THE PHYSIOLOGY OF MOVEMENT 45 



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. 



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

 oped in specialized organs located in the skin or mucous membranes and as 

 a result of the impact of various external agents, which for this reason are 

 termed stimuli. The nerve impulses thus developed are transmitted by the 

 afferent nerves to the efferent or motor nerve-cells which are in turn excited 

 to activity as a result of which, motor organs are aroused to action. 



In the case of both the volitional skeletal-muscle movements and the 

 affective or emotional viscero-muscle and vasculo-muscle movements as 

 well as the activities of glands, the nerve impulses which cause the move- 

 ments are discharged from certain motor or efferent nerve-cells in the gray 

 matter of the cortex of the cerebrum and transmitted by descending axons 

 or nerve-fibers direct to the nerve-cells in the spinal cord, by which they in 

 turn are excited to activity. 



The movements due to cerebral or psychic activity are, however, the 

 immediate or the more or less remote effects of sensations which have been 

 evoked in the sense areas of the brain, by the arrival of nerve impulses coming 

 through ascending axons, or nerve- fibers from peripheral sense organs, e.g., 

 skin, eye, ear, nose, tongue, and which have been developed by the impact of 

 objects in the external world. 



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 

 and the autonomic nerves which control them are apparently only influenced 

 in their activity by emotional states. Why this difference should exist it 

 is difficult to state. 



The nerve-cells and their related nerve-fibers, responding by the develop- 

 ment and conduction of nerve impulses are also said to be irritable. The 

 transformation of energy, however, manifests itself mainly as electricity and 

 molecular motion. The animal body in its entirety may therefore be regarded 

 as a machine for the transformation of potential energy into kinetic 

 energy, viz., heat and electricity, movements of muscles and bony levers, 



