STRUCTURE OF THE VERTEBRATES 285 



over myelinated sensory fibers to the cord and thence to the 

 brain where the sensation is registered; and (2) over unmyeli- 

 nated motor fibers from the ganglion to the cutaneous regions. 

 The latter cause blanching of the skin and profuse sweating. 

 The autonomic reactions are influenced by the sensitivity of the 

 nervous system, and to a certain degree by the conditioning 

 (experience) of the individual. The voluntary response follow- 

 ing the registration upon the brain is almost entirely controlled 

 by training. 



The study of nerve pathways differs from most anatomical 

 systems in that knowledge of structure depends very largely 

 upon physiological reactions. The function of a muscle is in- 

 dicated by its attachments; but a study of the cord and its 

 nerve tracts tells almost nothing as to regional function, and the 

 pathways have been traced through the degeneration of fibers 

 which follows injuries to the cord and brain. 



E. The Brain 



The brain of the lamprey (Cyclostomata) most nearly re- 

 sembles the embryonic brain with its three simple divisions. But 

 in this most primitive group there are clear indications of the 

 division of the brain into more complex regions. From the fore- 

 brain come two pairs of outpocketing, anterior evaginations to 

 form olfactory lobes, and dorsal cerebral hemispheres ; and the 

 hindbrain has a slight constriction which separates the anterior 

 cerebellum from the medulla. 



The embryos of the higher vertebrates develop a more com- 

 plex structure: (1) both the fore- and hindbrains are distinctly 

 divided into two regions; (2) complex evaginations appear in 

 both these regions, with folds and creases which increase the 

 surface area without greatly increasing the bulk of the entire 

 structure; and (3) certain areas increase in thickness due to the 

 growth of nerve nuclei and the invasion of nerve fibers. 



The forebrain is divided into an anterior telencephalon and a 

 narrow, thin-walled diencephalon. The former undergoes the 

 greatest differentiation. Paired anterior outpocketings develop 

 into the olfactory lobes and nerves, w^ith the lamina terminalis 

 of the forebrain between them. Laterally grow out two optic 



