TENDON REFLEXES 297 



from two or three of the main trunks proceeding from the plexus and is dis- 

 tributed to a group of muscles acting in harmony. 



Tendon reflex. A familiar example of tendon reflex is the 

 "knee jerk" or patellar reflex. This may be elicited by striking 

 the patellar tendon when partly stretched. The impression thus 

 produced quickly reaches the motor cells which innervate the 

 quadriceps muscle, and the leg is slightly and suddenly extended. 

 (There are several tendon reflexes.) 



Skin reflex. Irritation of the sole of the foot causes the 

 plantar muscles to contract, a plantar reflex. Scratching the skin 

 of the side of the abdomen causes contraction of abdominal 

 muscles, abdominal reflex. (There are other skin reflexes.) 



The spinal cord contains centers for controlling the tone of 

 vessel walls or vascular tone. Also for stimulating the action of 

 secreting glands, and for muscle action of viscera. These functions 

 are exercised through the sympathetic ganglia with which it is 

 widely connected; they will be referred to in Chapter XXII. 



Finally, it contains centers which influence (or control) certain 

 processes of nutrition trophic centers. 



It appears at once that the spinal cord is able, from the wide 

 distribution of its nerves, to provide for most of the activities of 

 the body. 



Taken as a whole it may be regarded as a great common center 

 of sensation and motion; and because of many connecting fibers 

 running upward, downward, and transversely, it can combine 

 and to some extent regulate the functions of many different parts, 

 so that systematic groups of movement, or series of movements, may 

 be executed by organs more or less distant in the body. 



In other words, the spinal cord can to some extent coordinate 

 the functions of the spinal nerves and skeletal muscles. 



To repeat the functions of the spinal cord, they are to preside 

 over: 



1. Reflex action. 



2. Muscle tone. 



3. Vessel tone. 



4. The action of secreting glands. 



5. Nutrition (trophic action). 



6. Coordination of skeletal muscles. 



