GENERAL ZOOLOGY 



the cerebral hemispheres, mesencephalon, and medulla. The gray matter of 

 the cord is, therefore, the location of adjustor neurons which connect dif- 

 ferent levels of the cord with one another and with parts of tne brain and 

 which conduct impulses from one side of the cord to the other. In addition, 

 the cell bodies of efferent neurons, the processes of which pass out in the 

 ventral roots of spinal nerves, are found in the gray matter of the cord. 

 Simple reflexes are adjusted in the spinal cord, and impulses are conducted to 

 and from the brain. 



The primitive brain, or brain stem, is composed of the telencephalon, dien- 

 cephalon, mesencephalon, metencephalon, and myelencephalon (see Fig. 5.21, 

 p. 156). Changes in the direction of greater brain complexity occur chiefly 

 in the regions of the telencephalon and metencephalon, from which the 

 cerebral hemispheres and the cerebellum, respectively, develop. The cerebral 

 hemispheres and cerebellum are the parts in which new functions are added; 

 the functions of the brain stem remain practically constant throughout the 

 vertebrate group. In contrast to its arrangement as a column in the spinal 

 cord, the gray matter occurs in the brain in masses known as centers, which 

 may be surrounded completely by white matter, as in the medulla, or form a 

 continuous peripheral layer, as in the cerebral cortex (Fig. 4.9). It is im- 

 possible to present here a detailed account of the functions of the parts of the 

 brain, but the more important localizations will be given. 



The medulla, into which the spinal cord merges, serves as a pathway be- 

 tween the cord and other parts of the brain. It also contains the centers that 

 adjust the reflexes of the tongue and of breathing. In the tongue reflexes, 

 impulses enter over processes of afferent neurons by way of the fifth and ninth 

 cranial nerves and pass out over processes of efferent neurons by way of the 

 twelfth nerve. The adjustor neurons lie in centers within the medulla. The 

 respiratory reflex depends on the sensitivity of the respiratory center in the 

 medulla to the amount of carbon dioxide in the blood (p. 124). Impulses 

 are conducted over processes of efferent neurons to muscles between the ribs 

 and in the diaphragm. The rhythm and depth of breathing and other re- 

 flexes of the viscera, pharynx, and larynx are also adjusted in the medulla. 



The ventral part of the metencephalon consists of fiber tracts that conduct 

 from side to side, as well as of those connecting lower and higher levels. In 

 the cerebellum, or dorsal part of the metencephalon, there are important 

 muscle-coordinating centers. These coordinations may involve the body as a 

 whole, as when reactions occur in response to stimuli received by the organs 

 of equilibration, the semicircular ducts of the ear. The adjustments that re- 

 sult in bilateral muscular coordinations are also made in the cerebellum. 

 Such bilateral coordinations are chiefly those of the movements of the limbs, 

 although the muscles of the eyes, facial expression, and mastication are 

 believed by some investigators to be bilaterally correlated in cerebellar centers. 



On the dorsal surface of the mesencephalon are found the optic lobes; here 

 are located centers in which certain important visual reflexes, such as the 

 constriction of the pupil of the eye in response to the stimulus of light on the 



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