THE ENDOSKELETON 68$ 



also called zygapophyses, both on the anterior and posterior sides of each 

 vertebra, and usually transverse processes extending laterally in the 

 planes of the original divisions between the muscles. 



Where true ribs occur, there are two additional transverse processes 

 to which these attach. 



The centrum where it meets \vith the intervertebral disc, has four 

 distinct forms (Fig. 404). 



If the face of the centra at each end where it is to meet with the 

 intervertebral disc of the centra lying immediately anterior and imme- 

 diately posterior to it (as in fishes), is hollow at both ends, it is called 

 amphicoelous (Fig. 404). If one end is like a ball, namely, convex, and 

 the other concave, so that the ball-like portion can fit into it, the condition 

 is known as precocious if the socket lies on its anterior surface, and 

 opisthocoelous if on the posterior surface, while if the ends of the centra 

 are flat, as they usually are in mammals, such a condition is known as 

 amphiplatyan. 



The arches of the vertebrae form first (Fig. 352), and the centra 

 later, and the sclerotome divides into a caudal and cranial half which 

 thus makes possible an advantageous condition to the animal in permit- 

 ting interaction of skeleton and muscles (Fig. 305). 



It must be remembered, however, in this connection, that in some 

 animals, normally, and in others abnormally, the two halves of the 

 sclerotome may unite (as in some fishes), and thus not have this inter- 

 play of muscles ; or two neural arches may form by the rudiment which 

 normally becomes one arch, dividing as does the sclerotome, and thus 

 produce a greater quantity of vertebrae than usual. And, not only may 

 this happen to the neural arches, but also to the centra. In fact, almost 

 any variation in the spinal column may be accounted for by an embryo- 

 logical condition remaining in the adult form. 



As the ventral nerve root usually penetrates the caudal division of 

 the halved sclerotome, and the dorsal root passes between the two divi- 

 sions of each sclerotome but penetrates the cranial portion, one can tell 

 in the adult, from following these nerve roots, which of the adult struc- 

 tures come from cranial and which from caudal halves. 



atlas of an adult Trionyx hurum. The second basi ventral ( intercentrum ) is 

 attached to the posterior end of the first centrum which, not being fused with the 

 second centrum, is not yet an odontoid process. (8) The complete atlas of an 

 adult Trionyx gangeticus, still typically temnospondylous. (9) The first and second 

 cervical vertebrae of an adult Platemys. (10) The complete atlas of a Chelys 

 fimbriata. Az, Anterior zygapophysis; B.D, basidorsal ; B.V, basi ventral ; 

 C v C a , C 3 , first, second and third centra, formed by the interventralia ; Cp l , Cp*. 

 articular facets of the capitular portions of the first and second ribs ; I, V, In- 

 terventral ; N v N 2 , N 3 , first, second and third neural arch ; formed by the basi- 

 dorsalia (B.D) ; Od, odontoid process (which is the first centrum) ; Pz, posterior 

 zygapophysis ; R lt R 2 , ribs ; Sp, detached spinous process of the first neural arch 

 t v t 2 , tubercular attachments of the first and second ribs ; 1, 2, 3, 4, "intercentra" 

 (which are the basiventrals) ; 7, 77, 777, position of the exit of the first, second 

 and third spinal nerves. 



777, Trunk vertebrae of a tropical Skate, h, haemal process ; t, intercalary 

 plate; n, neural process; r, rib; a, spinous process. (77, After Gadow. 777, from 

 Kingsley after Dumeril.) 



