HORSE-BACK RIDING. 51 



contraction which has for its result a mechanical 

 force, and we shall demonstrate that the same thing 

 takes place in the nervous system ; and that it causes 

 nervous action. But while the muscular action is 

 transformed into an external force, the nervous action 

 exhausts itself entirely in the interior of the economy, 

 and reduces itself to a simple intervention in the 

 functions of the organs which it animates. 



The arterial blood, red and rich in oxygen, pene- 

 trates the nervous system, while the venous blood 

 comes out of it black and charged with carbonic acid. 

 This incontestable fact alone proves that in a state 

 of repose the organic materials of the blood burn in 

 the capillaries of the nerves and nervous centres, as 

 it does in the vascular net-work of all the tissues of 

 the organism. 



When the nervous action is violent, as in anger, or 

 when the action is long continued, as in study, there 

 is an increase of the temperature of the body, and as 

 in muscular contraction, the nervous system when in 

 action absorbs oxygen and exhales carbonic acid; it 

 becomes fatigued and gives an acid reaction. It is 

 easy to convince ourself that after severe brain-work, 

 the proportion of urea in the urine is greatly in- 

 creased. 



Schiff remarks that when a nerve has been excited 

 from any cause, the propagation of this excitement 

 is accompanied by an appreciable elevation of tem- 

 perature along the course of the nerve. 



