Human Embryology and Morphology 12

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Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures

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Chapter XII. Development of the Face

Evolution of the Human Face

In our survey of the neural part of the human cranium we have seen that its outstanding features are the result of a great cerebral development. When, however, we turn to the facial and pharyngeal parts of the skull and head, we find that the factors which have determined their shape are related to the functions of smell, respiration and of mastication. It is unnecessary to again insist on the fact that the human embryo, in the latter part of the first month, shows a resemblance to a generalized type of fish ; it possesses the basis of a branchial arch system. As in the fish, the olfactory organ is represented by a pair of pits or depressions, which at first have no communication with the mouth. In some forms of fish — certain rays and sharks (Fig. 151) — a channel is formed between each olfactory pit and the mouth. The functional meaning of such a channel is evident ; the water imbibed is sampled by the nose before entering the mouth. When pulmonary breathing was introduced, as in Dipnoean fishes, the open naso-buccal channel became enclosed by the union of its bounding folds. In amphibians, reptiles and birds the naso-buccal channel becomes dilated to form a true respiratory nasal passage, and the parts bounding the passage unite on the roof of the mouth to form the primitive palate. In Fig. 152 the parts entering into the formation of the primitive palate are shown. • They are three in number : (1) a premaxillary and vomerine part developed between the nasal passages ; (2) a right and left maxillary part, laid down on the lateral or outer aspect of each passage. In mammals a fourth element is added to the primitive or reptilian palate, and in this way the mammalian mouth is separated from the nasal respiratory passage, and can serve the purposes of mastication and suction. Thus in the evolution of the face there have been three distinct stages : (1) a piscine, in which the nose and mouth were formed independently ; (2) an amphibian stage, where the nasal respiratory passage opened on the roof of the mouth ; (3) a mammalian stage, in which it opened in the naso-pharynx. In the development of the human embryo we see these three stages reproduced.[1]

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Fig. 151. The Naso-Buccal Grooves of a Dog-Fish. On the right side the nasobuccal channel is exposed.

Processes which form the Face

Towards the end of the 4th week of foetal life, five processes begin to spring from the base of the primitive cerebral capsule, which by the end of the second month have completely united together to form the facial part of the head.[2] In Fig. 153, a diagrammatic representation is given of the condition of these five processes about the end of the 6th week of development. Of the five, one, the nasal or fronto-nasal, composed of symmetrical right and left halves, is median, and projects beneath the fore-brain ; the others are lateral, two on each side, the mandibular and maxillary. The cavity which these five processes surround is the stomodaeum, a space ultimately destined to form the nasal and part of the buccal cavities. The representatives of these five elements are recognizable in certain fishes (see Fig. 151). The part of the adult face formed by each process is shown in Fig. 154.

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Fig. 152. Roof of the Mouth of a Lion-Pup, showing the condition of Cleft Palate recalling in form the Palate of ReptUes. On the right side the bones are exposed by removal of the soft parts.

Nasal Region of Face

In reality the core of the face is formed by the cartilaginous capsule which encloses the organ of smell. We have seen that the olfactory capsule occupies the terminal part of the prechordal plate, and in primitive vertebrates forms the entire snout or face (Figs. 130, 133). Hence the first step in the development of the human face represents the upbuilding of the nasal cavities. Three stages in this process are depicted in Fig. 155, taken from a recent research by Professor Frazer.[3] At the end of the 5th week the olfactory organ is exposed on each side of the fore-brain as a plaque surrounded by a growing raised margin or fold. The pituitary recess, opening from the stomodaeum (Fig. 155, A) lies in contact with the under surface of the fore-brain. At the end of the 6th week (B), the olfactory plaque has become a pocket by the upgrowth of the mesial and lateral nasal folds or processes which, being united above, rise up as a hood. Below, the olfactory pit communicates with the buccal cavity by an open naso-buccal channel — just as in the dog-fish. At the same time the maxillary process grows forward, and applies itself to and fuses with the substance of the lateral nasal fold. In the 7th week (C) the maxillary process has come in contact and fused with the globular end (globular process) of the mesial nasal fold, and thus the naso-buccal channel is covered over and we can now speak of anterior nares and a posterior opening or primitive choana — at first closed by an epithehal membrane (Fig. 165, C). As the olfactory pockets enlarge they come closer together under the fore-brain until the mesial folds and the tissues between them form the primitive septum of the nose — the lateral nasal fold and intermediate tissue of each side being sometimes called by a separate name^ the fronto-nasal process. Thus the nasal cavities which form the foundation of the face are built against the wall of the fore-brain and the nasal folds represent the margins of the outgrowing edifice.

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Fig. 153. Showing the formation of the Face by the Nasal, Maxillary and Mandibular Processes in an Embryo of the 6th week. After Wilhelm His (1831-1904)

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Fig. 154. Showing the parts of the Face formed from the Nasal, Maxillary and Mandibular Processes.

Malformations of the Face

These processes may fail to unite in the second month, and in this manner malformations of the face are produced. The most common anomaly is a partial failure of the nasal and maxillary processes to fuse, various degrees of hare lip and cleft palate being tlius caused. In hare lip[4] the cleft appears in the upper lip between the middle part formed by the mesial nasal processes and the lateral parts formed by the maxillary processes (Fig. 154). In cleft palate, the failure of union occurs between the deep parts of the nasal and maxillary processes (Fig. 171). The lateral or the mesial parts of the nasal process may fail to fuse with the maxillary processes, and these appear on the face as polypoid or irregular projections (Figs. 156, 157). In such cases the right and left maxillary processes may unite and form the whole of the upper lip. Macrostoma is due to a partial failure of the mandibular ta unite with the maxillary element. Any of these processes may be under or over-developed ; over-development of the nasal and under-development of the mandibular (micrognathia) are of common occurrence.

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Fig. 155. Three stages in the formation of the Nasal Cavities and Primitive Palate. (Prof. J. E. Frazer.)

The cleft in the lip of the hare is exactly in the middle line, and is due to a separation of the right and left parts of the mesial nasal process. The condition of median hare lip, which is rare in man, is represented in Fig. 158 ; in this case there was a partial cleft of the palate, and the pituitary body formed a tumour-like mass within the septum of the nose. A median cleft in the lower lip is also rare, and is due to a failure in the union of the right and left mandibular processes of the lower jaw (Fig. 159). Another remarkable condition — cyclopia — is shown in Fig. 202, p. 202, where the nasal processes have united together to form a single proboscis-like structure projecting above the eyes, which are partly fused.

In this condition the palate and upper lip are formed by the union of the maxillary processes. The condition is not uncommon, and shows how adaj)table the various embryological parts of the face are.[5]

The Method of Fusion

The manner in which embryological parts unite is similar in nature to the healing of wounds. Fig. 160 represents a coronal section of the head of a human embryo, in which the mesial nasal process containing the germinal epithelium of the upper incisor teeth is about to unite with the maxillary. The ectodermic coverings of the processes are in contact. Across the epithelial union thus formed the mesodermal tissue spreads, the two processes thus becoming intimately united. We know that the process of healing may be arrested by many pathological conditions ; the process of embryological union may be also arrested, but the exact causes of the arrest we do not yet know. If union of the facial processes fails to take place, then subsequent growth tends to move the processes apart, and union becomes impossible. The cleft in the lip or palate increases in width as the foetus becomes older. The tongue lies between the maxillary plates (Fig. 161), a normal position during the 2nd month. It is extruded as the palate is formed, the extrusion being due to the rapid growth of the mandibular and maxillary processes in the earlier part of the 3rd month.

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Fig. 156. Face of a Child showing the left Nasal Process and Pocket as a free Polypoid Body and the left Maxillary Process ununited with the Mesial Nasal (left Hare Lip). (After Kirchmayer.)

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Fig. 157. Face of Child in which the Nasal and Maxillary Processes are ununited. A, Polypoid Tubercle in line of Naso-Maxillary Cleft ; B, Eight Lateral Nasal Process ; C, Left Lateral Nasal Process ; D, Mesial Nasal Process ; E, F, Maxillary Process. (London Hospital Medical College.)

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Fig. 158. Median Hare Lip in a Child with Partial Cleft Palate and Ectopia of the Pituitary. (Mr. A. R. Tweedie's case.)

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Fig. 159. Median Cleft of the Lower Lip and Jaw. (Prof. MacCormick's case.)

Structures formed in the Mesial Nasal Processes

We have already seen that the mesial nasal processes, which represent the inner walls of the nasal pockets or cavities, grow out from the base of the fore-cranium, and when they grow together to form the primitive septum of the nose, the cartilage formed in their united substance represents a direct forward continuation of the prechordal plate. From the united substance of the mesial nasal processes are formed the septum of the nose (Fig. 162), the premaxillary part of the upper jaw, and the middle third of the upper lip "X^^gs. 152, 157) ; in their anterior inferior angles^re formed the premaxillae. The remainder forms the septum of the nose. Part of the cartilage of this septum remains unchanged as the septal cartilage (Fig. 162). In the septal wall are also developed the mesial limbs of the alar cartilages, which give form to the anterior nares. One element is added to the lower anterior part of the septum — just above the opening of the naso-palatine canal — the paraseptal cartilages (Fig. 142) which primarily serve for the protection of an isolated area of the olfactory membrane — Jacobson's organ — reduced to a vestige in man. In Fig. 155, C it will be observed that the lateral nasal process also fuses with the mesial ; the paraseptal cartilages are derived from the lateral nasal processes (Fawcett).

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Fig. 160. Coronal Section of the Head of a Human Embryo in the 6th week of development and 14 mm. long. (After J. L. Paulet, Archiv. fur Mik. Anat. und Entwickl. 1911, vol. 76, p. 658.)

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Fig. 161. Similar Section of the same Embryo further back, showing the Tongue in the Palatal Cleft. (J. L. Paulet.)

The vomer is developed in the membrane (perichondrium) which covers the primitive septum (Fig. 169). A centre of ossification appears at the end of the 2nd montli on each side near the lower border of the septum ; these fuse together under the palatal margin of the cartilage. Thus the vomer forms at first a shallow trough in which the cartilage of the septum appears to be imjDlanted (Fig. 163).

The Vertical Plate of the Ethmoid is formed by a direct ossification of the cartilage of the primitive septum. Ossification begins in the 4th month.

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Fig. 162. Showing the structures formed in the Mesial Nasal Processes.


^naso-pafatine canal premaxilla mesial port, of upper lip.

The crista galli, the intra-cranial part of the septum, is formed in part by the ossification proceeding into the attachment of the falx cerebri.

Fiemaxillary Bones

The two premaxillary bones form the sockets of the four upper incisor teeth. In the human foetus at birth the suture between the premaxilla and maxilla can be seen on the hard palate ; it runs on each side from the naso-palatine foramen to the alveolus between the lateral incisor and canine (Fig. 165). As is illustrated in Fig. 165, the relationship of this suture to the tooth sockets is variable, but the relationship just mentioned is the usual one. On the facial aspect, a suture between the premaxilla and maxilla is at no stage distinct, the maxillae appearing to overlap the premaxillary elements, almost completely excluding them from the face. The nasal spine is formed by the premaxillae. The palatal plates of the premaxillae represent the prevomers which are seen as distinct bones in the primitive palate of amphibia.

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Fig. 163. Showing the Trough-shaped Vomer of the newly-born.

In mammals generally the premaxillae are highly developed, separated throughout their whole extent by a suture from the maxillae and form the snout part of the face. In the higher Primates the face becomes less elongated, less prognathous, or projecting, and the premaxillae less developed. In the orang, for instance, the premaxillary sutures are distinctly seen on the face at birth (Fig. 164), but as the permanent canines begin to develop they fuse with the maxillae. The premaxilla is more reduced in man than in any other primate ; in him it is partly fused with, and overlapped by, the maxilla from the first appearance of ossification ; in apes fusion does not occur until the eruption of the permanent teeth. The vestigial character of the premaxilla in man is due to the reduced size of his masticatory apparatus and the consequent retrogression in the development of the facial part of the skull.

Relationship of the Premaxilla to Cleft Palate

It is usual for the sockets of all four incisor teeth to be formed by the premaxilla. In many cases of cleft palate[3] (see Fig. 167) only the two central incisors are situated on the premaxilla, the sockets of the lateral incisors being attached to the maxilla. Even in the normal palate (Fig. 165, B) this may be the case. Albrecht supposed that each premaxilla was made up of two bones— an outer and an inner — and that in cleft palate the fissure might lie between the elements of the premaxillary or to their outer side. We now know (1) that cleft palate is not due to a failure of ossific centres to join, but to a non-union of two embryological masses — the mesial nasal and maxillary ; (2) that the partial suture, which may divide the palatal part of the premaxilla, is due, not to two centres of ossification, but to the formation of the palatal part by two processes — one corresponding to the middle incisor socket, the other to the lateral incisor ; (3) the germ or bud of the lateral incisor is formed at the point of union of the mesial nasal and maxillary processes. If these processes fail to join, the bud of the lateral incisor, as the processes move apart during subsequent growth, may be carried away by the maxillary or premaxillary element, or, as I have seen, be left stranded in the cleft between the processes. If the lateral incisor remains attached to the maxillary process, then its socket is formed by that element ; if by the premaxillary, then the cleft appears in the more usual situation, and the socket forms part of the premaxilla. The late Mr. Clement Lucas observed that the lateral incisor is often small or even absent in families subject to cleft palate.

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Fig. 164. Showing the suture on the face between the premaxilla and maxilla in the Skull of a Young Orang.

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Fig. 165. Palate at birth, showing varieties of the suture between maxilla and premaxilla. On the right side (A) the suture between the palatal processes of premaxilla and maxilla ends at the socket of the canine ; on the left (J5) between the mesial (Z^) and lateral (/*) incisors ; D, naso-palatine foramen, in which the anterior end of the vomer appears ; E, F, palatal processes of the maxillary and palate bones.

Naso-palatine Foramen

The naso-palatine foramina are formed where the mesial nasal and two maxillary processes unite to form the palate (Fig. 172). In animals with well-developed premaxillae the two naso-palatine (anterior-palatine) foramina are large, and through each passes the naso-palatine duct, which allows a communication between the buccal and nasal cavities. The odour of the food within the mouth thus reaches the organ of Jacobson, which is situated on the septum, close to the nasal orifice of the duct. In man the upper ends of the ducts remain open ; they terminate blindly below, behind the mesial incisor teeth, in the naso-palatine or incisive papilla.

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Fig. 166. Facial part of the Skull of a Cyclops Foetus, in which the nasal processes formed a free proboscis, the eyes a median structure and the maxillary processes the palate. A, orbital plates of frontal; ' B, fused optic foramina; C, orbital plate of sphenoid ; C, basi-sphenoid ; E, orbital plate of maxUla ; F, ear ; G, superior maxilla ; C, canine ; ill', first milk molar.

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Fig. 167. Case of Cleft Palate, in which the maxillary and premaxUlary processes have remained ununited on the left side. A, septal process of premaxilla ; B, nasal septum ; C, canine ; B, palatal process of premaxilla ; E, palatal process of maxilla. The left lateral incisor was absent.

Nasal Duct

The lachrymal sac and nasal duct, through which tears pass from the eye to the inferior meatus of the nasal cavity, are formed between the lateral nasal and maxillary processes (Figs. 154, 155, 157). At the end of the 6th week, when the furrow between the maxillary and nasal processes is obliterated, the nasal or naso-lachrymal duct is represented by a solid bud or core of ectoderm embedded at the inner angle of the eye and in the site of the upper part of the naso-maxillary groove or fissure. This bud becomes cord-like, one extremity growing towards the nasal cavity, which it reaches at the beginning of the 3rd month, while the orbital extremity expands to form the lachrymal sac. The canaliculization of the duct begins in the 3rd month, but is not complete until late in foetal life. In Fig. 157 the lateral nasal and maxillary processes have not fused ; the eye is separated by two folds from the nasal cavity ; the outer represents the semilunar fold, the inner a fold in which the lachrymal canaliculi and caruncula lachrymalis are formed.

Structures formed in the Lateral Nasal Process

The lateral nasal process is developed to form the outer wall and roof of the chamber containing the olfactory organ. Within it develops a plate of cartilage which represents the greater part of the cartilaginous olfactory capsule of lower vertebrates. In the human embryo the process of chondrification begins near its lower border and spreads up towards the roof (Frazer), where it joins the upper edge of the septal cartilage, developed on the united mesial nasal processes and also spreads backwards to enfold the hinder part of the olfactory chamber and to become continuous with the presphenoid part of the prechordal plate. The cribriform area is the last to be formed (see Figs. 143, A, B, p. 148). In front, the lower border of the lateral nasal process joins the septal process, adding to it the paraseptal cartilage (p. 163).

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Fig. 168. Showing the structures formed in the Lateral Nasal Processes.

What becomes of the Cartilage of the Lateral Nasal Process^ (Fig. 168).— It forms on each side: (1) The cribriform plate around the olfactory nerves as they issue from the olfactory bulb ; (2) The lateral mass of the ethmoid, at first merely a plate of cartilage ; the superior and middle turbinate processes are developed from the plate ^ See Fawcett, loc. cit. p. 135.

(Fig. 169) ; ossific centres appear in the cartilage of the lateral mass and turbinate processes during the fourth, month of foetal life ; (3) The inferior turbinate bone (Fig. 169) (maxillo-turbinal). The body of the superior maxilla is developed on its outer side in the maxillary process (Fig. 169) ; (4) The lateral and part of the alar cartilages of the nose ; (5) In the membrane over the cartilage, between the ethmoid behind and the cartilages of the nares in front, are developed the lachrymal and nasal bones, and the ascending process of the superior maxilla. The cartilage beneath these bones disappears after birth (Fig. 168). Ossification of the nasal bone appears at the beginning of the 3rd month ; the centre for the lachrymal appears late — at the beginning of the 4th month (Mall).

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Fig. 169. Coronal Section of the Skull of a 7th month Human Foetus to show the cartilages of the Lateral and Mesial Nasal Processes and the bones formed round them.

Arteries and Nerves of the Nasal Processes

A knowledge of the development of the face assists one to unravel the complicated distribution of its arteries and nerves. Each process carries its own vessels and nerves.

1. Mesial Nasal Process. The chief artery and nerve of this process are the naso-palatine, but branches also come from the nasal nerve and its accompanying artery, the anterior ethmoidal.

2. Lateral Nasal Process. The nerves of the lateral nasal process are derived from Meckel's ganglion and from the descending palatine nerve. Vessels accompany these nerves from the descending palatine artery. The nasal nerve and anterior ethmoidal artery supply the process in front.

The Parts formed from each Maxillary Process

The maxillary process springs from the base of the mandibular process at the end of the 4th week of development, and sweeping forwards below the eye, separates that structure from the mouth (see Figs. 44^ 45, 154). In front it comes in contact and fuses with the lateral nasal process, which it assists to form the outer wall and floor of the nasal cavity, and, in the 7th week, with the globular process of the mesial nasal which forms the premaxillary part of the palate and the middle part of the upper lip. The part of the face formed by the maxillary process is shown in Fig. 154. The hard palate (with the exception of the j)reniaxillary part) is formed by a palatal plate which begins to grow inwards from the maxillary process in the 6th week (Fig. 170) and fuses with the plate of the opposite side about the 10th week. The palatal processes separate the buccal from the nasal cavities, forming the roof of the one and the floor of the other. The palatal plates meet first with the premaxillary part (Fig. 171) ; behind that they come in contact with each other ; the process of fusion spreads backwards, and before the end of the third month the hard and soft palates form a complete naso-buccal septum.

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Fig. 170. Showing the ingrowth of the Palatal Plates of the two Maxillary Processes at the end of the 6th week. The openings erroneously indicated as " posterior nares," are the primitive choanae. (After Kollmann.)

Cleft Palate

To understand the manner in which the various forms of clefts arise in the palate it is necessary to note the manner in which the septum of the nose grows and the fate of the primitive choanae. At the end of the 6th week (Fig. 170) the nasal septum is seen to be relatively short and wide and to form the mesial borders of the primitive choanae or posterior nares (see Fig. 197), By the 9th week (Fig. 171) the septum has grown greatly in length, pushing the primitive palate forwards away from the base of the skull, and thus presenting a long posterior or palatal border which still forms the mesial edges of the primitive choanae ; the choanae stiU extend from the primitive palate to the sphenoidal end of the septum. The dorsum of the foetal tongue lies against the palatal margin of the septum with the palatal folds tucked under its lateral margins (Fig. 161) until the 9th week, when the forward growth of the primitive palate lifts the nasal septum off the dorsum of the tongue and allows the palatal folds to come in contact with each other and with the lower margin of the septum. The manner in which the palatal folds are applied to the septum is shown in Fig. 171, 5 ; by a process akin to the healing of wounds the palatal folds unite with each other and with the palatal border of the septum. The process begins behind the premaxilla and passes backwards, but the posterior part oiE the septum is left free to form the partition between the permanent posterior nares. Thus in the formation of the palate a Y-shaped cleft has to be united ; the short limbs lie on each side of the premaxilla in the primitive palate, the long limb in the middle line of the permanent or mammalian palate. All three parts may remain united as in Fig. 157, or the long limb with one short as in Fig. 167, or only the long Hmb as in Fig. 152. Further, it sometimes happens that one or both primitive choanae are closed permanently by the plug of epithelium which temporarily occludes them becoming organized and forming membrane or bone. As the septum and choanae expand this occluding membrane or partition is stretched and gives rise to atresia of the posterior nares. The wide gap and bent septum seen in nearly all cases of cleft palate are due to changes produced by growth in the later months of foetal life. An asymmetrical growth is a result of the failure in the union of the processes.

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Fig. 171. Development of the Maxillary Palate. A, stage reached in 9th week; B, schematic figure to illustrate the manner in which the maxillary folds are applied to the nasal septum. (Prof. Frazer.)

The Soft Palate

While the hard palate is derived from the palatal plates of the maxillary processes, the soft palate (Fig. 171, A) is derived from a fold which arises as a prolongation backwards of each horizontal plate into the pharynx.[6] Into the palatal folds spread derivatives of the superior constrictor to form the palato-pharyngeus, palato-glossus and azygos uvulae, and possibly also the levator palati. The posterior pillars of the fauces are continuations of the palatal folds within the pharynx. A divided uvula represents a failure of the final stage in the formation of the palate.

Bones formed in each Maxillary Process

The zygomatic process of the temporal, the malar, and the greater part of the superior maxillary are formed directly in the connective tissue within the process.[7] They are membrane-formed or dermal bones. The centre foT the maxilla appears at the beginning of the 7th week in that part of the process which lies under the eye. Very soon, after the various processes of the face are fully united, an extension passes upwards over the lateral nasal cartilage towards the frontal bone (frontal process) ; the orbital, alveolar, and palatal processes are later extensions from the single centre of ossification (Mall, Fawcett).

Palato-Quadrate Bar

In lower vertebrates the maxillary process is supported by a skeletal bar of cartilage known as the palato-quadrate bar, because it stretches from the palate to the quadrate bone^ situated at the base of the mandibular arch (Fig. 173). Although in the human embryo this cartilaginous bar is at no time clearly differentiated (Fawcett), there can be no doubt that two bones have arisen in connection with it — namely the palate and internal pterygoid (Fig. 174). The internal pterygoid plate — the first part of the sphenoid to ossify — is formed early in the 3rd month in membranous tissue which overlies the position of the middle part of the bar, while the vertical plate of the palate is developed in membrane over its more anterior part. Ossification extends to the horizontal plate, within the horizontal plate of the maxillary process, at the end of the 2nd month.

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Fig. 172. Showing the Hard Palate at birth. The premaxillary part is formed from the Mesial Nasal Processes ; the remainder by the Palatal Plates of the Maxillary Processes.

The mandibular process has also a cartilaginous bar developed within it known as Meckel's cartilage (Fig. 173). Thus each of the processes which grow out to form the face has a basis of cartilage, but while the cartilages within the nasal processes are continuous with the base of the skull, the cartilage within the maxillary process comes in contact by its posterior extremity with Meckel's cartilage (Fig. 173). The quadrate bone, which is well seen as a separate element in birds and reptiles, forms a movable base on which the lower jaw articulates. This form of joint gives birds and reptiles an easy faculty of swallowing unmasticated food.

With the development of grinding and chewing teeth in the very early ancestry of mammals a more stable form of temporo-mandibular articulation was evolved, the mandible during the change coming to articulate with the temporal bone, thus leaving the upper end of Meckel's cartilage and the quadrate free to be utilized as the malleus and incus by the organ of hearing.

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Fig. 173. The Cartilages in the Nasal, Maxillary and Mandibular Processes of a Shark.

The simplest condition of the cartilages of the maxillary and mandibular processes is seen in certain fishes. In the common base of these two processes, there is developed a cartilage which binds the basal ends of the palato-quadrate bar and Meckel's cartilage to the skull. The cartilage of the hyoid arch is also bound to it, and hence it is known as the hyo-mandibular cartilage. (Compare Figs. 132, 133, 150 and 173.) Nerves and Arteries o£ the Maxillary Process.— A knowledge of the manner in which the maxillary process is developed explains the distribution and course of its arteries and nerves. The second division of the 5th, represented by the infra-orbital, descending palatine, pterygo-palatine, and Vidian nerves, forms its nerve supply. Its main artery is the internal maxillary.

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Fig. 174. Diagram to show the position of the bones in the Skull of the Human Foetus which are formed in connection witli the palato-quadrate bar.

Formation of Foramina and Canals in Bone

The development of canals and foramina in the bones of the maxillary process illustrates the manner in which these are formed in the skull generally. Many foramina and canals occur between elements which unite in the course of development (see p. 150). The Vidian nerve lies between the internal pterygoid plate (a separate bone) and the external pterygoid, a plate which is formed in the maxillary process as a prolongation of the great wing of the sphenoid. The pterygo-palatine canal is situated between the pterygoid and palatal parts of the palato-quadrate bar. The descending palatine nerve lies between the palate bone and superior maxilla. These are canals formed between different elements. The infra-orbital nerve at first passes forwards in a groove on the orbital aspect of the superior maxilla, but in the later months of foetal life, upgrowths from the centre of ossification of the maxilla meet over the nerve and convert the groove into a canal.

The foramen rotundum and foramen ovale are at first notches on the edge of the great wing of the sphenoid, but in the course of foetal growth the notches are converted into foramina. Hence wherever a nerve foramen or canal is found one may conclude that it marks the junction of two elements, originally distinct, or that it is originally a groove or notch on the edge of the bone (Bland-Sutton). The foramina for nerves in the malar bone appear to be exceptions to this rule. Only one centre appears for the ossification of this bone (7th week), and the nerves evidently become involved during the ossification of the membranous basis. The malar bone is occasionally ossified from two centres which may fail to unite ; the bone is then divided by a suture passing from the orbit to the temporal fossa. A divided malar occurs rather more frequently in Japanese and Mongolian skulls, hence the name of Os Japonicum.

Palatal Rugae

In all classes of mammals the mucous membrane on the hard palate is ridged transversely ; three or four of these transverse ridges are seen on each side of the palate of the newly born child ; they tend to disappear in the adult. Food is triturated between them and the rough papillae on the palatal aspect of the tongue. Their disappearance in man is probably due to the soft nature of his food.

Maxillary Sinus

It will be seen from Fig. 175 that the maxillary process is at first a thin plate, lying between the orbit and mouth, containing the canine and molar tooth buds. It rests on the outer aspect of the lateral nasal process, and to some extent assists that process to form the outer wall of the nasal cavity (Fig. 169). In the third month of foetal life the mucous membrane in the middle meatus begins to bud outwards, presses before it and bursts through the lateral nasal plate of cartilage and begins to distend the maxillary process. At birth the sinus is only a shallow recess on the outer wall of the middle meatus, above the germ of the first milk molar (Fig. 175). It continues to grow until the 25th year, and is the only one of the air sinuses developed from the nasal cavity which is more than a rudiment at the time of birth. In the years of adolescence the sinus expands until it inflates the maxillary part of the malar. As it expands backwards the posterior border of the maxilla, which contains the buds of the permanent molar teeth, undergoes a rotation downwards, so that what was situated on the posterior border conies to be situated on the alveolar border (Fig. 176). If the processes of growth and rotation are arrested, the last molar (wisdom) tooth is left on the posterior border of the maxilla, where it may give rise to pain and supjDuration. The maxillary sinus or antrum is peculiarly large in man and in the anthropoid apes. It is small in monkeys, a greatly expanded inferior meatus taking its place.[8]

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Fig. 175. Coronal Section of the Nasal Cavities of a Newly-Born Child, showing the development of the hiatus semilunaris and air sinuses.

Mandibular Process and Arch

The two mandibular processes unite in the middle line and form the mandibular or first visceral arch. The arch forms the lower or hinder boundary of the stomodaeum (Fig. 177). The right and left processes are in contact in the 4th week of development, but the process of fusion, which may be arrested (Fig. 159), is not complete until the middle of the second month.

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Fig. 176. Showing the manner in which the development of the Maxillary Antrum alfects the size of the palate and position of the molar teeth.

Parts formed from the Mandibular Arch

Besides the lower jaw, there are formed from this arch the soft parts over and under the jaw, the lower lip, the muscles of mastication, the mylo-hyoid and anterior belly of the digastric, the tensor palati, and the tensor tympani. The anterior two-thirds of the tongue, the sublingual and submaxillary glands are formed from the floor of the primitive pharynx between the mandibular and the second or hyoid arch. These parts are supplied from the nerve of the mandibular arch, and are therefore probably derived, in part at least, from the substance of the arch. The whole arch and its derivatives are set apart primarily for the purpose of mastication. Only in mammals are the lips separated from the alveolar processes. In the human embryo the lower lip is demarcated from the alveolus by the downgrowth of an epithelial groove (the labio-alveolar plate or groove) about the middle of the 7th week.

The Mandibular Arch bounds the stomodaeum behind, and is the foremost of the visceral arches which encircle and form the walls of the primitive pharynx. Meckel's cartilage[9] forms its skeletal basis (Figs. 173, 174).

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Fig. 177. The Mandibular Arch and Stomodaeum (primitive mouth) in a Human Embryo of the 5th week. (After Rabl.) A, from the front ; B, from the side.

The 3rd division of the 5th is its nerve, but its artery, the first aortic arch, has only a transient existence, although the inferior dental may represent part of it.

Development and Ossification of the Lower Jaw

In Fig. 178, which represents the condition of the human mandible[10] at the beginning of the 4th month, the primitive cartilaginous skeleton of the mandibular arch can still be followed from the symphysis to the tympanum. Only one part of the cartilage takes a direct share in the formation of the mandible — that part which lies near the symphysis and assists to form the section of the mandible which carries the first premolar and canine teeth. The ventral extremities persist through foetal life as cartilaginous nodules ; they may become ossified. The proximal end of Meckel's cartilage forms the malleus ; all the rest of the bar disappears, although the long internal lateral ligament occupies the site of part of the cartilage. In rare instances the cartilage may undergo complete and independent ossification. Thus the lower jaw, which shares with the clavicle the distinction of being the first bone in the body to ossify, is a membrane or dermal bone. Late in the 7th week a centre of ossification appears in each half, on the outer side of the Meckel's cartilage, and near the site of the future mental foramen. Each half of the lower jaw is ossified by the extension of a single centre. Processes grow up on either side of the inferior dental nerve which, with the tooth buds, comes to lie in a primitive alveolar trough. During the third month the ascending ramus begins to form. In the condylar and coronoid processes a formation of secondary cartilage occurs ; thus the condyle and coronoid are ultimately laid down in cartilage. The two halves of the mandible unite at the symphysis during the second year ; in some animals, such as the kangaroo, the symphysis remains open.

Evolution of the Mandible

To interpret the appearances seen during the development of the human mandible we must suppose that Meckel's cartilage is the primitive skeleton of the mandible — a condition we know to occur in various forms of fishes (see Fig. 173). The malleus formed the upper end of the skeleton of the jaw, the joint between the malleus and incus representing the mandibular joint. The second stage in the evolution of the jaw is the formation of membrane or dermal bone to strengthen the cartilaginous rod and form supports for the teeth. This stage is also seen in fishes. The third and final stage is the formation of an ascending ramus and the evolution of a new joint between the condyle of the ascending ramus and the squamosal part of the temporal. This stage evidently occurred in the early ancestry of the mammals. In all other vertebrates — amphibians, reptiles and birds — the primitive joint persists.

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Fig. 178. Meckel's Cartilage and Mandible of a Foetus in the 4th month of development, viewed on the inner or lingual aspect. (From a drawing and reconstruction by Dr. Alex. Low.) A and B, cartilaginous ossicles at symphysis ; C, termination of Meckel's cartilage.

Growth Changes in the Jaw

The mandible undergoes great changes in the course of growth. As the permanent teeth erupt behind the milk set, increased alveolar space is required. This is obtained (see Fig. 179) by new bone being deposited along the posterior border of the ascending ramus, while absorption takes place at the anterior border. Growth in the vertical height is obtained by the deposition of new bone along the upper border of the ramus. Growth of the upper jaw and of the antrum of Highmore, by pushing downwards the body of the lower jaw, leads to an elongation of the ascending ramus, and to its assuming a more vertical position to the body of the jaw (Fig. 179). In old age, when the teeth drop out and the alveolar margins are absorbed, the ascending ramus again becomes oblique, to allow the lower jaw to come in contact with the upper during mastication. The mental eminence is present at birth, and is a human characteristic. In apes the genioglossal muscles arise from a fossa, in place of a tubercle as in man, on the lingual aspect of the symphysis. In primitive races this simian fossa occasionally occurs.^[11]

As the teeth erupt, growth occurs both at the lower and alveolar borders, and also over the mental eminence or chin. These growth changes are well exemplified in the subjects of acromegaly (Fig. 179). In this disease growth of the jaw proceeds after adult years are reached. The deposition of new bone at the condylar process leads to the chin and teeth being pushed forwards in front of the upper jaw and teeth. The chin and lower border also increase in size.

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Fig. 179. Mandibles of a Child at Birth, of a Normal Adult and of a Man, the subject of the disease of growth known as Acromegaly, superimposed to show the manner in which growth takes place.

The Temporomandibular Articulation

Two types of this joint are found in mammals, one (see Fig. 180, A), exemplified in the carnivora, in which only a hinge action is permitted, and hence the jaws act like scissor blades ; the second (see Fig. 180, C), in which a gliding movement is allowed, the teeth being thus able to act as grinders. The second type occurs in all vegetable feeders. The human articulation combines the characters of both types (Fig. 180, B), the gliding action taking place between the interarticular cartilage and the skull, the hinge action between the cartilage and the condyle. In rodents the glenoid cavity is a narrow gutter in which the plate-like condyloid process glides backwards and forwards. The interarticular cartilage is developed in all the Mammalia except the monotremes, and one or two marsupials (Parsons).[12] At the end of the third month the cartilage appears as a condensation of fibrous tissue between the coronoid process and root of the zygoma. There is at this time no articular cavity ; the disc appears to arise from tissue caught between the condylar process and future glenoid cavity ( Vino grad off).

Development of the Tympanic Plate and Articular Eminence

If the chin be depressed the condyle of the jaw moves on to the articular eminence (Fig. 180, B) ; if over-depressed it springs over the eminence, and a dislocation is produced. This is impossible in the early years of life, for at birth there is no eminence and no glenoid cavity (see Fig. 181, A). At birth the membrana tympani lies exposed on the surface of the skull behind the condyle, supported in a fine osseous hoop, the tympanic ring. The ring is imperfect above, and there the flaccid part of the membrane occurs. By the second year the ring has grown into a plate by sending out two processes, which, as they grow out, unite and leave a gap between (Fig. 181, B). This, as a rule, is soon filled up. By the 20th year the tympanic plate is three-quarters of an inch long, forming the bony floor of the external meatus and the posterior wall of the glenoid fossa, which in man is remarkably deep. It protects the meatus from the condyle, and must be regarded as an accessory part of the mandibular joint. Every year until the 20th the bony meatus gets longer, while the fibro-cartilaginous part becomes relatively shorter. In the adult the bony part forms two-thirds of the meatus. As the tympanic plate grows outwards, the membrana becomes less easily accessible to the surgeon (Fig. 181, C). The plate also grows inwards to form the floor of the bony part of the Eustachian tube and downwards to form the vaginal process, to which the upper end of the carotid sheath is attached (Fig. 181, C).

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Fig. 180. The chief types of the Temporo-MaxUlary Articulation. A. Carnivorous Type. B. Omnivorous Type. G. Herbivorous Type.

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Fig. 181. Showing the chief changes after birth in the form of the Temporo-Maxillary Articulation.

A. At Birth. B. At Two Years. C. In the Adult.

Fate of the Stomodaeum

Having described the manner in which the three developmental masses — nasal, maxillary and mandibular — -are involved in the upbuilding of each side of the face, it may be profitable to look back and see what has become of the primitive oral cavity — ^the stomodaeum. A diagrammatic section of this cavity is given in Fig. 182 ; up to the 5th week it is separated from the primitive pharynx by the oral membrane ; the pituitary evagination — Rathke's pocket — is seen arising from the stomodaeum at the dorsal margin of the membrane. When the prechordal plate of cartilage is formed below the fore-brain, the -pituitary body thus becoming an intracranial organ, its stalk comes to be situated at the hinder or sphenoid end of the nasal septum or vomer. This vomerine point may be regarded as stationary during the development of the nasal and buccal cavities. In Fig. 183 the position is shown which the oral plate would assimie were it to persist until adult life. The lips and teeth are developed in front of it, and therefore within the cavity of the stomodaeum. The hard palate is developed in front of it but only part of the soft. The nasal cavities are not derived from the stomodaeum. It is true tliat the nasal processes grow within and fill up the primitive space as it expands, but the cavities within the nasal processes represent expansions of the primary olfactory pockets. The tongue and floor of the mouth arise in the pharynx, behind the oral plate.

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Fig. 182. Sagittal Section showing the Stomodaeum and position of the Oral Plate in the 4th week. (Schematic.)

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Fig. 183. Showing the fate of the Stomodaeum. The relative position of the Oral Plate is indicated.

In this chapter an account has been given of the various embryological elements which go to form the face. In the chapters dealing with the eye, nose, teeth and tongue further details will be described. The chief feature of the human face is its power of expression — due to the high differentiation of its subcutaneous musculature, and to the elaborate nervous mechanism controlling that musculature. The muscles of expression, we shall see, arise in connection with the hyoid arch ; their wide distribution on the face occurred with the evolution of the pulmonary respiratory system.

  1. See Professor J. E. Frazer, Lancet, 1916, vol. 2, p. 45 ; Berry and Legg, Harelip and Cleft Palate, 1918 ; Keith, " Malformations of the Head and Neck," Brit. Med. Journ. 1909, vol. 2, p. 310.
  2. K. Peter, Anat. Anz. 1911, vol. 39, p. 41 (Development of Face).
  3. 3.0 3.1 M. Inouye, Anat. Hefte, 1912, vol. 45, p. 471 (Premaxilla in Man); 1912, vol. 46, p. 1 (Dev. of Palate, Mammals) ; E. Gaupp, Anat. Hefte, 1910, vol. 42, p. 311 (Evol. of Palate) ; G. Schorr, Anat. Hefte, 1908, vol. 36, p. 69 (Dev. of Palate) ; E. Fawcett, Journ. Anat. and Physiol. 1906, vol. 40, p. 400 (Ossific. of Palate). See also references, p. 159,
  4. 1 For an account of the development of the lips see : L. Bolk, Anat. Hefte, 1911, vol. 44, p. 227 (describes curious pits seen in abnormally developed lower lips) ; M. Ramm, Anat. Hefte, 1905, vol. 29, p. 55 ; W. L. H. Duckworth, Journ. Anat. and Physiol. 1910, vol. 44, p. 349 (Lips of Primates).
  5. F. P. Mall, Contributions to Embryology, 1917, vol. 6, No. 15 ; R. J. Gladstone, Journ. Anat. 1920, vol. 54, p. 196 ; Davidson Black, Journ. G. Neur. 1913, vol. 23, p. 193.
  6. See J. Ernest Frazer, Journ. Anat. and Physiol. 1911, vol. 45, p. 190.
  7. E. Fawcett, Journ. Anat. and Physiol. 1911, vol. 45, p. 378 (Ossification of Maxilla). See also Mall, Amer. Jour. Anal. 1906, vol. 5, p. 449.
  8. See Keith, Proc. Anat. Soc. Great Brit, and Ir. May, 1902, Brit. Journ. Dent. Sc. 1902, vol. 45, p. 529 ; J. Parsons Schaeffer, Amer. Journ. Anat. 1912, vol. 13, p. 1 ; Ibid. Amer. Journ. Anat. 1912, vol. 13, p. 1 (Formation of Nasal Duct) ; Ibid. Amer. Journ. Anat. 1910, vol. 10, p. 313 (Formation of Antrum). See also references, p. 135.
  9. E. Gaupp, Anat. Anz. 1911, vol. 39, pp. 97, 433, 609 (Morphology and Mandible).
  10. I have followed the account given by Dr. Alex. Low, Jour?}, of Anat. and Physiol. 1910, vol. 44, p. 83. See also Professor Fawcett's account in Journ. Amer. Med. Assoc. 1905, vol. 45, p. 695. For abnormal ossification of Meckel's cartilage see Keith, Journ. Anat. and Physiol. 1910, vol. 44, p. 161.
  11. For the morphology of chin and symphysis see Professor Arthur Thomson, Journ. Anat. 1916, vol. 50, p. 43.
  12. " Joints of Mammals," Journ. of Anat. and Physiol. 1900, vol 34, p. 41.

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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures