Book - Vertebrate Zoology (1928) 21

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXI The Vertebral Column, Ribs, and Sternum

The primitive skeletal stiffening of the body is the notochord. In Amphioxus this extends to the extreme anterior end of the body ; in Petromyzon it does not reach further forwards than the region of the infundibulum, but in this position it persists throughout life. In the remaining vertebrates, the notochord usually disappears in the skull region.

Surrounding the notochord are two sheaths, the so-called elastica interna and the elastica externa. These are of import- ance in some forms in connexion with the formation of the vertebral column.

Amphioxus has no structures comparable to vertebrae, but they appear first in Petromyzon in the form of little paired pegs or struts on each side of the nerve-cord, rising up from the notochord. There are two pairs of these pegs to each segment as a rule. The notochord in Petromyzon is continuous and unconstricted, a primitive feature.

A properly formed vertebral column appears first in the Selachii. Each vertebra is composed of a neural arch formed from a pair of basidorsals, and a pair of basiventrals which in the region of the tail form a haemal arch. Between them, the basidorsals and basiventrals form the body of the vertebra or centrum, which constricts the notochord, and usually obliterates it altogether except between one centrum and the next. Alternating with the basidorsals are the interdorsals, and in some, interventrals are present. The basidorsals and basi- ventrals perforate the elastica externa and cartilage-cells invade the notochordal sheath. Such centra are called chordal, and they occur in the Selachii, in the sturgeons and in the Dipnoi. In all other forms the vertebrae arise outside the notochord and do not invade its sheath. These are called perichordal vertebrae.

The vertebrae of the higher bony fish are compact bony structures obliterating the notochord. Amia is interesting with regard to its vertebral column, for in the region of the tail there are what look like vertebrae with neural and haemal arches alternating with vertebrae without. Those vertebrae with the neural and haemal arches are the basidorsals and basi-ventrals ; those without are the interdorsals and interventrals. The neural arches are always formed from basidorsals and the haemal arches from basiventrals.


Fig. 152. — Transverse sections through the developing vertebral column of Scy Ilium embryos.

A, early stage ; B, late stage (in the region of the tail), b d, basidorsal bv, basiventral ; ca, caudal artery ; cv y caudal vein ; ee, elastica externa ; ha, haemal arch ; hs, haemal spine ; n, notochord ; na, neural arch ; nc, nerve-cord ; ns, neural spine.

The vertebrae are formed from the sclerotome, which is segmented. The anterior part of the sclerotome in each seg- ment gives rise to the interdorsals and interventrals, while the posterior part produces the basidorsals and basiventrals. Later it is found that the basidorsals and basiventrals of one segment fuse on to the interdorsals and interventrals of the next posterior segment. The vertebrae are therefore inter- segmental in position, which enables the myotomes, which of course are intrasegmental, to be attached to two vertebrae.

The most posterior haemal arches are enlarged to form the hypurals which support the ventral lobe of the tail-fin.

In the most primitive amphibia, the Embolomeri of the Labyrinthodonts, the vertebrae have neural arches and two centra. The anterior centrum of each vertebra is formed from the basiventrals and is called the hypocentrum ; the posterior centrum is formed from the interdorsals and interventrals, and is called the pleurocentrum. While in the later amphibia the hypocentrum has been enlarged and the pleurocentrum reduced, in the reptiles, birds, and mammals the opposite has occurred, and the vertebras of these animals have centra which correspond to pleurocentra. In consequence of this,


Fig. 153. — Origin of the vertebral column, A, in Scyllium ; B, diagram showing the relations of the vertebral elements to the nerves ; C, the vertebral column in the tail-region of Amia ; D, the vertebral column in the Embolomerous Stegocephalia. dr, dorsal nerve-root ; he, hypocentrum ; id, interdorsal ; iv, inter- ventral ; m, myotome ; pc, pleurocentrum ; vr, ventral nerve-root. Other letters as Fig. 152.


Fig. 154. — Vertebral columns of A, a bony fish (tunny) ; B, crocodile (in the tail-region) ; C, Sphenodon (trunk region).

c, centrum ; cb, chevron-bone ; ha, hasmal arch ; he, hypocentrum ; na y neural arch ; pc, pleurocentrum ; pz, prezygapophysis ; r, rib ; tp y transverse process.

the haemal arches in amphibia are always attached to the centra themselves ; whereas in the amniota, when they occur (mostly in the tail-region), the haemal arches are attached to separate little elements called intercentra which represent the hypo- centra. These intercentra and haemal arches in the amniotes are called " chevron-bones " ; they are never found in the amphibia. Primitive reptiles like Seymouria and Sphenodon have a complete set of intercentra all the way along the vertebral column, and some primitive mammals (hedgehog ; mole) have intercentra in the lumbar region.

The transverse processes are lateral extensions of the vertebrae from the base of the neural arches. The dorsal (or tubercular) head of the ribs is attached to the transverse process, and in all land-vertebrates except the most primitive Stego- cephalian amphibia (such as Eogyrinus),the transverse processes of at least one vertebra are attached to the ilia of the pelvic girdle forming the sacrum. The sacrum is of course not formed in animals which do not possess hind limbs.

The first vertebra in amphibia is modified to carry the head, and the vertebral column (which in fish is divisible only into trunk- and tail-regions) is now divisible into regions corre- sponding to neck, thorax, sacrum, and tail. In the amniotes the first vertebra (the atlas) becomes detached from its centrum, which becomes attached to the second vertebra or axis, and forms its odontoid peg. There are therefore two vertebrae specially modified in connexion with the neck, and a variable number of normal cervical vertebrae which differ from the thoracic in that their ribs are short and do not reach the sternum. The vertebrae between the thoracic (whose ribs reach the sternum) and the sacral are the lumbar. In primitive forms the sacrum affects only one vertebra, to the ribs or transverse processes of which the ilia are attached. In higher forms, and especially in birds, there are several sacral vertebrae.


Fig. 155. — View of the anterior region of the vertebral column of the croco- dile seen from the left side.

C2, centrum of the 2nd or axis vertebra ; hi, hypocentrum of the 1st or atlas vertebra ; mi, neural arch of the 1st or atlas vertebra ; M2, neural arch of the 2nd or axis vertebra ; op, odontoid peg, or pleurocentrum of the atlas vertebra which has become attached to the axis vertebra ; pa, proatlas ; n, 2, 3, ribs of the 1st to 3rd vertebrae ; V3, 4, 3rd and 4th vertebrae.

The vertebrae of land-animals bear facets by means of which they articulate with one another, and so enable the vertebral column to bend with considerable flexibility without diminishing its strength. These facets are the pre- and postzygapophyses. In some groups such as the lizard and snakes, additional facets may be developed. The faces of the centra are either flat or slightly concave or convex, but in the birds a special saddle-like shape has been developed, which allows of very great flexibility.

In the mammals, the number of cervical vertebrae is seven in all species with only three exceptions. These are the Edentates Bradypus which has nine, and a species of Choloepus which has six or seven, and the Sirenian Manatus which has six.

Ribs are extensions of the basiventrals, and they may be of two kinds. Those which pass just on the outside of the splanchnocoelic cavity are pleural or ventral ribs, and they occur in Dipnoi. " True " or dorsal ribs pass in the horizontal septum which separates the myotomes into dorsal and ventral portions, and they occur in Selachii and in the Tetrapods. Both kinds of ribs are present in Polypterus and some Teleosts.

In the land- vertebrates, the ribs primitively articulate with the vertebrae by a broad head which touches the centrum and the neural arch. These holocephalous ribs as they are termed are present in Labyrinthodonts, Cotylosauria, and Sphenodon.

Later, that portion of the head which touches the centrum (capitulum) became distinct from that which abuts against the transverse process (tuberculum), by the reduction of the intervening part of the head. In this way, the typical double- headed or dichocephalous ribs arose. Between the two heads of the rib and the vertebra there is a canal through which the vertebral artery passes ; and this vertebrarterial canal is conspicuous in the cervical vertebrae on to which the cervical ribs are usually fused.

In many forms, the articular heads of the ribs are degenerate and either the capitulum or the tuberculum may be lost. This secondary single-headed condition must be distinguished from the primitive holocephalous type.

In the Chelonia, the ribs are expanded into broad flat plates which touch one another and are fused to the dermal bones (osteoscutes) to form the carapace. In Sphenodon, crocodiles and in birds the ribs bear uncinate processes, which extend backwards and overlap the next posterior rib. In many cases, the ribs are in two portions : a dorsal or vertebral, and a ventral or sternal portion. The hindmost ribs do not usually reach the sternum, and they are known as floating ribs. Primitively, all the vertebrae as far back as the middle of the tail bore ribs. In higher forms they do not extend behind the sacrum.

The sternum first appears in Amphibia. It arises from paired rudiments of cartilage which may become replaced by cartilage-bone. In the Amphibia which are alive to-day, the sternum has no connexion with the ribs. The sternum in the Amniotes is however connected with the ribs, and this was probably the condition in the Stegocephalia also. The sternum is also usually in contact with the coracoids and clavicles. In the mammals, the sternum is often broken into a number of pieces or sternebrae. In the birds (with the exception of the Struthiones : ostrich and its allies) the sternum bears a median projection forming the " keel " or carina to which the flight muscles are attached. Analogous but not homologous keels are developed on the sterna of Pterosaurs and bats.

Overlying the sternum on the ventral side there is in many forms a dermal bone, the interclavicle. It is present in the Stegocephalia but has been lost in the living amphibia. Among the reptiles, it is present in all except the snakes. In birds it is apparently absent, unless it is represented by the keel of the sternum. Only the Monotremes preserve the interclavicle among the mammals.

Fig. 156. Diagram showing the relations of dorsal (" true ") and ventral ribs, as seen in transverse section. c, centrum of vertebra ; co, coelom ; dr, dorsal rib ; e, epaxonic muscles ; g, gut ; h, hypaxonic muscles ; ns, neural spine ; vr, ventral rib.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology

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