532 TEXT-BOOK OF PHYSIOLOGY. 



complicated and involved. The functions 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. collectively 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 cerebellum, 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 nerve impulses which are 

 transmitted through efferent axons to various peripheral organs skeletal 

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

 by the same influences as excite the efferent cells of the spinal cord: e.g., 

 variations in the composition of the blood or lymph; the arrival of nerve 

 impulses coming through afferent pathways in the spinal cord and through 

 afferent cranial nerves; the arrival of nerve impulses 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 impulses coming through lower afferent pathways and transmit 

 them through their related axons to the cortex of the cerebrum, where they 

 evoke sensations. 



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

 come the efferent or motor cranial nerves. 



The afferent cells give origin to fibers which pass to the cerebral cortex. 

 Around both groups of cells, the afferent or sensor cranial nerves terminate 

 in tuft-like expansions. (In a subsequent section the origin, course, dis- 

 tribution, and functions of the various cranial nerves will be considered). 

 But as the function of the nerve is only to transmit energy from the cell of 

 which it constitutes a part, the function ascribed to the nerve may without 

 impropriety be transferred to the cell itself. 



Since it is by means of nerve-cells and their associated fibers that many 

 important functions of organic life are initiated and maintained, it would 

 naturally be expected from its extensive nerve connections that this region 

 of the nerve system plays an extensive r61e in this respect. As the accomplish- 

 ment of these functions requires the cooperation and coordination of a number 

 of separate but related structures, it is evident that there must exist in the 

 medulla and pons a number of coordinating mechanisms consisting of nerve- 

 cells and nerve-fibers which are associated in various ways for the accomplish- 

 ment of definite functions. To such a coordinating mechanism the term 

 " center" has been given: e.g., respiratory, cardiac, deglutitory, etc. * 



1 By the term center as here employed is meant a collection of nerve-cells and nerve-fibers 

 occupying an area of greater or less extent, though its exact anatomic limits may not be accurately 

 defined. That an area may merit the term center, it is necessary that its stimulation should 

 increase, its destruction should abolish or impair, functional activity. 



