Talk:Paper - Observations on the development of the human vertebral column
OBSERVATIONS ON THE DEVELOPMENT OF THE HUMAN VERTEBRAL COLUMN
By G. M. WYBURN, Department of Anatomy, University of Glasgow
The modern conceptions and current descriptions of the development of the human vertebral column are largely based on the observations of Bardeen (1905) and Bardeen & Lewis (1901) in the early part of this century. Their conclusions were in turn influenced by the notable researches of Remak (1855)—responsible for the resegmentation theory—and v. Ebner (1888), who recognized the significance of the intrasclerotomic fissure. Froriep (1886), Gaupp (1896), Weisse (1901) and Schauinsland (1906) are other names associated with valuable contributions
-to our knowledge of the embryology and morphology of this region.
The present communication is not an exhaustive study of the development of the human vertebral column, but has concentrated rather on those aspects of the subject which seem to require further elucidation. In particular, attention is focused on the thoracic vertebrae and the relation of the costal processes to the developing vertebral bodies. ‘The ribs are essentially intersegmental and the intervertebral discs are segmental’ (Goodrich, 1930). Some such statement is to be found in most textbooks of human anatomy and embryology, but as far as can be ascertained no explanation has been offered to account for the ultimate segmental position of the head of the rib—how the midthoracic ribs achieve an articulation with the preceding vertebrae—and why this additional attachment is denied to the first and last two or three thoracic costal processes.
It is sometimes felt that the interpretations of the earlier embryologists are over-much influenced by a too slavish allegiance to the comparative morphologist. The phylogeny of the vertebral column has therefore been deliberately neglected in this work, and, where of necessity it has obtruded itself, the relatively simple terminology devised by Gadow has been closely adhered to in an effort to evade the semantic confusion of controversial nomenclature which flourishes in vertebral mor phology. MATERIAL AND METHODS
Embryos of the Glasgow Collection were utilized and wax reconstructions were made of the upper, middle and lower thoracic regions of 12:5, 16-1, 23 and 42mm. embryos. Spalteholtz specimens were prepared of the thoracic vertebral column of a 66 mm. embryo, one of. 44 months, of 5 months
and of 6 months, and these were treated by Lundvall’s method whereby the cartilage stains blue with toluidin blue and the bone red with alizarin. °
Embryo 4-5 mm. Flanking the neural tube and dorso-lateral to the notochord are the somites, clearly differentiated into lateral myotome and more medially placed sclerotome. There are no well-marked lateral or dorsal mesodermal processes from the sclerotome, but in the mid-line around the notochord there is diffuse, sparse connective tissue continuous with the ventro-medial aspect of the sclerotome (PI. 1, fig. 1).
The sclerotome is divided by a cleft—the intrasclerotomic fissure—into cranial and caudal halves: the cranial and caudal sclerotomites. There is little difference in the tissue density of the two halves (Pl. 1, fig. 2). ,
Embryo 7mm. A frontal section just dorsal to the notochord (PI. 1, fig. 3) shows in the mid-line alternating clear and dark bands of mesodermal tissue. The position of the intersegmental arteries can be made out. The dark area consists of dense mesoderm in continuity with the now denser caudal sclerotomite. It forms the perichordal disc and arises as a fusion of ventral processes from the caudal sclerotomite of each side around the notochord (PI. 1, fig. 4). The perichordal discs are almost ‘entirely derived from the caudal sclerotomite, but receive a small tissue reinforcement from the cranial sclerotomite of the same somite (Pl. 1, fig. 3, R). Two formations are now evident arising from the caudal sclerotomite: (a) dorsal processes extending lateral to the neural tube—the future neural processes, and (b) lateral processes pushing ventrally—the costal processes (Pl. 1, fig. 4). In the mid-thoracic region the costal processes are more cranially situated relatively to the neural processes than in the cervical region. The tissue of the lighter areas between the perichordal discs is in continuity with and probably derived from the cranial sclerotomite. The limits of the various mesodermal processes are not sufficiently distinct to permit a reconstruction.
Embryo 8mm. This specimen had sustained a tear which prevented satisfactory reconstruction.
The perichordal discs surround the notochord. The myotomes are well marked and clearly define the limits of the somite. The intrasclerotomic fissure is still present, and a prominent feature of the Observations on the development of the human vertebral column
frontal section (PI. 1, fig. 5) is the contribution from the cranial sclerotomite to the perichordal disc. The now strongly growing neural and costal processes are evident in transverse section (Pl. 1, fig. 6). The costal processes of the mid-thoracic region, as in the 7 mm. embryo, are more cranially situated than those of the cervical region.
Embryo 12:5 mm. In this embryo the axial skeleton is represented by a continuous column of. embryonic mesoderm in the centre of which is the notochord. This mesodermal column is half-moonshaped in transverse section in the thoracic region and at intervals gives off neural and costal processes. It consists of alternating darkly staining areas of dense tissue—the perichordal discs—and lighter staining areas of more diffusely arranged mesenchyme.
Each perichordal disc is now divided into three zones, a central dense area C and lighter strips A
Text-fig. 1. A drawing of a wax-plate reconstruction of the upper thoracic region of a 12-5 mm. embryo. The dark area represents the perichordal disc.*
and B—cranial and caudal. Areas A and B are distinct from the clear ring D which separates: the perichordal discs (PI. 2, fig. 7).
In the mid-thoracic region the attachment of the costal processes to the perichordal discs is more cranial than in the upper and lower thoracic regions. (In an examination of serial transverse sections the Ist rib appears some eight sections before the corresponding neural process, the midthoracic costal processes are twelve sections in advance of the neural processes, while in the lower thoracic region the number is again eight.) There is an opening up of tissue in the light areas between the perichordal discs. This is a prechondral change (Pl. 2, fig. 9) and is the first indication of the formation of a cartilaginous vertebral body. A
- For simplicity the costal processes are shown in all the
text-figures in continuity with the vertebral column, although in the older embryos (16-1 mm. onwards) there are signs of synovial cavity formation.
similar change is apparent at the roots of the neural processes and the vertebral ends of the costal processes. Prechondrification advances most rapidly in the lower thoracic region and quickly involves the caudal portion of the perichordal discs and in consequence in this region brings the attachment of the neural processes and a small part of the costal processes on to the vertebral body (Text-fig. 3). Elsewhere the costal processes are still in fibrous continuity with the perichordal discs (Text-figs. 1, 2).
Embryo 14 mm. The three areas of the perichordal discs can still be distinguished. There is a broadening of the intervening light area at the expense of the tissue of the perichordal discs (Pl. 2, fig. 8).
Prechondral change has extended and now encroaches on both aspects (areas A and B) of the perichordal discs throughout the thoracic region. The chondrification of the caudal portion of the
Text-fig. 2. A drawing of a wax-plate reconstruction of the mid-thoracic region of a 12-5 mm. embryo. The dark area represents the perichordal disc. :
preceding perichordal discs now includes almost the entire area of attachment of the neural processes. The more caudal position of the upper and last two or three thoracic costal processes brings a small part of their vertebral attachment within the succeeding prechondral area. This is better marked in the lower thoracic region where the precartilaginous change advances more rapidly. In the mid-thoracic region the costal processes remain attached to the perichordal discs. Contemporary chondrification is advancing in the neural and costal processes.
Embryo 16-1 mm. The formation of the cartilaginous vertebral body has advanced considerably from that of the 14 mm. embryo. In a ‘frontal section of the lumbo-sacral region the perichordal disc is a narrow strip of dense mesoderm separating wide cartilaginous areas (PI. 2, fig. 10). This narrow strip is the area C of the perichordal discs of the 12-5 and 14 mm. embryos and there is only a faint indication of areas A and B which are now absorbed in the cartilage. Cartilage formation is less advanced in the cervical and upper thoracic regions where the discs are broader, but relatively narrower than in the corresponding regions of the 12:5 and 14 mm. embryos. The progressive invasion of the
Text-fig. 3. A drawing of a wax-plate reconstruction of the lower thoracic region of a 12-5 mm. embryo. The dark area represents the perichordal disc.
perichordal disc area by cartilage has brought the whole attachment of the neural processes on to the vertebral body. The first thoracic costal process is attached in part to the perichordal disc and caudally to the cranial portion of the succeeding cartilaginous body, i.e. its own vertebra (Text-fig. 4).
the perichordal disc.
The last two or three thoracic costal processes have a more’ extensive contact with their own vertebra because body formation is more rapid in this region (Text-fig. 6).
In the mid-thoracic region the attachment of the more cranially situated costal processes is confined to the perichordal discs (Text-fig. 5).
Embryo 23 mm. In this embryo the costal processes of the mid-thoracic region articulate with the remains of the perichordal disc and the body of the vertebra above and below the disc, i.e. they are approximating to the adult condition (Text-fig. 8). This is a result of the inclusion of cranial and caudal
Text-fig. 4. A drawing of a wax-plate reconstruction of the upper thoracic region of a 16-1 mm. embryo. The dark area represents the perichordal disc.
portions of the perichordal discs (Pl. 2, fig. 7, areas A and B) in the expanding vertebral bodies. A vertebral body is now made up of the light area’ D between the perichordal discs, plus area B of the cranial perichordal disc, plus area A of the caudal perichordal disc. The first and last two or three
Text-fig. 5. A drawing of a wax-plate reconstruction of the mid- Text-fig. 6. A drawing of a wax-plate reconstruction thoracic region of a 16-1 mm. embryo, The dark area represents
of the lower thoracic region of a 16-1 mm. embryo. The dark area represents the perichordal disc.
thoracic costal processes remain attached to the intervertebral disc and the upper end of their own vertebra (Text-figs. 7, 9). Their more caudal attachment to the perichordal discs in the first instance leaves them unaffected by the caudal spread of the body of the vertebra above.
Embryo 80 mm. This embryo was sectioned in Observations on the development of the human vertebral column
the sagittal plane. Pl. 2, fig. 11 shows the heads of the ribs lying on the intervertebral discs and the sides of the bodies of adjacent vertebrae. The section through the upper part of the column shows the proportionate size of the intervertebral disc and vertebral body.
Embryo 42 mm. The first rib has now an ex
Text-fig. 7. A drawing of a wax-plate reconstruction of the upper thoracic region of a 23 mm. embryo. The dark area represents the perichordal disc.
tensive articulation with the body of its own vertebra (Text-fig. 10). In the mid-thoracic region the vertebral bodies on each side of the intervertebral disc claim a much larger share of the rib articulation (Text-fig. 11), while the lower ribs tend to forsake even the intervertebral disc in favour of the upper end of their own vertebral body (Text-fig. 12).
Text-fig. 9. A drawing of a wax-plate reconstruction of the lower thoracic region of a 23 mm. embryo. The dark area represents the perichordal disc.
The cartilage is more mature than in the 23 mm. embryo. In the centre of the bodies of the vertebrae the cartilage spaces with enclosed cells are becoming
enlarged, the matrix is more deeply staining and the appearance suggests that a pre-osseous change is not far distant. Embryo 45 mm. The sections of this embryo were cut so obliquely both in the antero-posterior and horizontal planesthat satisfactory reconstruction was impossible. As a result of the plane of the sections, however, Pl. 2, fig. 12 shows the articulation of a mid-thoracic rib with the intervertebral disc and adjacent vertebrae. Ossification has commenced in the vertebral bodies, ribs, and neural processes.
Text-fig. 8. A drawing of a wax-plate reconstruction of the mid-thoracic region of a 23 mm. embryo. The dark area represents the perichordal disc.
Older embryos. Spalteholtz preparations of the vertebral columns of a 65 mm. embryo, a 44 months’, a 5 months’, and a 6 months’ foetus, were examined. With the differential. stain the red osseous portion contrasts sharply with the blue cartilaginous portion and the extension of ossification can be followed. Dixon (1920) states that the costal facets ‘do not actually belong to the body but are
Text-fig. 10. A drawing of a wax-plate reconstruction of the upper thoracic region of a 42 mm. embryo. The dark area represents the perichordal disc.
extensions of the epiphysial plates’. The epiphysial plates are not present until after puberty, but the costal facets are quite a prominent feature of the vertebral column of the child and are carried by that part of the cartilaginous body which has been ossified from the neural arch. In the post-pubertal vertebral column the continuity of costal facet and 98
epiphysial plate is due to the continuity of the covering hyaline cartilage. The facets are not ‘extensions of the epiphysial plates’ but are situated on bone ossified from the neural arch
Text-fig. 1. A drawing of a wax-plate reconstruction of the mid-thoracic region of a 42 mm. embryo. The dark area represents the perichordal disc.
Text-fig. 12. A drawing of a wax-plate reconstruction of the lower thoracic region of a 42 mm. embryo. The dark area represents the perichordal disc.
Sclerotomes. With the conversion of Gadow (1988) to the theory of the resegmentation of the vertebrae, all opposition to the ‘Neugliederung der Wirbelsaule’, as originally described by Rémak (1855), might be said to have ceased. Although discarded by Froriep in 1886 the views of Remak were supported and confirmed by v. Ebner (1888), Schauinsland (1906), Bardeen (1905), and Piiper (1928); a full discussion of this subject with relevant literature can be found in a recent work by Dawes (1930).
The intrasclerotomic fissure is present in the 4-5 mm. embryo and divides the sclerotome into cranial and caudal halves—the sclerotomites.* In
- The term ‘scleromere’ is avoided as too elastic. Bardeen
defines a scleromere as the condensed sclerotogenous tissue of the caudal sclerotome half. Gadow (1933) in his Fig. 2 depicts a scleromere as the union of the two sclerotomites this embryo there is little difference in the tissue density of the two sclerotomites although in a slightly older specimen (5mm.), described by Bardeen, there is a marked increased density of causal sclerotomite, and also in the 7 mm. embryo of the present series (Pl. 1, fig. 3).
Dawes (1930), in his description of the development of the vertebral column of the white mouse, divides each sclerotomite into a dorsal and ventral half on the basis of cell orientation—no such differentiation could be observed in the human embryos, where, as Sensenig (1943) stated of the sclerotome of the deer mouse, ‘only an arbitrary division can be made by regarding that area of the sclerotome above the notochord as dorsal and that area below the notochord as ventral’. This is also in agreement with the findings of von Bochmann (1937) in Mus according to whom the sclerotome is a continuous homogeneous structure from dorsal to ventral tip and is not differentiated into dorsal and ventral components as has been reported in the lower vertebrates.
Perichordal discs and membranous vertebral column. The perichordal discs are first apparent in the 7 mm. embryo. In their formation as a ventral process of the caudal sclerotomite with an accession of tissue from the cranial sclerotomite of the same somite, i.e. from the tissue around the intrasclerotomic fissure, they resemble the perichordal discs of the developing mouse vertebral column (Dawes, 1930), the horizontal plate of Weisse (1901), and the primitive discs of Bardeen (1905). Bardeen describes the primitive discs as ventral processes of the caudal sclerotomite strengthened by a condensation of tissue immediately surrounding the intrasclerotomic fissure. What he describes as ‘interdiscal membrane’ and labels as such in his Fig. 3 (a frontal section of a 6 mm. embryo) appears to be joining and reinforcing the disc tissue rather than forming a definite structure, and corresponds to R of our figs. 3 and 5 (Pl. 1). Bardeen’s interdorsal membrane is a somewhat tenuous distinction. Certainly there is some loose connective tissue bridging the interval between consecutive neural processes, but it does not seem justifiable to regard this as a separate entity. In the earlier embryos tissue boundaries are loose and ill-defined and do not lend themselves to faithful reconstruction. Bardeen admits that his reconstructions of the developing vertebral column of the younger human specimens are largely diagrammatic.
The division of the perichordal discs into three zones—a central dark area with lighter strips on each side (PI. 2, fig. '7, 4, B, C)—was first observed
to form the final vertebra, while Dawes interprets Remak’s ‘Urwirbelkern’ as a scleromere ‘out of which are developed the cranial part of a vertebra with its arches plus the caudal end of the next anterior vertebra’—this would correspond to a sclerotome. Observations on the development of the human vertebral column
in the 12-5mm. embryo. In the 23 mm. embryo the perichordal disc is reduced to the central dark area C—now the intervertebral disc—while areas A and B are incorporated in the cartilage of adjacent vertebral bodies, area A into the caudal portion of the vertebra on the cranial side of the disc, area B into the cranial portion of the vertebra on the caudal side.
Dawes describes three zones in the perichordal disc of the mouse embryo and states that the definitive mouse centrum ‘equals the posterior zone of the disc, plus the earlier centrum rudiment, plus the anterior zone of the disc next posterior. The central area is the intervertebral ligament.’ His observations are corroborated in a more recent study of the developing vertebral column of the deer mouse by Sensenig (1943). Piiper’s (1928) conception of the development of the avian vertebral column, adversely criticized by Gadow (1933), has been restated by Williams (1942) in an investigation of the chick vertebral column. Both authors describe an ‘interstitial body’ formed from a dense collection of cells around the intrasclerotomic fissure, which alternates with a lighter area— the ‘vertebral ring’. The interstitial body divides into three zones—the anterior is called opisthospondylous zone because it joins with the posterior portion of the adjacent vertebral ring. The posterior or prospondylous zone coalesces with the anterior portion of the other adjacent vertebral ring which by this fore and aft addition becomes the final vertebral body. There is here a very close resemblance to the method of formation of the mammalian perichordal disc and the ultimate construction of the vertebral body.
According to Bardeen, the growing vertebral body incorporates cranially the posterior part of the primitive disc which meanwhile levies a contribution from the caudal end of the vertebral body on its cranial side, with which it forms the intervertebral disc.
The account which is given here of the perichordal disc is in agreement with the observations of Dawes (1930) and Sensenig (1943) in the mouse and finds an interesting parallel in the interstitial] body described by Piiper (1928) and later Williams (1942). f
While in their formation and position the perichordal disc and primitive disc would appear to be the same structure, the further development and final fate of the primitive disc, as described by Bardeen, conflict with the present findings.
In the 12-5 mm. embryo the vertebral column consists of axial mesoderm with alternating light and dark areas. This membranous anlage receives its tissue from the segmental sclerotomes and contains the primordia of vertebral body and intervertebral disc, but it is misleading to suggest, as in Gray’s Anatomy (1942), a mesenchymal vertebral column consisting of 35 or more segmental units. Not until the completion of the cartilaginous stage of development, about the end of the second month, is it possible to detail vertebral units, although from an early stage the neural and costal processes adumbrate this division.
Cartilaginous vertebral column. The first phase of the cartilaginous stage of vertebral formation is the prechondral change in the light area D of the 12-5 mm. embryo. Thereafter the cartilage extends until it involves the whole of area D, spreads into area B of the preceding perichordal disc and area A of the succeeding disc. Thus, in its final form the cartilaginous vertebral body occupies the territory of the cranial sclerotomite and has overflowed into the caudal sclerotomite above and below. The clear-cut text-book description of the union of a caudal sclerotomite with the succeeding cranial sclerotomite to form a vertebra is a simplified version of this rather more complicated process.
Cartilage formation commences and progresses more rapidly in the lower thoracic and lumbar regions.
The perichordal discs are never entirely obliterated, but the narrow central area C persists as the intervertebral disc. This seems to attain its minimum size (relatively) somewhere between the 20-40 mm. stages, but there is no evidence of the existence at any time of cartilaginous continuity throughout the vertebral column such as is described by Schultze (1896).
It has seemed preferable to speak of the cartilaginous vertebral body rather than centrum. Gadow defines the centrum, simply, as the axially placed mass of a vertebra which carries the neural arch. The osseous centrum therefore lies between the neuro-central sutures and excludes the costal facets. At the cartilaginous stage the central mass, often referred to as centrum, includes the area of costal attachment and is co-extensive with the osseous vertebral body rather than the bony centrum. Gegenbaur (1898) expresses the same opinion rather differently. He states: ‘The process of ossification extends from the arch over a not inconsiderable portion. of the vertebral body so that this in its osseous condition may be considered as being formed by a portion of the arch.’ In this series of embryos there was no neuro-central synchondrosis in the position of the future neurocentral suture such as is depicted in Gray’s Anatomy (Fig. 100 B), where a cartilaginous, centrum is shown which includes only that portion of the axially placed mass ossified from the body centre. The nascent cartilaginous vertebral bodies of the mid-thoracic region extend cranially and caudally into the tissue of the perichordal discs and come to include first the attachment of the neural arch, e.g. 16 mm. embryo, and only at a later stage, e.g. 28 mm. embryo, are the more cranially situated costal processes brought on to the cartilaginous bodies.
Neural and costal processes. In the 4-5, 7 and 8 mm. embryos the neural and costal processes are attached to the perichordal disc. Throughout the thoracic region the costal processes are cranial to the neural processes. The attachments of the first and last two or three thoracic costal processes are however more caudally situated than in the midthoracic region. With the invasion of the caudal portion of the perichordal disc the cartilaginous body acquires its neural arch, e.g. 14 and 16 mm. embryos. Further penetration will bring the caudal part of the attachment of its own costal process within the body. This will take place at an earlier stage, e.g. 14 mm. embryo, and ultimately to a greater extent, in the case of first and last two or three thoracic costal processes. The contemporary chondrification of the perichordal disc on its cranial aspect allows the mid-thoracic ribs to articulate with the lower end of the preceding vertebral body, but the articulation of the first and last two or three ribs, because of their more caudal position, is restricted to their own vertebral body. The shaft of the rib pushes into the myoseptum and might therefore be described as intersegmental. The head of the rib, however, is never intersegmental, as the costal processes are outgrowths of the caudal sclerotomites from which they come off near the intrasclerotomic fissure. °
Bardeen is not concerned with the relation of the head of the rib to the vertebral bodies. The costal processes in his earlier embryos are attached to the primitive disc. In his 14 mm. embryo they are described and shown in relation to the intervertebral discs which, according to the author, are only in part formed from the primitive discs.
Morphology. Gadow suggests a fundamental scheme for the composition of the ideally complete vertebra from which any known vertebral modification can be derived. This in the main consists of a division of the caudal sclerotomite into dorsal and ventral portions—the basidorsals and basiventrals respectively. A similar division of the cranial sclerotomite yields interdorsal and interventral— the so-called interarcualia. The implication that biological processes can evolve only as a variant of some fundamental scheme is a debatable premiss, and the results of a too close analysis of the . mammalian vertebral unit is evident in the diverse values accredited to its various parts by different morphologists. Goodrich (1930) and Cope (1886) are of the opinion that the body of the mammalian vertebra represents the interdorsal element. Gadow emphasizes the interventral derivation of the vertebral body and the complete absence of the interdorsals. According to Dawes (1930) and Sensenig (1943) basiventral, basidorsal, interventral and interdorsal all make their contribution to the body of the mouse vertebra. Bardeen makes no specific evaluation of the human vertebra, but from his description of its development the body would appear to incorporate elements from both sclerotomites.
The neural process of the human vertebra might be identified as basidorsal, but it is impossible to exclude tissue contribution from the cranial sclerotomite as is described by Dawes for the neural process in the mouse. Gadow classifies the mammalian vertebra as gastrocentrous in the sense that the neural arch equals basidorsal and the body equals interventral. The formation of the vertebral body from both sclerotomites makes this classification unacceptable. The observations made in this series of human embryos offer little encouragement to pursue the effort to reduce all vertebra to a common formula, but rather serve to emphasize the comment of Goodrich: ‘Recently attempts have been made to compare in detail different types of vertebral column and refer them to a single scheme of homologous parts. It is doubtful whether such a proceeding is altogether justifiable since at least some of the types may have been independently evolved.’
Perhaps a more fruitful line of investigation into the growth pattern of the vertebral column is suggested by the interesting transplantation experiments of Williams in the chick embryo. On the evidence of the results of his experiments Williams agrees with Feller & Sternberg (1934) that there are two developmentally different parts in the vertebral column: (1) arches whose normal morphogenesis depends on the central nervous system, and (2) centra controlled in thetearlier stages by the inductive capacity of the notochord.
The mammalian rib has a variable attachment to the vertebral column, not only in different species but in the different regions of the same species. The head of the rib may articulate with joint facets on the bodies of neighbouring vertebrae, as in the mid-thoracic region in many mammals, or forsake this position and transfer its head caudally upon the body of its own vertebra. The articulation may shift dorsally when the rib is carried entirely by the diapophysis as in the lower thoracic region in the sperm whale.
The morphologist regards the rib as a process of
- the basiventral to which it remains attached. The
final position of the head of the rib would then be determined by the fate of the basiventral, but, as there is little agreement on the role of the basiventral in mammals, there is no straightforward morphological explanation of adult costo-vertebral relations.
The embryological explanation advanced in this paper is based on two facts. First, the division of the perichordal discs into three areas and the inclusion of the caudal and cranial areas in the Observations on the development of the human vertebral column
vertebral bodies, and second, the more cranial position of the attachment of the mid-thoracic costal processes with its consequent inclusion in the chondrified cranial portion of the perichordal disc absorbed into the preceding vertebral body.
This account would be incomplete without some reference to the functional significance of the more cranial position of the heads of the middle ribs which results in their contact with the preceding vertebral body. The mechanical advantage of this arrangement would seem to be due to the increased vertical inclination of the ribs. There is in consequence a greater forward movement of the sternum with the elevation of the ribs in inspiration, and as the axis of rotation is through the heads and tubercle of the rib, i.e. lies obliquely in the frontal plane, a contemporary greater outward displacement of the lateral part of the ribs.
Dr Orr, of the Engineering Department of Glasgow University, was consulted and he’ suggested and carried out the calculations necessary to give quantitative expression to any decrease in the vertical angle of the rib. It was calculated that if the head of the fifth rib were 4 in. lower, which would exclude the articulation with the fourth thoracic vertebra, the vertical angle of the rib would be decreased by some 20% with a proportionate limitation of the forward movement of the sternum and the outward displacement of the rib.
The net result is that the 4 in. or so which the head of the fifth rib gains by its more cranial position imparts of the order of a 20% increase to the ability of that rib to increase the capacity of the thorax.
1. The membranous vertebral column consists in its earliest phase of an axial column of mesoderm with alternating dark and light areas—the dark areas are the perichordal discs.
2. The perichordal discs are formed from ventral processes of the caudal sclerotomites with a tissue contribution from the cranial sclerotomite of the same somite.
8. Neural and costal processes of mesoderm are in fibrous continuity with the perichordal discs.
4. Perichordal discs divide into three areas.
5. Cartilage formation begins in the light area between the perichordal discs, i.e. area corresponding to cranial sclerotomite, and finally involves the caudal area of the preceding perichordal disc and the cranial area of the succeeding disc.
6. The attachment of the thoracic costal processes to the perichordal discs is cranial to the attachment of the neural process. This is most marked in the mid-thoracic region and less so in the first and last two or three thoracic segments.
7%. The inclusion of perichordal disc tissue in adjacent vertebral bodies brings the mid-thoracic costal processes into contact with the upper end of their own vertebra and the lower end of the © vertebra above. The first and last two or three ribs, with their more caudal attachment to the peri- chordal disc are confined in their articulation-to their own vertebra.
8. The more cranial position of the heads of the mid-thoracic ribs which brings them into relation with the bodies of the preceding vertebrae adds considerably to theig ability to increase the capacity of the thorax.
My thanks are due to Prof. Blair for his helpful criticism and suggestions, and to Dr Bacsich for his assistance and advice, particularly in the preparation of the Spalteholtz specimens. The photographs and microphotographs were made by Mr George Marshall of this Department. The drawings of the wax-plate reconstructions are the work of Dr W. J. McCallien and Mr A. Rettie of this University.
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EXPLANATION OF PLATES
Fig. 1. Transverse section of 4-5 mm. embryo. S =sclerotome.
Fig. 2. Frontal section of 4-5 mm. embryo. x98. I = intrasclerotomic fissure.
Fig. 3. Frontal section of 7mm. embryo. x80. P.D.=perichordal disc; R =tissue reinforcement to perichordal disc.
Fig. 4. Transverse section of 7 mm. embryo. x80. N.P.=neural process; C.P.=costal process; P.D.=perichordal disc.
Fig. 5. Frontal section of 8 mm. embryo. R as in Fig. 3.
Fig. 6. Transverse section of 8 mm. embryo. N.P. and C.P. as in fig. 4.
x66. P.D. and
PLatE 2 Fig. 7. Frontal section of 12-5 mm. embryo. x57. A, B, C=the three divisions of the perichordal disc; D=light area between the perichordal discs. Fig. 8. Frontal section of 14 mm. embryo. x57. A, B,C and D as in Fig. 7.
Fig. 9. Transverse section of 12:5 mm. embryo. x 57,
Fig. 10. Frontal section of 16-1 mm. embryo. x43. C as in fig. 7.
Fig. 11. Sagittal section of 30mm.embryo. x8. H=head of rib; M =intervertebral disc. Fig. 12.. Transverse section of 45 mm. embryo. x8.
Journal of Anatomy, Vol. 78, Part 3
Plate 1 Plate 2 Journal of Anatomy, Vol. 78, Part 3