ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



mparativelv thin thread. In the skate it becomes interrupted 

 rntirdy. Tin- thickening of the cartilaginous wall of the cylinder dies 



literiorlv and posteriorly and is separated from the next thickened 

 portion l>y a ring of unthickened cartilage which gradually loses its original 

 character and becomes converted into tough fibrous tissue. In this way 

 tin- originally continuous cylinder of cartilage becomes segmented up 

 3 of pieces known as vertebral centra, bound together by strong 

 intervertebral ligaments. It will be understood that each centrum has 



Ily tin- form of a short cylinder, and that it contains two deep 



&c. 



FIG. 132. 



:iii through front portion of tail region of Acanthias. A, Dorsal aorta (caudal 

 in.il aivh ; m, myotomes separated by thin septa of connective tissue; n, noto- 

 wural an h ; S .I, primary sheath ; s.II, secondary sheath ; s.c, spinal cord ; sk, skin ; 

 ' vein. 



anterior and one posterior in the form of two cones 



to apex and opening into one another at the point where 



tin- t\vo iipircs meet. Such a vertebral -centrum with a conical cavity 



"1 is termed amphicoelous and this form of centrum is charac- 



IC "l ;ill onlinarv fishes. In the fresh condition the conical cavities 



ply but are filled by the notochord which traverses them. 

 ' ir distal ends the neural arch rudiments gradually extend until 

 i'i tl.r mesial plane over the spinal cord (Fig. 132) and the 

 ; 1 portion so formed continues its outward growth to form a neural 

 spine. 



An ' i >omt to be noticed in the Dogfish and in various other 



