166 



APPLIED PHYSIOLOGY 



bundles of a limb are bound together by a very thick sheet 



of the same tissue. You can see these bundles and their 



tough coverings in any meat. 



330. Action of muscles. A nerve thread touches every 



muscle cell. When it brings an order to act, each muscle 



cell makes itself thicker and 

 shorter. Thus the whole muscle 

 becomes shorter and pulls upon 

 whatever is fast to its ends. 

 The usual action of a muscle 

 is to bend a joint. 



The messages sent to the mus- 

 cles come from the cells of the 

 spinal cord. They send mes- 

 sages to the muscles either in 



c connective tissue binding the 

 cells together. 



A thin slice of a voluntary mus- 

 cle, cut lengthwise ( x 100) . 

 a muscle cell. n 



b capiiia-ries surrounding the cells, a reflex way or when told to do 



so by the cells of the brain. 

 Muscle and brain cells are the 

 only cells of the body which can be made to act when- 

 ever we wish them to. 



331. Involuntary muscles. There is a kind of muscle 

 which we cannot make act by an effort of the will, but 

 which the spinal cord keeps in action in a reflex way 

 without our knowledge. Such muscles are found in the 

 stomach, intestine, arteries, and skin. They are the 

 muscles which aid the digestion of food and the flow of 

 blood. The spinal cord and sympathetic system send them 

 orders without our knowledge. It is well that nature has 

 put these muscles beyond our control, for we might 

 forget to attend to them. They are called involuntary 

 muscles, because we cannot make them act. These mus- 

 cles are made of cells with long pointed ends. Instead of 

 being solid masses, they form thin leaves around tubes. 



