160 



FUNCTIONS OF THE NERVOUS SYSTEM. 



Fig. 28. 



Human embryo of sixth week, enlarged 

 about thret times; a, vesicle of corpora quad- 

 rigemina; b, vesicle of cerebral hemispheres; 

 c. vesicle of thalami optici and third ventricle ; 

 rf, vesicle for cerebellum and medulla oblon- 

 gata; e, auditory vesicle; f, olfactory fossa; 

 A, liver; **, caudal extremity. (After Wag- 

 ner.) 



animal, and even in Man himself, bears a 

 very close correspondence. The Erice- 

 phalon consists, at this time, of a series of 

 vesicles, arranged in a line with each other, 

 of which those that represent the Cerebrum 

 are the smallest, whilst that which repre- 

 sents the Cerebellum is the largest. The 

 latter, as in Fishes, is single, covering the 

 fourth ventricle on the dorsal surface of the 

 Medulla Oblongata. Anterior to this, is 

 the single vesicle of the Corpora Gluadri- 

 gemina, from which the Optic nerve chiefly 

 arises : this has in its interior a cavity, the 

 ventricle of Sylvius, which exists even in 

 the adult Bird, where the Corpora duadri- 

 gemina are pushed, as it were, from each 

 other by the increased development of the 

 Cerebral hemispheres. In front of this 

 the vesicle of the Third Ventricle, 



is 



which contains also the Thalami: as de- 

 velopment proceeds, this, like the preced- 

 ing, is covered by the enlarged hemi- 

 spheres; whilst its roof becomes cleft 

 anteriorly on the median line, so as to 

 form the anterior entrance to the cavity. Still more anteriorly is the double 

 vesicle, which represents the hemispheres of the Cerebrum: this has a cavity 

 on each side, the floor of which is formed by the corpora striata. The cavity 

 of the cerebral vesicles has at first no opening, except into that of the third 

 ventricle ; at a later period is formed that fissure on the inferior and posterior 

 side, whick (under the name of the fissure of Sylvius) enables the membranes 

 enveloping the brain to be reflected into the lateral ventricles. 



215. Thus it will be seen that the real analogy between the brain of the 

 Human foetus and that of the adult Fish, is not so close as, from the resem- 

 blance in their external form, might have been supposed. In the small pro- 

 portion which the Cerebral Hemispheres bear to the other parts, there is 

 evidently a very close correspondence ; and this extends also to the general 

 simplicity of their structure, the absence of convolutions, and the deficiency of 

 commissures. But there is a much nearer analogy between the fatal brain 

 of the Fish, and the fatal brain of the Mammal ; indeed, at the earliest period 

 of their formation, they could not be distinguished ; during their advance to the 

 permanent condition, however, each undergoes changes, which are so much 

 more decided in the higher animals than in the lower, that in the latter there 

 seems but little departure from the festal condition, whilst in the former the 

 condition appears entirely changed. Hence it is not correct to assert, as is 

 frequently done, that the Brain, or any other organ, in the higher animals, 

 passes through a series of forms, which are parallel to the permanent forms of 

 the same organ in different parts of the animal scale : since the Ja.ct is rather, 

 that the more nearly all are traced back to their first origin, the closer will 

 their conformity be found to be ; the subsequent development of each taking 

 place not only in various degrees, but in different modes or directions ; so that 

 the resemblances presented by the higher, at different epochs of their evolution, 

 to the permanent conditions of the lower, are often far from being complete.* 



* For a fuller examination of this interesting question, see Gfeneral and Comparative 

 Physiology, 244. ^ 



