THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 49 



region of the brain up to the region of the pineal eye and ganglion 

 habenultc is one large membranous bag, except for the single con- 

 striction where the fourth nerve on each side crosses over. The 

 explanation of this peculiarity is given in Chapter VII., and follows 

 simply from the facts of the arrangement of that musculature in the 

 scorpion-group which gave rise to the eye-muscles of the vertebrate. 



In Ammocoetes both cerebellum and posterior corpora quad- 

 rigemina can hardly be said to exist, but upon transformation a 

 growth of nervous material takes place in this region, and it is seen 

 that this commencing cerebellum and the corpora quadrigemina arise 

 from tissue that is present in Ammocoetes along the course of the 

 fourth nerve. 



Here, then, again Embryology does its best to tell us how the 

 vertebrate arose. The formation of the two primary constrictions 

 in the dilated anterior vesicle whereby the brain is divided into 

 fore-brain, mid-brain, and hind-brain is simply the representation 

 ontogenetically of the two nerve-tracts which crossed over the 

 cephalic stomach in the prevertebrate stage, in consequence of 

 the mid-dorsal position of the pineal eyes and of the insertion of 

 the original superior oblique muscles. 



The subsequent constriction by which the prosencephalon is 

 separated from the thalamencephalon is in the position of the 

 anterior commissure, that commissure which connects the two supra- 

 infundibular nerve-masses, and is one of the first-formed commis- 

 sures in every vertebrate. This naturally is simply the commissure 

 between the two supra-oesophageal ganglia; anterior to it, in the 

 middle line, equally naturally, the anterior end of the old stomach 

 wall still exists as the lamina terminalis. 



The other division in the hind-brain region, which separates the 

 region of the cerebellum from the medulla oblongata, is due to the 

 growth of the cerebellum, and indicates its posterior limit. In such 

 an animal as the lamprey, where the cerebellum is only commencing, 

 this constriction does not occur in the embryo. 



From such simple beginnings as are seen in Ammocoetes, the 

 higher forms of brain have been evolved, to culminate in that of man, 

 in which the massive cerebrum and cerebellum conceals all sio-n of 

 the dorsal membranous roof, those parts of the simple epithelial tul >e 

 which still remain being tucked away into the cavities to form the 

 various choroid plexuses. 



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