480 TEXT-BOOK OF PHYSIOLOGY. 



able that these transverse fibers are commissural in character, bring- 

 ing into relation opposite but corresponding regions of the cerebellar 

 cortex. In addition to the commissural fibers other transverse fibers 

 associate the cerebellar cortex with the gray matter in the pons on 

 both the same and opposite sides. In this way the cerebellum is 

 brought into relation with longitudinal fibers coming from and going 

 to the cerebrum. 



FUNCTIONS OF THE MEDULLA OBLONGATA, ISTHMUS, AND 

 BASAL GANGLIA. 



Microscopic examination of the white and gray matter of these 

 various parts of the central nerve system shows that they are com- 

 posed of nerve-cells and nerve-fibers which morphologically do not 

 differ in essential respects from those found in the spinal cord, though 

 their arrangement is far more complicated and involved. The func- 

 tions of these closely related structures are in consequence equally 

 complex and involved and but imperfectly known. 



In a general way it may be said that by virtue of the presence 

 of nerve-cells and definite tracts of nerve-fibers these structures col- 

 lectively may be regarded as consisting : 



1 . Of centers for reflex actions ; and 



2. Of conducting paths by which the various parts are brought into 



relation one with another and with the spinal cord, the cerebel- 

 lum, and the cerebrum. 



The Medulla Oblongata and Pons. The gray matter situated 

 in these structures i. e., just beneath the floor of the fourth ventricle 

 contains nerve-cells arranged in more or less well-defined groups 

 which may be divided into efferent and afferent. 



The efferent cells are the immediate sources of energy which 

 is transmitted through efferent axons to various peripheral organs 

 muscles, glands, and blood-vessels. Their activity may be excited 

 by the same influences which excite the efferent cells of the spinal 

 cord: e. g., variations in the composition of the blood or lymph; the 

 arrival of nerve energy coming through afferent pathways in the 

 spinal cord and through afferent cranial nerves; the arrival of nerve 

 energy coming through efferent pathways from the cerebrum. The 

 peripheral activity resulting from their excitation may therefore be 

 automatic or autochthonic, peripheral (reflex) or cerebral (volitional) 

 in origin. 



The afferent cells are sentient or receptive in function, inasmuch 

 as they receive nerve energies coming through lower afferent pathways 

 and transmit them through their related axons to the cortex of the 

 cerebrum, where they are translated into conscious sensations. 



The efferent cells give origin to nerve-fibers which pass ventrally 

 and become the efferent or motor cranial nerves. 



