224 



Life and Health 



descent will be made with ease and safety if the spinal 

 cord is allowed entire charge of the act, but the chances of 

 stumbling or of tripping are very much increased if each 



step be taken as the result of the 



will power. 



Experiment 97. To illustrate reflex 

 action by what is called knee-jerk. Sit 

 on a chair and cross the right leg over 

 the left one. With the tips of the fingers 

 or the back of a book strike the right 

 ligamentum patellae. The right leg will 

 be raised and thrown forward with a jerk, 

 owing to the contraction of the quadriceps 

 muscles. An appreciable time elapses 

 between the striking of the tendon and 

 the jerk. The presence or absence of 

 the knee-jerk may be a most significant 

 symptom to the physician. 



7-342. The Sympathetic System. 



small cells of this part of the Running along each side Q f the 

 cord (3). In some unknown 



way this impulse passes across spine, from the base of the skull 



the gray part of the cord to to the coccyx is a cha in of nerve 

 the large cells of the anterior J 



root (5), the cells of this part knots, or ganglia. These ganglia, 

 being connected b y their axis twenty-four on each side, and their 



cylinder with the efferent fibers 



(6). These convey the stim- branches form the sympathetic 

 ulus to the fibers of the muscle S y st em, as distinguished from the 



(7) which accordingly contract. ,_, 



Where the brain is concerned cercbro-spmal system. The sym- 

 in the action the circuit is pathetic ganglia are connected with 



longer through S and M. , . , , 



each other and with the sensory 



roots of the spinal nerves by a network of gray nerve fibers. 

 At the upper end the chain of each side passes up into 

 the cranium and is closely connected with the cranial 

 nerves. In the neck, branches pass to the lungs and the 

 heart. From the ganglia in the chest these nerves form 



FIG. 122. Dr. Waller's Dia- 

 grammatic Illustration of 

 the Reflex Process. 



From the sentient surface ( I ) an 

 afferent impulse passes along 

 (2) to the posterior root of the 

 spinal cord, the nerve fibers of 

 the posterior root ending in 

 minute filaments among the 



