44 TEXT-BOOK OF PHYSIOLOGY 



takes place, one of which passes toward and into the spinal cord and becomes 

 related in part with the nerve-cells in the ventral horn of the gray matter, 

 the other of which passes to the periphery and becomes connected with 

 the skin and mucous membrane covering the surfaces of the body (Fig. 13). 

 The surfaces of the body thus become associated anatomically and hence 

 physiologically with the motor organs. 



With the growth and development of the brain and especially of the 

 cerebrum, nerve-cells make their appearance in the outer or cortical por- 

 tion. From the cells of certain specialized regions nerve-fibers pass down- 

 ward as far as the medulla oblongata, where for the most part, they cross 

 over to the opposite side and then descend the spinal cord to give off branches 

 or fibers at different levels which become related histologically with the 

 nerve-cells in the ventral horns of the gray matter. The cerebrum is thus 

 brought into relation through the intermediation of the spinal cord, with 

 skeletal, vascular and visceral muscles and gland epithelium (Fig. 12). 



Coincident with the development of the nerve-fibers descending from the 

 cerebrum to the spinal cord, nerve-cells are developing in the more central 

 regions of the gray matter of the spinal cord from which nerve-fibers arise 

 that cross to the opposite side of the cord and then ascend directly or in- 

 directly to the cerebrum where their terminal branches come into physiologic 

 relationship with groups of specialized nerve-cells in different regions of the 

 cortex. Of the fibers of the dorsal roots some, as previously stated, pass 

 forward to the gray cells in the ventral horn; others, however, become 

 associated with the more centrally located nerve -cells above alluded to. 

 The surfaces of the body are thus brought into relationship through the 

 intermediation of the spinal cord with the cortex of the cerebrum. 



The statements, made in the foregoing paragraphs in reference to the 

 spinal nerves, hold true for the twelve cranial nerves which correspond 

 physiologically at least with the ventral and dorsal roots of the spinal nerves. 

 Their relation to the cortex of the cerebrum is similar. 



The nerve-fibers connecting the cerebrum with the motor organs con- 

 stitute the outgoing or efferent side of the nerve system. The nerve-fibers 

 connecting the surfaces of the body with the cerebrum constitute the in- 

 going or afferent portion of the nerve system. 



The Efferent Spinal Nerve-cells. It has been experimentally demon- 

 strated that each nerve-celt in the ventral horn not only generates but under 

 given conditions discharges a form of energy termed a nerve impulse, which 

 is transmitted by the nerve-fiber arising from it and by way of the ventral 

 roots of the spinal nerves directly to skeletal muscles and indirectly through 

 the sympathetic ganglia and their branches to glands, blood-vessels and 

 walls of viscera. (See Fig. 13.) 



The arrival of the nerve impulse at once calls forth the form of activity 

 characteristic of the structure stimulated. Thus the muscle, for example, 

 passes from the passive to the active sjtate, that is, the muscle becomes shorter 

 and thicker, and the bone to which it is attached is moved. This is at 

 once followed by a return of the muscle to the passive state; that is, it 

 lengthens, becomes narrower, and resumes its original form; the bone at 

 the same time returns to its former position. Coincident with this change 

 of shape there is a liberation of heat and electricity. The nerve impulse 

 which occasions this transformation of potential into kinetic energy is the 



