THE PHYSIOLOGY OF MOVEMENT. 43 



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 nerve-cells which are in turn excited to activity. 



In the case of volitional skeletal-muscle movements, the nerve impulses 

 which cause the movements 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. Fig. 12. 



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 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, 

 secretion, sensation and other forms of nerve activity. When muscles and 

 bones are applied to the overcoming of opposing forces, mechanic work is 

 accomplished. In the following chapters some of the problems connected 

 with the activities of the primary mechanisms, the skeletal, muscle and 

 nerve tissues will be first considered and subsequently some of the problems 

 connected with the activities of the secondary mechanisms. 



