464 TEXT-BOOK OF EMBRYOLOGY. 



The general features of the vertebrate nervous system which especially 

 illuminate conditions met with in the human nervous system are the following: 

 (i) The correlation between the peripheral structures (receptors and effectors) 

 and the nervous system. (2) The distinction between the epichordal and pre- 

 chordal portions of the brain. The latter (fore-brain) is, in accordance with its 

 anterior position (comp. p. 457), the most highly modified part of the neural 

 tube. (3) The distinction between the segmental and suprasegmental parts 

 of the brain (Adolf Meyer).* The segmental part of the brain is that portion 

 in more immediate connection with peripheral segmental structures. Its epi- 

 chordal part is spinal-like and most clearly segmental. Its prechordal part, 

 both as to its peripheral and central portions, is so highly modified that its 

 segmental character is more obscure. It and the rest of the prechordal brain 

 are most conveniently treated together as fore-brain. The suprasegmental 

 parts of the brain, or higher coordinating centers, are the cerebellum, mid- 

 brain roof and the pallium (cerebral hemispheres). Their general functional 

 significance has been mentioned (p. 457). Some of their general structural 

 characteristics are : First, that they are each expansions of the dorso-lateral 

 walls of the neural tube; second, that in them the neurone bodies are placed 

 externally and in layers (cortex), the nerve fibers (white matter) lying within; 

 third, that each appears to have originally had an especially close relation with 

 some one of the three great sense organs of the head, the olfactory, visual or 

 acustico-lateral system; fourth, that each is connected with the rest of the brain 

 by bundles of centripetal and centrifugal fibers, and often there are specialized 

 groups of neurone bodies in other parts of the brain for the origin or recep- 

 tion of such bundles. Each higher center has also its own system of association 

 neurones. 



It will accordingly be most convenient to consider: (i) the spinal cord, (2) 

 the segmental part of the epichordal brain, (3) the cerebellum, (4) the mid- 

 brain roof, (5) the prosencephalon. 



Spinal Cord and Nerves. 



As already brought out, there are two principal morphological differences 

 between the afferent and efferent peripheral neurones. First, the neurone 

 bodies of the former are located outside the neural tube, while the neurone 

 bodies of the latter lie within the walls of the neural tube. Second, the afferent 



*This distinction apparently ignores the fact that the primitive neuromeric segmentation of the ;' 

 neural tube involves its dorsal as well as its ventral walls and thus " suprasegmental " as well as " seg- !i 

 mental " structures were originally segmental. This may be granted, but while the demonstration 

 of the primitive segmentation of the neural tube may be valuable as showing the primitive mechan- 

 ism which has undergone later modifications, the importance of such later modifications renders the 

 above distinction necessary. The main significance of the nervous system is its associative character i 

 and its progressive development is not as a segmental, but as a more and more highly developed 

 associating mechanism. 



