THE MEDULLARY CANAL. 



trunk, but in existing Vertebrata all trace of these, except in so 

 far as they are indicated by the visceral clefts, has vanished in 

 the adult. The cranial nerves however, especially in the embryo, 

 still indicate the number of anterior somites ; and an embryonic 

 segmentation of the mesoblast has also been found in many 

 lower forms in the region of the head, giving rise to a series of 

 cavities known as head-cavities, enclosed by mesoblastic walls 

 which afterwards break up into muscles. These cavities corre- 

 spond with the nerves, and it appears that there is a praeman- 

 dibular cavity corresponding with the third nerve (fig. 193, \pp) 

 and a mandibular cavity (zpp) and a cavity in each of the 

 succeeding visceral arches. The fifth nerve, the seventh nerve, 

 the glossopharyngeal nerve, and 

 the successive elements of the 

 vagus nerve correspond with the 

 posterior head-cavities. 



The medullary canal. The 

 general history of the medullary 

 plate seems to point to the con- 

 clusion that the central canal of the 

 nervous system has been formed by 

 a groove having appeared in the 

 ancestor of the Chordata along the 

 median dorsal line, which caused 

 the sides of the nervous plate, 

 which was placed immediately 

 below the skin, or may perhaps at 

 that stage not have been distinctly 

 differentiated from the skin, to be 

 bent upwards ; and that this groove 

 subsequently became converted into 

 a canal. This view is not only 

 supported by the actual develop- 

 ment of the central canal of the 

 nervous system (the types of Tele- 

 ostei, Lepidosteus and Petromyzon 

 being undoubtedly secondary), but 

 also (i) by the presence of cilia in the epithelium lining the canal, 

 probably inherited from cilia coating the external skin, and (2) by 



FIG. 193. TRANSVERSE SECTION 

 THROUGH THE FRONT PART OF THE 

 HEAD OF A YOUNG PRISTIURUS 



EMBRYO. 



The section, owing to the cranial 

 flexure, cuts both the fore- and the 

 hind-brain. It shews the prseman- 

 dibularand mandibular head-cavities 

 ipp and ipp, etc. 



fb. fore-brain; /. lens of eye; m. 

 mouth ; pt. upper end of mouth, 

 forming pituitary involution; \ao. 

 mandibular aortic arch; ipp. and 

 ipp. first and second head-cavities ; 

 ivc. first visceral cleft ; V. fifth 

 nerve ; aun. ganglion of auditory 

 nerve ; VII. seventh nerve ; aa. dor- 

 sal aorta ; acv. anterior cardinal 

 vein ; ch. notochord. 



