EXTRACT XXXII. B. 



ON THE EVOLUTION OF THE SYSTEMIC NERVOUS 



SYSTEM. 



THE systemic nervous system is evolved from the wall of 

 the posterior, or dorsal, limb, of the neurenteric canal, 

 which has been separated from, and reunited to, the anterior, 

 or ventral, limb, by the interposition of metamorphic 

 textures, which afterwards serve to keep them in continued 

 modified structural, and functional, relationship. The 

 metamorphic textures, here meant are those forming the 

 outlet channels surrounding the olfactory projections of the 

 anterior cerebral vesicle, together with the optic, and otic, 

 outlets, the pituitary outlet from the cerebral infundibulum, 

 and the coccygeal outlet, or termination, of the spinal 

 meningeal cavity, and the thecal investment of the cord, 

 represented by the filum terminale, and its coccygeal 

 termination all of which outlets persist to the end of life, 

 and maintain, or afford, facilities, for the excretionary dis- 

 posal of superabundant cerebro-spinal fluid, and waste, 

 cerebral and neural, material. This excretional provision 

 is supplemented, and completed, by the sweat glands, 

 through their connections with the inter-neurilemmar, 

 spaces, and lymph channels. 



In the wall of this posterior limb, of the neurenteric 

 canal, are laid down the structural elements of the budding 

 systemic nervous system, at first in most rudimentary form, 

 but, subsequently, by evolutionary stages, in increasingly 

 organised, and specialised, form, until the finished systemic 

 nervous system appears, in all its completeness of struc- 

 tural detail, and functional capability. The wall of this 



