LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM 119 



of carbon dioxide in the blood. Efferent neurones pass to the mus- 

 cles between the ribs and those of the diaphragm. The rhythm 

 of breathing is likewise controlled in the medulla by way of afferent 

 neurones from the walls of the lungs. The medulla also controls 

 other reflexes of the viscera, pharynx, and larynx. At the level of 

 the auditory nerve, certain reflex adjustments of the head and 

 neck to the reception of sound stimuli are made in the medulla. 



The ventral part of the metencephalon consists of fiber tracts 

 that transmit from side to side as well as those connecting lower 

 and higher levels. In the cerebellum, or dorsal part of the meten- 

 cephalon, there are important muscle-coordinating centers. These 

 include adjustments involving the body as a whole, such as the 

 reactions occurring in response to stimuli received by the organs of 

 equilibration, the semicircular canals. The adjustments that 

 result in bilateral muscular coordinations are also made in the 

 cerebellum. Such bilateral coordinations are chiefly those of the 

 voluntary movements of the limbs, although the muscles of the 

 eyes, facial expression, and mastication are also believed by some 

 investigators to be bilaterally correlated by centers in the cere- 

 bellum. Finally, the normal tension of voluntary muscle, or what 

 is known as " muscle tonicity," is governed by cerebellar centers. 



On the dorsal surface of the mesencephalon are found the optic 

 lobes. In all vertebrates, the optic tracts, which extend from the 

 optic chiasma, end here. Centers are present which control certain 

 important visual reflexes, such as the constriction of the pupil of 

 the eye in response to the stimulus of light on the retina. In the 

 higher vertebrates, certain reflexes following sound stimuli are con- 

 trolled in the roof of the mesencephalon. The lateral and ventral 

 regions of the midbrain contain many groups of neurones that pro- 

 vide for numerous connections, and fiber tracts over which impulses 

 are relayed from one level to another. 



In the diencephalon are many fiber tracts connecting various 

 centers in other parts of the brain with the cerebral cortex. The 

 optic nerves and tracts, carrying impulses from the retinae to the 

 optic lobes, form the floor and part of the lateral walls of the dience- 

 phalon (Fig. 67). In the lower vertebrates, all the fibers from one 

 retina cross the optic chiasma to enter the opposite optic lobe. The 

 crossing in higher vertebrates involves only the medial half of the 

 fibers of each retina (Fig. 65), while the fibers of the lateral halves 

 do not cross. Correlations resulting from olfactory stimuli are 



