SUPPORT, MOTION, AND SENSATION 327 



this triple process some of the thousands of pressure endings that 

 are scattered over the body come into phiy. Many of these, in t\m 

 case of man, are conveniently concentrated in the finger tips which 

 relay messages to the brain. It is readily seen that the process of 

 getting food requires co-operative action of the skeletal, muscular, 

 and nervous systems. 



The limb action involving stooping, standing, and reaching calls 

 into play different sets of voluntary or skeletal muscles. This empha- 

 sizes one of the fundamental principles of the study of muscles 

 (myology), namely, that for every muscle group there is an opposing 

 set which performs the opposite type of movement. Muscles are 

 effective during contraction and not during relaxation. We speak 

 of the muscles that extend the arm or leg as extensors and those 

 which bend them as flexors. Such muscles are very different from 

 the smooth, involuntary muscles in the walls of the intestines. Here 

 the food undergoes rhythmic segmentation and is broken up into 

 boluses by the intermittent contractions of smooth muscle cells. 

 Fortunately, the control of these involuntary muscles is taken off 

 the hands of the voluntary or central nervous system. Such routine 

 functions are put under the control of the autonomic nervous system, 

 which frees the brain of the necessity of "willing" all these things to 

 happen and leaves the central nervous system free for "higher evolu- 

 tionary adventures" by taking over the "drudgery of living." In 

 order to understand these processes, commonly taken as a matter of 

 course, we must investigate carefully the "why and how" of loco- 

 motion and then try to see how this complicated performance is 

 controlled. 



SECTION A. SKELETAL DEVICES 



The Interdependence of the Parts 



The material covered in this unit consists of representatives of three 

 well-defined and anatomically separable systems, namely, the skeletal, 

 muscular, and nervous systems. Although they are frequently con- 

 sidered separately for the sake of clearness it should be kept in mind 

 that, physiologically, the muscles, skeleton, nerves, and blood supply 

 are all intimately interwoven. In the human body, there are numer- 

 ous muscles most of which are under voluntary control and as such 

 are concerned with posture, with maintaining the relationship of the 

 various skeletal parts to one another, or with some sort of movement. 

 H. w. H. — 22 



