296 ANATOMY AND PHYSIOLOGY 



For example, we shiver with cold, or tremble from excitement; 

 these are purely reflex acts of which we may be conscious although 

 we are unable to control them; the action of the heart is some- 

 times very evident to the senses, as in palpitation, etc., but beyond 

 our power to regulate. 



In each lateral half of the cord the cell tissue is grouped in 

 crescents. Fibers in the posterior tracts transmit sensory impulses 

 from various parts of the body to cells in the posterior horns of the 

 crescents. Fibers in the anterior tracts transmit motor impulses 

 from cells in the anterior horns to various parts of the body (their 

 axons arise in cells of the anterior horns) (Fig. 190). 



Here we have the simple reflex arc, or the apparatus for 

 reflex muscle action. A sensory or afferent nerve receives an 

 impression, and transmits a series of impulses to the spinal cord. 

 These are received by a cell which in its turn is stimulated, and 

 liberates energy to be conducted by a motor or efferent nerve to a 

 muscle, and the muscle contracts; or to a gland the gland 

 secretes; or to a vessel wall the caliber is changed. These acts 

 are comparatively simple. 



Most muscle activities, however, are complex, requiring the 

 combined action of several organs; in these cases many motor cells 

 and nerves must be stimulated, and this is accomplished by means 

 of additional neurons within the cord, whose fibers associate the 

 activities of different regions. For instance, an unsuspected blow 

 upon the hand is followed instantly by a drawing back of the hand 

 and arm most of the muscles of the upper extremity will have 

 been called into action; in other words, many motor cells (in the 

 lower cervical region of the cord) have been stimulated to a sudden 

 liberation of energy, showing the effect of one stimulus when 

 conducted by connecting fibers to many cells in the cord. 



Walking was in the beginning a purely voluntary act, but the 

 centers which control it become, by education, independent, and 

 it takes its place among the reflexes. So with piano-playing, and 

 many other acts. 



The complicated motor response is provided for by the arrangement known 

 as a nerve plexus, which is formed by interlacing branches of nerve trunks. 

 An illustration is seen in the brachial plexus, where five large trunks are 

 reduced to three while passing under the shoulder joint; then, by branching 

 and interlacing, the fibers are so arranged that each nerve contains fibers 



