632 



PRINCIPLES OF GENERAL PHYSIOLOGY 



to be found in the urine after vigorous muscular exercise. When the oxygen 

 tension in the blood has fallen below about 60 mm., the nerve centres are excited 

 and convulsions ensue. This result is due to the formation of asphyxial products, 

 probably of an acid nature, in the centres themselves. Mathison (1910, 1911) 

 studied the stimulation of the spinal and bulbar centres by deprivation of oxygen 

 without increase of carbon dioxide, that is, by respiration of nitrogen, and came 

 to the conclusion that it is acid formed in the centres themselves which excites 

 them. The bulbar (vasomotor) centre is more easily excited than the spinal 



A 





Curve U 



Curve X 



FIG. 194. RESPIRATORY REFLEXES PRODUCED BY INFLATING THE LUXUS. 



A, The dotted line is the tracing of a mercury manometer connected with the trachea, a 



rise indicating- distension of the lungs. 

 The curve below it is that of a lever attached to the diaphragm slips of the rabbit, and 



represents the movement of the diaphragm as a whole. Thus inspiration is 



shown by an upward movement of the curve. 



The tracing at the top is from a control lever attached to the walls of the chest. 

 Note that distension inhibits inspiration. 



B, Similar effect of short inflation with pure hydrogen. To show that the result of the 



preceding experiment was not due to cessation of respiration owing to increased 

 supply of air. 



(Head, 1889, PI. 2, Figs. 2 and 10.) 



centres. Thus, the former reacts to thirty seconds' deprivation of oxygen, to 

 5 per cent, carbon dioxide, or to 2 c.c. lactic acid. The spinal centres require 



two minutes' deprivation of oxygen, 30 per cent, of carbon dioxide or 5 c.c. - lactic 



6 



acid. The production of acids other than carbon dioxide is a sudden one and 

 occurs at the point when the cell mechanisms are beginning to be disorganised ; 

 hence the process is not of use to the organism, as far as the centres referred to 

 are concerned, although the acids formed are very potent stimuli. 



For further details with regard to the chemical regulation of respiration, the 

 essay by Douglas (1914) may be consulted. 



The Nervous Mechanism. From what we have seen above (pages 500, 511, and 

 540) as to the importance of afferent impulses from contracting muscles (proprio- 

 ceptive impulses) in the regulation of their activity, it would be surprising if the 

 respiratory mechanism were devoid of similar control. 



In the present case the receptors are mainly in the lungs and in connection 



