248 NUTRITIONAL PHYSIOLOGY 



the brain quite apart from attention or desire on the part 

 of the subject. 



This kind of subconscious control of organs is well illus- 

 trated in the case of the respiratory center. About a 

 hundred years ago it was found by French workers that 

 cutting across the nervous axis at the point where it leaves 

 the skull that is, where the brain passes into the cord 

 instantly stops the breathing. A similar cut a little higher 

 up does not have this immediately fatal effect. It was in- 

 ferred that the part of the brain next adjoining the cord 

 must contain the special cells which stand in charge of the 

 respiratory muscles. This section of the brain, just within 

 the skull, is called the medulla. Later experiments have 

 confirmed the early belief that the breathing is governed 

 from this region. The muscles employed are not in them- 

 selves automatic. Every contraction which they make is 

 referable to a metabolic change in the cells of the medulla. 

 So there is a fundamental difference between the breathing 

 movements and the beating of the heart. The former 

 are due to central causes; the latter, to an innate quality 

 of the contractile substance. 



The respiratory center is frequently involved in reflex 

 action. In fact, it is easy to convince one's self that hardly 

 any considerable reflex occurs without some disturbance of 

 the breathing as an incident. Any sort of shock will in- 

 fallibly change the depth, regularity, or some other feature 

 of the movements. Outcries following such shocks are 

 merely respiratory reflexes, but the center is not prompted 

 to each successive discharge by afferent impulses; it shows 

 us the possibility of another means of regulation. This 

 is through the influence upon the nerve-cells of the 

 chemical composition of the blood and lymph in the 

 vicinity. When exercise is taken the breathing is involun- 

 tarily deepened. The cause of this adjustment is found in 

 the increase of carbon dioxid in the circulation. The 

 center is remarkably sensitive to any rise in the percentage 

 of this gas. Conversely, it is temporarily paralyzed by a 

 reduction of .the circulating carbon dioxid to an unusually 



