42 TEXT-BOOK OF PHYSIOLOGY. 



in each half is arranged in the form somewhat of a crescent united in the 

 median line by a transverse band or commissure, the whole forming a 

 figure resembling the letter H. Though varying in shape in different 

 regions of the cord, the gray matter in all situations presents on either side 

 an anterior or ventral and a posterior or dorsal horn. 



In the ventral horns of the gray matter are located large nerve-cells 

 which give origin to nerve- fibers; these fibers in their growth pass forward 

 through the cord and emerge as ventral roots; continuing to grow, these 

 fibers gradually reach and become connected with the motor organs to 

 which they are by heredity directed. 



It has been experimentally demonstrated that each nerve-cell 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 ganglia of the sympathetic nerve system 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 state, 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 

 normal or the physiologic stimulus. The glands in response to the nerve 

 impulse pour out a secretion, the blood-vessels and viscera change their 

 caliber; all these tissues responding to the nerve impulse in a characteristic 

 manner are said to be irritable. 



The nerve-cells in the ventral horns of the gray matter of the spinal cord 

 are therefore the sources of the energy requisite for the physiologic excitation 

 of the motor organs. If they are destroyed either experimentally or by 

 pathologic processes, the energy is no longer discharged and the motor organs 

 become incapable of performing their functions in a physiologic manner. 



The nerve-cells, though extremely irritable, do not possess spontaneity 

 of action, but require for their excitation the arrival and stimulating action 

 of other nerve impulses. These may come (i) from the periphery through 

 afferent nerve- fibers by way of the dorsal roots of the spinal nerves; and 

 (2) from motor nerve-cells in the cortex of the cerebral portion of the brain, 

 through descending axons or nerve-fibers. 



In the first instance the resulting movements taking place in response 

 to a peripheral or surface stimulation and independently of volitional or 

 emotional activity are termed reflex movements; in the second instance the 

 resulting movements taking place in response to volitional or emotional 

 activities are termed volitional or emotional movements. 



The only organ that can be properly said to be excited to action by a 

 volitional act is the skeletal muscle; the glands, blood-vessels, and viscera are 

 apparently only influenced in their activity by emotional states. 



In the case of reflex movements, the nerve impulses are primarily devel- 



