66 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



coelom incloses the viscera, some of which will be observed to be suspended by a 

 delicate membrane, the dorsal mesentery, from the median dorsal line of the coe- 

 lomic wall. Study the vertebra, which appears in the middle of the myotomes. 

 It consists of centrum and neural arch, similar in appearance to those of tail 

 vertebrae; but the haemal arch appears to be absent. It is represented by a 

 pair of small cartilages at the sides of the ventral part of the centrum. These 

 cartilages, known as transverse processes, represent the stumps of the haemal arch, 



s 



A 



B 



FIG. 22. Diagrams to show the composition of the vertebrae in the different vertebrate classes, 

 according to Gadow. A, elasmobranch , vertebra consisting of chordal centrum a, basidorsals b, inter- 

 dorsals c, and basiventrals d. B, teleost, vertebra composed of all of the arcualia fused together. 

 C, pseudocentrous vertebra of a urodele; i, early stage showing the arrangement of the arcualia; 

 2, adult stage, showing fusion of the arcualia along the dotted lines; note splitting of the interdorsals 

 and interventrals to form the ends of the vertebra. D, notocentrous vertebra of anurans; /, early 

 stage, showing the arcualia which form the vertebra; 2, adult stage, arcualia fused along the dotted 

 lines; note end of the procoelous centrum formed by the interdorsal. E, gastrocentrous vertebra of 

 reptiles, birds, and mammals, centrum formed of the interventral e, the basiventral d functioning as 

 the intervertebral cartilage. F, composition of the atlas and axis of the alh'gator, according to Gadow; 

 the centrum of the atlas z functions as the odontoid process of the axis, a, chordal centrum; b, basi- 

 dorsal or neural arch; c, interdorsal (intercalary arch in A); d, basiventral or haemal arch; e, inter- 

 ventral (centrum in E and F); x, atlas; y, axis; z, odontoid process (interventral of atlas). (F after 

 Gadow in the Cambridge Natural History, courtesy of the Macmillan Company.) 



which may be regarded as having opened out and shifted to a more lateral posi- 

 tion, the arch portion disappearing. Examine the horizontal skeletogenous sep- 

 tum carefully and find within it, by picking away the muscles if necessary, a 

 slender cartilage on each side, which articulates with the transverse process. 

 These cartilages are ribs, and since they are located in the horizontal skeletoge- 

 nous septum they are intermuscular ribs. There is, of course, a pair of such ribs 

 to each vertebra. Draw the section. 



