76 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



caudally, the last vertebrae consisting of centra only. Very small haemal 

 arches (chevron bones) are present in the tail vertebrae of the cat, but are missing 

 on prepared skeletons. Man has three to five caudal vertebrae fused into a 

 single piece, the urostyle or coccyx. 



7. General remarks on the whole column.— The ends of the centra of most 

 mammalian vertebrae, as can be seen by inspection of isolated vertebrae, are 

 more or less flat. Such centra are called amphiplatyan. Between the ends of 

 the vertebrae are found in life cartilaginous disks, the intervertebral cartilages. 

 On the mounted skeleton observe an opening on each side between successive 

 centra. Through these openings, the intervertebral foramina, the spinal nerves 

 pass out from the spinal cord. The spinal cord occupies the continuous canal 

 formed by the neural arches. Haemal arches (basiventrals) are absent except 

 in the atlas and axis and in the tails of some mammals, where they form chevron 

 bones. The vertebrae of mammals are gastrocentrous. 



J. SUMMARY OF THE VERTEBRAL COLUMN AND RIBS 



i. The vertebrae arise at the intersection of the myosepta with the mesenchyme 

 surrounding the notochord and neural tube. Each vertebra is produced by the union of the 

 posterior halves of the two sclerotomes of one segment with the anterior halves of the two 

 sclerotomes of the succeeding segment. Owing to this manner of origin the vertebrae alter- 

 nate with the myotomes. 



2. The vertebrae and ribs are first formed in cartilage, produced by the activity of the 

 mesenchyme. In the elasmobranch fishes they remain permanently in the cartilage stage. 

 In most other vertebrates they ossify during development. They belong, consequently, to 

 the category of cartilage bones. 



3. Each vertebra begins as four pairs of cartilages or arcualia surrounding the notochord: 

 dorsally an anterior pair of basidorsals and a posterior pair of interdorsals ; ventrally an 

 anterior pair of basiventrals and a posterior pair of interventrals. 



4. In primitive vertebrae (occurring in some fish, extinct Amphibia and reptiles) these 

 pieces remain more or less separate in the adult. In the majority of vertebrates, however, 

 some of them are lost and the remainder fuse into a single structure. 



5. The parts of a single vertebra thus formed by the fusion of originally separate pieces 

 are: the centrum or main body of the vertebra, which incloses the notochord; a dorsally 

 directed arch, the neural arch, inclosing the spinal cord; and a ventrally directed arch, the 

 haemal arch, inclosing blood vessels. The neural arch consists of the fused basidorsals, the 

 haemal arch, of the fused basiventrals; the centrum is of different origin in different groups. 



6. The centrum in the elasmobranch fishes is formed within the sheath of the notochord; 

 it is called a chordal centrum. The centra in nearly all other vertebrates are produced by 

 the fusion of certain of the arcualia and are known as perichordal or arch centra. In different 

 groups of vertebrates different arcualia contribute to the centra. 



7. The ends of the centra are variously shaped: amphicoelous, or concave at each end 

 (fishes, urodeles); procoelous, concave in front, convex behind (Amphibia, reptiles); opistho- 

 coelous, convex in front, concave behind (Amphibia, reptiles); heterocoelous, saddle shaped 

 (birds); and amphiplatyan, flat at each end (mammals). 



8. In addition to centrum and arches, vertebrae commonly bear projecting processes 

 or pophyses. The most common of these are the transverse processes and the zygapophyses 



