THE ELASMOBRANCII FISHES 



237 



SPINAL COED 



The spinal cord extends from the })rain practically to the tip of the tail. Unlike 

 that in the higher forms it does not possess marked pectoral and pelvic swell- 

 ings produced by the i)assage of the nerve bundles to the limbs. In its external 

 aspect and in its internal structure the cord may be spoken of as a simplifi- 

 cation of the medulla. The place occupied in the medulla by the nucleus of the 



A B 



Fig. 217. Transverse sections of the spinal cord. (From Sterzi.) A. Acanthias vulgaris. B. 

 Baja clavata. 



ca., calcification; d.li., dorsal horn; d.r., dorsal root; er., endorachis; nc, neurocoele; pm., 

 paracentral mass; ps., perimeningeal space; v.h., ventral horn. 



sixth nerve and the grey matter of the formatio-reticularis {f.r., fig. 216) is 

 occupied in the cord by the ventral horn {v.h., fig. 217) ; while the general 

 cutaneous nucleus of the medulla (g.c.n.) gives place to the dorsal horn of the 

 cord (d.h., fig. 217) ; and the lobes of the vagus and the visceromotor nuclei 

 are supplanted by the paracentral mass (pm.). Furthermore, the enlarged 

 fourth ventricle of the medulla becomes the small neurocoele of the cord (tic.) . 



A transverse section shows the spinal cord lying within the neural canal 

 from which it is separated by a considerable perimeningeal space, especially 

 in the rays (fig. 217b). The endorachis (er.) lines the neural canal and is 

 further surrounded by calcification (ca.). Directly surrounding the cord is 

 the meningeal lining, through which ventrally the spinal blood-vessels pass. 

 The cord itself (Acanthias, fig. 217a) is much like that of Heptanchus (fig. 

 201) in that the dorsal horns (d.h.) are close together and the ventral horns 

 (v.h.) are almost at right angles to the central mass. Above the ventral horns 

 are the outgrowing paracentral masses (pm.) . In the rays, however, the cen- 

 tral grey matter of the cord is more diffuse. 



The relation of the grey and white matter, cells and fibers, in the Elas- 

 mobranchs is essentially like that in higher forms, although the proportion 



