N. facialis consequently passes l)acl~:w'm-ds medial to the muscle, whereas in other Mammals it passes backwards lateral to it.

It would thus appear that the name “ mastoideo-hyoideus " has been applied by various investigators to three muscles of difiering origin and morphological nature : (1) to astapedius muscle which has preserved a levator hyoidei stage, as in Monotremes; (2) to the dorsal portion of a hyoicl ventral constrictor which has divided into dorsal and ventral portions, as in B1'a.dypus marmoratns, Dasypus novemcincta, Choloepns didactylns; and (3) to the part of the dorsal portion of a hyoid ventral constriction which is inserted into the stylo-hyal, as in the pig.

The name “ stylo-hyoideus” was applied by Owen and by ',l‘oldt to (2) above, and also by Toldt to the hyoid ventral constrictor of Monotremes.

On the Homologies of the Posterior Digastric Muscle in Non- l\Ia.mmals.~—According to Gegcnbaur and Ruge the posterior digastric is homologous with the depressor mandibulze of lower vertebrates, and the latter thought that the change of insertion, from mandible to hyoid, was related to the formation of the new squamoso-mandibular joint. fiirbringer and Bijvoet rejected this theory, the former being of opinion that the old depressor inandihulze was either altogether gone or would be found as a rudimentary structure attached to the nialleus, the latter finding it difiicult to understand ho\v the insertion of a depressor mandibnlee should be transferred to the hyoid.

Bijvoet’s theory was that the posterior dig-astric is derived from a ventral constrictor of the hyoid segment, and is serially homologous with the iutermandibularis s. mylohyoid of the mandibular segment. This becomes all the more probable from the investigations detailed in this paper, which show that the priinary upper attachment of the muscle is to the hyoid bar.

The serial homology is well brought out in fig. 4, which, being drawn from a slightly oblique section of Dasyurus in

stage 13, shows the posterior digastric on one side and the intermaudibulaa-is on the other.

On the Homologies of the Stapedius in Nonl\Ia.mmals.—fiirbringer was of the opinion that the depressor mandibnlze of non~mammals has entirely disappeared in mammals, or will be found as a rudiment attached to the malleus—the hind end of the primitive jaw. No such rudiment, however, was found in the embryos investigated.

Graupp stated that the disappea.ra,nce of the depressor is a fact: “ Der Schwuud des alten M. depressor mandibulae bei den S'augern und damit der \'Vechsel in dem Muskelmechanismus bei den Oefinung des Mundes sind Tatsachen.”

But, perhaps, after all, the muscle has not disappeared. It has been shown above that the stapedius muscle of Mammals is primarily a levator hyoidei arising from the outer wall of the auditory capsule and inserted into the upper part of the hyoid bar. Now, the depressor mandibulee of Dipnoi, Urodelal and Sauropsida, as I have shown in a. former paper, is at first a levator hyoidei inserted into the hyoid bar———a condition which persists in Protopterus-—and only subsequently becomes attached to the hind end of Meckel’s cartilage. It may be inferred that the stapedius muscle of Mammals is homologous with the levator hyoidei of Urodela. and Sauropsida though not with its later stage of depressor mandibulae—there being no developmental or other evidence that it ever gained that secondary attachment.

The origin of the superficial facial and platysma muscles has been investigated by the methods of comparative anatomy and by direct observation of the phenomena of development.

Rabl stated that the platysma develops in the territory of the hyoid arch, grows forward, and upwards behind the Anlage of the external ear, and gives rise to the \vhole of the mimetic facial musculature and that of the epicranium. He did not further particularise exactly where and how it develops, nor the relationship of the Anlage of the platysm a. to the stapedius, posterior digastric and stylo-hyoid.

Futamura stated that in man and pig the facialis musculature is derived from an aggregate of myogenic cells round the motor facial nerve. The proximal part consists of two layers separated by the N. facialis, of which the lateral forms the Platysma colli.


  • The condition in the Anura is more complicated, the levator

hyoidei of the tadpole forming only the superficial portion of the depressor mandibulae of the adult form.


There is no trace of the superficial facial and platysma muscles "in Stages A and B of Dasyurus. Indifferent mesoblast cells lie between the lateral surface of the hyoid ventral constrictor and the epidermis (fig. 5). In stage C (fig. 13) the Anlage of the sphincter profuudus and platysma. muscles is developed in this mesoblast, about midway between the lateral surface of the hyoid ventral constrictor and the epidermis, and extending dorsally lateral to the submaxillary gland. No evidence is afforded by the sections that these embryonic muscles are proliferated or delaminated from the hyoid ventral consti-ictor~they are developed in the subepithelial mesoblast. In stage D (fig. 18) the embryonic muscle-cells are better developed and have spread forwards into the mandibular segment. In stage E (figs. 20 and 21) they have spread still further forwards in the mandibular segment, and down the neck. In the mandibular and hyoid segments they show evidence of division into two layersan inner, of cells elongated in the transverse plane, the sphincter profundus ; and an outer, of cells elongated in the longitudinal plane, the platysma. In stage F (figs. 22 and 23) these two sheets are more widely separated, and the platysma has spread dorsally over the temporal muscle and forwards, forming the Anlage of the sphincter palpebrarum ; whilst the sphincter profnndus does not extend dorsally above the level of the zygomatic arch. The sphincter profundus is not developed behind the external ear; its hindmost fibres form the depressor am-is s. pars auricularis of the constrictor profundus. The Anlage of the auriculamandibnlaris is formed as a forward and downward growth from the platysma sheet, opposite the incns (fig. 23). In stage H (fig. 28) and in the 10 mm. stage of Didelphys aurita (fig. 39) it has separated from the platysma and extended forwards and backwards; its anterior end is attached to the posterior edge of the mandible and its posterior end to the internal surface of the aural cartilage.

In stage H the buccinator and maxillo-labialis are formed from the sphincter profundus.

The muscles of the external ear begin to be differentiated in stage F, and are well marked in stage H; the depressor auris, as stated above, is developed from the sphincter profuudus; all the other muscles are developed from the platysma. sheet, which is solely present behind the ear, and in front solely present above the zygomatic arch. They consist of sculutaris, post-auricularis, anricnlo-occipitalis, three little muscles on the outside, and two—the transversus and obliquus —on the medial side of the cartilage.

These developmental phenomena. in Dasyurus confirm the opinion of Boas and Paulli that the orbicularis palpebrarum is derived from the platysma sheet, the opinion of Ruge that the buccinator is derived from the sphincter profundus, and Show that the maxillo-labialis is derived from the sphincter profnndns.

In an 8'5 mm. embryo of Ornithorhynchus the Anlage of the sphincter superficialis and platysma muscles is present in the hyoid segment outside the hyoid ventral constrictor (figs. 29-33). It has spread forwards into the mandibular segment, and backwards into the neck. It shows slight indications of separating into the sphincter superficialis and platysma sheets.

In 14 mm. embryos of the pig there is no trace of the Anlage of the sphincter profundus and platysma. muscles; the space between the outer surface of the Anlage of the stapedius and hyoid ventral constrictor and the epiblast is packed with mesoblast, in which no trace of differentiation can be seen. In 15 mm. embryos (figs. 56, 58) the Aulage of the sphincter profundus and platysma can be seen in the ventral part of the hyoid segment, close to the epiblast, and separated by an interval filled with indifferent mesoblast cells from the Anlage of the hyoid ventral constrictor.

The auriculo—1na,ndibula.ris muscle—either muscular or tendinous—-is present in many Mammals. It takes origin from the cartilaginous portion of the external auditory meatus or cartilage of the outer ear, and is inserted into the ramus ascendens of the mandible (Bijvoet). It is innervated by the N. facialis, either by a branch from the rami aux-iculares



634 F. H. EDGEWORTH.

posteriores, e. g. Canis (Chaine), or by a more distal branch than that for the posterior digastric, e . g. Tatusia, Marsupials (Bijvoet).

Two views have been advanced as to the derivation of the muscle. Ch-aine held that it is homologous with the depressor mandibulae of lower Vertebrates; fiirbringer and Bijvoet that it is a derivation of the facial musculature. The abovedescribed phenomena in Dasyurus prove the correctness of the latter theory; they show that it is derived from theplatysma. The muscle is one which was identified by some observers as a single—bellied digastric inserted into the lower jaw, e. g. in Dasyurus, Tatusia, Manis. This question is discussed above (pp. 627, 628).

On the Derivation of the Muscles of the External Ear .—-The comparative anatomy of the muscles of the external ear in Mammals has been investigated by Rage, Baum and Kirsten, and by Boas and Paulli; their development by Killian, Dobers and Futamura.

Ruge was of the opinion that all the auricular muscles were derived from the platysma; none from the sphincter profundus.

Boas and Paulli agreed with this opinion, except in regard to the depressor am-is, which they described as the pars.

auricuiaris of the sphincter profundns.

Killian stated that the muscles in front of the ear— attraheus, attolens (anterior part), helicis major, helicis minor, tragicus—were derived from an upgrowth of the platysma in front of the ear; and those behind—1'et1-a.hens,. hinder part of the attolens, obliquus, transversus—from the hinder superficial layer of the dorsal portion of the hyoid arch musculature.

Dobers stated that in the pig the muscles develop from twogroups which “zwar gemeinschaftlicher Abstannnung sind, sich jedoch sehr frfihzeitig trennen.” One group lies from the very beginning caudo-dorsal to the ear; the second group lies more ventral, and can be called the embryonic platysma, for it also gives rise to the pla-tysma myoides.


From the second group develop the tragicns, adductor

Ll1l‘iS inferior and externns and helicis major ; from the first

group the other muscles.

Futamnra.’s statements difier much from the above; they are that, in the pig, the scntularis, helicis major and minor are developed from the platysiiia, .and the other muscles from the sphincter auris, which is derived from the sphincter colli.

In Dasyurus the superficial muscle sheet spreads upwards from the site of its formation in the mesoblast external to the hyoid ventral constrictor, and divides into two layers, each consisting of embryonic muscle-cells easily distinguishable from one another and from surrounding uiesoblast. The sphincter profnndus does not spread above the zygouiatic arch, nor is it present behind the ear. Only the depressor uni-is is developed from the sphincter profundus ; all the other muscles are derived from the platysma. sheet. This method is also evident in Didelphys a.urita.. The embryological phenomena. in these two Marsupials thus confirm the theory of Boss and Paulli.

The development of the cervical portion of the platysma. by g'ra.dua.l descent from the hyoid region is of special interest owing to the discovery by Kohlbrugge tha.t in many Marsupials and Eclentates‘ the R. cervicis of the N. fa.cia.lis is entirely replaced by the Nu. cutanei of the cervical nerves so that it is entirely restricted to the face. Kohlbrugge advanced the following theories in explanation: “Entweder tiinscht der R. colli des Menschen nur eine primitive Luge vor uud erreichte die Halsregion nur durch ein Herabriicken der Muskulatur welches bei den Marsupialieren dann noch nicht hei den Monotremen wohl eingetreten ware, oder wir niiissen eine secundéire Reduction dieser Theile bei Ma.rsupia.liern (und Manis) annehmen, die Gregend welche sonst derFa.cia.lismusl(ula.tur angelifirte wéire da.nn Von den spinalen Nerven uud ihrer Muskeln erobert worden.” He was inclined to accept the latter theory.

‘ Cuscus orientalis, Cnscus maciilatus,Parrtdoxnrns hermztphroditzt, Mztcropus brunii, Mania ja.v:1nica..


There is a. third p0Ssibility—that the superficial muscles of the neck may be of facial origin and receive a. secondary innervation by cervical nerves; this is confirmed by the developmental phenomena in Dasyurus, where the platysma originates in the hyoid region and spreads down the neck.

This phenomenon is one which can be assimilated with others occurring in the muscles of the head. I have previously given a list of thirteen in various Vertebrates, which appear to be referable to the law that if a muscle grows into one or more neighbouring segments, that portion tends to be innervated by the corresponding nerve or nerves.

It is noteworthy that in Monotremes (McKa._v, cited by Kohlbrugge) the cutaneous muscles of the neck are innervated by both the N. facialis and the first four cervical nerves—i. e. a condition exists re innervation intermediate between that present in Marsupials and that in which the innervation is solely by the seventh nerve. It is similar to the double innervation of the trapezius by the N. accessorius and cervical nerves.

Kohlbrugge also found that in Marsupials the cervical nerves innervate the Platysnia sheet in front of the ear, the N. facialis only innervating the sphincter profundus. The cervical nerves thus spread beyond their segmental territory and innervate muscles which did not originate in it. This must be a very rare occurrence as regards motor nerves, though it is parallelled by the N. vagus.

On the Homologies of the Sphincter superficialis s. colli, Platysma, and Sphincter profundus, in NonMamma.ls.——Ruge, who investigated the adult form of the hyoid muscles in all classes of Vertebrates, stated that the sphincter colli of Monotreuues is homologous with the muscle Cgvd of Selachians, and the Platysma group with the muscle Cgmd. '

Now in Ornithorynchus, Dasynrus and pig these muscles are formed from a common Anlage developed in the mesoblast outside and not as an extension backwards of the hyoid ventral constrictor—so that the homologies suggested by Ruge appear impossible. Similarly, they cannot be homologous with the constrictor colli of Sauropsida, for this is developed as an extension backwards of the levator hyoidei and interhyoideus, nor with the backward extension of the inter-hyoideus which occurs in many Amphibia.

It is therefore doubtful whether these skin muscles of Mammals have any homologies in non-mammals; they have probably been developed within the Mammalian phylum.

The phenomena recorded above suggest (1) that the sphincter profundus, platysma, and sphincter superficialis muscles have been developed within the Mammalian phylum. (2) That the other muscles developed in the hyoid segment are derived from (a) a levator hyoidei, arising from the outer surface of the auditory capsule and inserted into the upper part of the hyoid bar; (b) a ventral constrictor or inter-hyoideus, arising from the hyoid bar and passing to a. median raphé.

When the primitive conditions of the mandibular and hyoid muscles in Mammalia are compared with those of Amphibia and Sauropsida, it becomes clear that they point to the same conclusion as that arrived at by fiirbriuger in his “ Abstainmung der Saugethiere ”—~that Mammals are descended from a pro-amphibian stock.

I have, in conclusion, to offer many thanks to Prof. J. P. Hill for the loan of sections of Dasyurus viverrimus and Ornithorhynchus, to Prof. VV. N. Parker for the loan of sections of an adult Echidna, to Dr. Assheton for the loan of specimens of Echidna (stages 47 and 50) and Phascolarctos, to Herbert Dennison, Esq., for the head of a Phoca vitulina, to Fraulein Snethlage for an embryo and head of an adult Bradyp us marmoratu s, and to the Director of the Calcutta Museum for the head of a Manis pentadactyla.

The expenses of the \vork and its publication have been defrayed by a grant from the Committee of the Bristol University Colston Society.

September Qncl, 1913.


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EXPLANATION OF PLATES 38-15,

Illustrating Dr. F. H. Edge‘-vortl1’s paper “ On the Development and Morphology of the Mandibular a.nd Hyoid Muscles of Mammals.”

ABBREVIATIONS.

Ala temp. Ala. temporalis. alisphen. b. Alisphenoid bone. An. hy. gloss. &' slyl. gloss. Anlage of the hyo-glossns and stylo-glossus muscles. An. pal. A} pty. b‘. Anlage of palette and pterygoid bones. An. of i'nte'rm,aml. m. Anlage of intermandil:-nlm'is muscle. An. of must. ms. Anlnge of masticatory muscles. An. par. 11. Anlnge of parietal bone. An. Sph. sup. J: platys. Anluge of sphincter superficialis and plntysum. muscles. An. stap. rt" hy. 1:. cows. Anluge of stapedius and liyoid ventral constrictor. Au. tens. pal. it tens. ty. Anl-age of tensor veli palatini and tensor tympani muscles. An. zygo. 1% mass. Anlnge of zygoinntico-ma.ndibulai'is and n1a.sseter muscles. Ammlus m. Annulus muscle. ant. dig. 121.. Anterior digastric muscle. and. cap. Auditory capsule. aud. epith. Epithelium of auditory vesicle. czur. mancl. m. Auriculo-ma-udibularis muscle. cur. temp. n. Auricnlo-tempor-.Ll nerve. ba.sz'br. c. Basibra.nchia.l cartilage. 1 In-. bar. first bl“r].1lCl1l‘cLl bar. b1‘.]ly. m. Branclnio-liyoid muscle s. cerato-lxyoideus s. inter-liyoideus. Zn‘. IX to g. s. 13. Branch of ninth nerve to great superficial petros-a.l nerve. lmcc. m. Bnccinator muscle. lmccal n. Buccal nerve. charzla ty. Cllordu. tyinpani nerve. cmn. br. IX to VII. Communicating branch of ninth nerve toseventh nerve. cons. ph. m. Constrictorpharyngis muscle. cm's£cc]1u1'. Crista. parotica. cut. br. an/ylohy. 11. Cutaneous branch of the mylohyoid nerve. dep.wwr1Is m. Dep1'essora.nris muscle. dep.maml.ant.an. Depres~ sor mandibulae anterior muscle. detrah. mami. m. Detraliers mandibular: muscle. E. A. M. External auditory meatus. ezct. pty. m. External pterygoid muscle. ext. rect. m. External rectus muscle of eye. Eust. tb. Eustnchizm tube. Gasa. 9. Gasseri-u.n ganglion. gen. gloss. m. Genieglossus muscle. gen. gloss. pout. ext. Genioglossus posticus e.\'te1-nus muscle. gen. gloss. post. int. Genioglossus posticus internus muscle. gen. hy. m. Grenio-hyoid muscle. genie. g. Geniculate ganglion of seventh. g. s. 10. Great superficial petrosal nerve. ham. Hamulus. hy. tap. in. Hyo-epiglotticus muscle. hyagloss. m. Hyo-glossus muscle. hi/.11. cons. Hyoid ventral constrictor muscle. hy. v. cons. 11.12. Hyoid ventral constrictor muscle, dorsal portion. 111/. 1;. cans. v. p. Hyoid ventral constrictor muscle, ventral portion. inf. (1. Zing. rt" mylahy. 11.. Trunk which (livides into inferior dental, lingual and Inylo-hyoid nerves. int. car. (1. Internal carotid artery. int. hyal. Inter-hyale. inter/J.rt. men. Intemrticular meniscus of jaw joint. 1.'nlerma,nd. m. Intermandihuleu-is muscle. int. ply. 11:. Internal pterygoid muscle. jug. liy. m. J1Igulo— hyoideus muscle. lam. use. a. t. Lmniim ascendens alaa temporalis. lury. gloss. m. Lzu-yn-go-glossus muscle. lcf..hya.l. Latero-hyal cartilage. lat.-m. Lateral masticatory muscle. le1;.ILy. an. Levatorhyoidei muscle. len. pal. -m. Levutor veli palatini muscle. ling. n. Lingual nerve. ling. &‘ chonlc ly. United lingual and chords. tympani nerves. liwg. Dr. IX. Lingual branch of ninth. mal. b. Malzmr bone. mall. Malleus portion of Meckel’s cartilage. .M'a.m. ply. b. Ma.mma.lia.n pterygoid bone. -maml. Mandible. mass. m. Musseter muscle. mcssnn. Masseter nerve. muaml/. llrlaxilla hone. Me. Meckel’s cartilage. med. m. Median masticatory muscle. memb. ob. Membmna. ol)tura.t0ria. mid. cerv. 7;. Middle cervical vein. M071. ply, b. Monotreme pterygoid bone. mylohy. 7L. Mylohyuicl nerve. otic. g. Otic ganglion. orb. par. com. Orbito-parietal commissure. pal.b. Palate bone. pal. ns. Palatine nerves. pmxb. Parietal hone. plalys.m.. Plzitysma muscle. phar. Pharynx. post. d2'g.m. Posterior digastric muscle. post. dig. m. cl. 1). Posterior digastric muscle, (lorszrl pant. post. dig. an. 12. p. Posterior dig-ustric muscle, ventral part. pi-esph.c. Presphenoid cartilage. premtmd. s. Prem-.mdibulur segment. pi-ac. ulm-is. Process uluris. ply. bo. Pterygoicl bone. r. lat. V2. Ramus lateralis of the mandibular division of fifth. 1'. med. V._.. Rmnus medialis of the mauidibulzu‘ division of fifth. sph. prof. Sphincter profuudus. xph. sup. Sphincter superlicialis. spl1.eu.pal. g. Spheno-pa.la.tine ganglion. sq. b. Squamous hone. aitap. m. Stupedius muscle. slernogloss. an. Sterno-glossus muscle. stcmohy.1n. Stemo-hyoid muscle. st. ma-ucl. an. Sterne-mandibularis muscle. st. mast. m. Sterno-mastoid muscle. slylagloss. m. Stylo-glossus muscle. slglhyal. Stylo-hyal cartilage. stylph. m. Style-pharyngeus muscle. styloid. m. Styloicleus muscle s. hyoid ventral constrictor. sub. mam. gl. Suilnmrxillary gland. temp. m. Temporal muscle. lend. past. dig. Tendon of posterior (ligastric muscle. tend. tens. pal. Tendon of tensor veli pulatini muscle. lens. pal. m. Tensor veli pa.latini muscle. tens. ly. in. Tensor tympani muscle. thyraliy. m. Thyro-hyoid muscle. thyro. cart. Thyroid ca.rti]a.ge. trap. in. Trnpezius muscle. tymp. b. Tympanic bone. rid. n. Vidi-an nerve.



THE MANDIBULAR AND l:lY0lD MUSCLES OF MAl\l'.\lALS. 5413

zyqo. maml. 111. Zygmnntico-111a11dih11l1i1'is muscle. Ro1n:111 1l\1n1&l‘:‘I.iS. Cranial nerves.

Dzisyurus vivei-1-imus, figs.1-28.

figs. l—3.—F1~om transverse sections, stage A (just born; g1'ez1test length, in spirit, 56 111111., l1e:Ld length 2'5 111111,); fig. l is the most a.nterio1'. fig, 1, slide 1, row 1, 1111u1ber 17; fig. ‘2. s. 1. 1'. 7, 11. 3 ; fig.3, s. 1,1‘. 8, 11. 12.

fig. 4.~—Fro111 trm1sVe1'se section, stage B (few hours old; g'1'e:1.test length 5'75 to 6 111111. heat] length 3 mm.) : the section is 21. little oblique and tl1e right side is amtei-ior to the left ; s. 2, 1-. 1, 11. 1.

fig. 5.~—Po1-tion of tra11s\'e1'se section. stage B ; s. ‘2. 1'. 1, 11. 20. showing undiifereiitizitecl inesoderin between posterior cligusti-ic muscle and epiblzist. '

figs. G—10.-F1'o111 sagittztl sections stage C (twenty-six hours old; g1'ea.test length 6 111111. l1e21c1le11gtl1 3225111111.); fig. G is the 111ost:111te1‘io1‘. fig. 6, s. 1, 1'. 2, 11. 1; fig. 7. s. 1, 1'. 3, 11. G; Hg. 8, s. 1, 1'. 4, 11.1; fig. 9, E. 1, 1*. 4,11. 7; fig. 5, s. 1,1'. 5, 11. 9.

fig. 1l.—F1-om t1-1111sve1'se section stage C ; s. 2, 1-. 2, 11. 15.

fig. 12.—Portio11of tr.1.ns\'e1'se section, stage C, s. 2, r. 3,118, showing Htnpedius muscle.

fig. 13.—Po1-tion of t1'a.ns\'e1‘se section, stage C, s. 2. 1-. 4, 11. 11, showing Anlage of spl1incter p1'ofL1nclnsa.nd plntysinau muscle.

figs. 14—18.—F1'o1n tra.11sve1'se sections, stage D (p1-ob-.1l.>lytl11'ee days old; greatest length 7 n1111.,hez1d length 4 111m.); fig. 14 is the 111051 zuiterior. fig.14, s. 1, 1'. 8, 11. 1; fig. 15, s.1,1‘. 8, 11. 9; fig. 16, s. 1, 1'. 8, 11.13; fig. 17,s. 2,12 1,11. 3; fig.18, s. 2,1-. 2, 11.4.

fig. 19.——Po1'tion of t1':111sve1'se section, stage D, s. 9, 1'. 3. 11. 1‘_’, showing stnpedins muscle.

figs. ‘.10, ‘21.—F1'o1n t1'a11sve1'se sections, stage E (5 01- G clays old; greeitest length 8 111111.,l1e:1(l length 4'5 111111.); fig. 90 is the more :111te1~ior; fig. 20, 11.2, 1'. 4,11. 12; fig. 21, s. ‘2, 1'. 5,11. 5

figs. 22, 23.—F1'on1 transverse sections, stage F (-.ibo11t 7 days old: greatest length 8'5 to 9 1.nm., head length 5 to 5'5 111111.) ; fig. '22 is H11more '.111terio1'. fig. 22, s. 2, 1'. 8, 11. 10; fig. 23, s. 3, 1'. 2, 11. 13.

figs. 24-—27.—Fro111 t1':1.nsverse sections, stage H (about 14- days old; greatest length 135 111111., head length 8 to 85 111111.); fig. 24, is the most ~.111te1'io1'. fig. 221, s. 5,1: 3,11. 10; fig. 15, s. 5, 1: 5,11. 2; fig. 26,s.5,1-. 6.11. 8; fig. 27, s. 6. 1'. 2, 11. 10.

fig. 2S.—From t11111sve1'se section, stage J (35 days old; gl'€'11te5i. length 20 111111., head length 125 mm); s. 3, 1'. 2, 11. 5.


Ox'nithorhy1.1chus, figs. 2933.

figs. 29£1.—F1-omlongitudinal sections, embryo 8'5 mm.; fig. 29 is the most later-3.1. fig. 29, s. 3, r. 2, n. 4; fig. 30, s. 4, r. 1, n. 3; fig. 31, s. 4. 1'. 3, 11. 4.

figs. 32, 33.-—Fron1 Transverse sections, embryo 8'5 mm. ; fig. 32 is the more anterior. fig. 32. s. S. r. 1, n. 12; fig. 33, s. 7, r. 1, u. 3.

Echiclna, figs. 34-38.

figs. 3~L—3T.—F1-om transverse sections, 25 mm. long ( = stage 50 of Semon) ; fig. 34 is the most anterior.

fig. 3S.—Fro1n transverse section, young adult 12".? cm. long.

fig. 39.—Transverse section. Didelphys a.urii:a, 10 mm.

fig. 40.—Sketch copied from Bijvoet, showing cligastric muscle of Bradypus tridnctylus.

fig. 41.—Sketch showing (ligzistric muscle of Bra dypns marmoratns.

fig. 42.—From transverse section, Dzisypus noven1cincti1.,301nn1. fig. 43.—Sketch of side of head of Mzmis pentadactyla. fig. 4~L.—Sketch of muscles on inside of jaw of Phoca vitulinn.

Rabbit, figs. 45-54-.

fig. 45. Transverse section. embryo 5-} mm. crown-rump length. figs. 46—i8.—Transverse sections, embryo 16 mm. crown-rump length; fig. 49 is the most anterior.

figs. 49-53.—L0ngitL1dinaJ horizontal sections, embryo '23 min. crownrump length; fig. 54 is the most dorsal.

fig. 5«L.—Longit.udimiI horizontal section, embryo 33 mm. crown-rump length. '

Pig, figs. 55-64.

figs. 55—5S. Transverse sections embryo 15 mm. crown-rump length; fig. 60 is the most anterior.

fig. 59-63. Transverse sections, embryo 21 mm. crown-rump length; fig. 68 is the most anterior.

fig. 64. Transverse section, embryo 32 mm. crown-rump length.


figs. 65—67,—Sketches of model of developing iiiasticatoi-y muscles of rabbit, embryo 13 min. cro\vn»1'ump length ; fig. 65 from inside, fig. 66 from front, fig. 67 from outsifle.

figs. 68, 69.—Sl:ebelies oi model o£ developing masticatory muscles of mhlait, embryo 16 mm. crown-rump length ; fig. 68 from inside, fig. 69 from outside.

figs. 70, 71.—-Sketches of moclel of developing masticatory muscles of pig, embryo 19 mm. crown-1-nnip length; fig. 70 from inside, fig. 71 from olitsifle.

figs. 72, '?3.—Sketches of model of developing mastimtory muscles of pig, embryo 21 mm. c1'own-rump length; fig. 72 from inside, fig. 73 from oubsifle.

figs. 1-28, 39-41, 43 were drawn by Mr. E. E. Sliellarcl. figs. 65-73 were (lmwn by Mr. C. W. Slmrpe.