564 TEXT-BOOK OF PHYSIOLOGY 



not only with one another but with the cerebrum, the cerebellum and 

 the spinal cord. 



A. The Medulla and Isthmus, as Local Nerve-centers. The emis- 

 sive or efferent cells situated in the gray matter beneath the floor of the 

 fourth ventricle and beneath the aqueduct of Sylvius and arranged in more 

 or less well-defined groups, are the sources of. the nerve energy that excites 

 activity in the skeletal muscles, glands, vascular and to some extent visceral 

 muscles of the head, neck, thorax and abdomen. 



The efferent fibers emerging from these cells constitute in part, the trunks 

 of the efferent encephalic or cranial nerves, the origin, course, distribution 

 and functions of which will be considered in a subsequent chapter. 

 The discharge of their energy may be caused: 



1. By variations in the composition of the blood or lymph by which they 



are surrounded or as the outcome of a reaction between the chemic 

 constituents of the lymph on the one hand and the chemic constituents 

 of the nerve-cell on the other hand. The excitation of the cell thus 

 occasioned is termed automatic or autochthonic excitation. 



2. By the arrival of nerve impulses, coming through afferent cranial and 



spinal nerves from the general periphery, skin, mucous membrane, 

 etc. 



3. By the arrival of nerve impulses descending from cells in the cortex of 



the cerebrum or subordinate regions. 



The excitation in the former instance is said to be reflex or peripheral 

 in origin; in the latter instance direct or cerebral in origin. In the direct or 

 cerebral excitations the skeletal muscle movements are due to psychic states 

 of a volitional or an affective or emotional character; the gland discharges and 

 vascular and visceral muscle movements to emotional phases of cerebral 

 activity only. 



The activities of these centers are thus called forth by autochthonic, reflex 

 or peripheral, and direct or cerebral excitations. The forms of activity excited 

 in peripheral organs resemble those excited by the spinal nerve-centers. 

 See page 451. 



Special Nerve-centers. 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 accomplishment of these functions requires the co- 

 operation 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 co- 

 ordinating mechanisms consisting of nerve-cells and nerve-fibers which are 

 associated in various ways for the accomplishment of definite functions. 

 To such a Coordinating mechanism the term "center" has been given: 

 e.g., respiratory, cardiac, deglutitory, etc. 1 



Experimentation has shown that the medulla and pons contain a 

 number of such centers, the more important of which are as follows: 

 i. The cardiac centers, which exert (i) an accelerator action over the heart's 

 pulsations through nerve-fibers emerging from the spinal cord in the 



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. 



