NERVES OF RESPIRATION. 173 



is much more active than that of reptiles. Under an equal Respiration of 

 weight they consume nearly as much oxygen as mammalia : insects - 

 the comparative lowness of their temperature is due to the relatively 

 great surface and moist exterior they present to the air. It is to be re- 

 marked that we are here comparing the respiration of insects with that 

 of mammalia whose weights may lie from 2000 to 10,000 times as 

 great. 



llth. The respiration of animals of different classes, in an air con- 

 taining two or three times as much oxygen as the atmos- Effect of in- 

 phere, does not differ from existing respiration ; indeed, the creasin s tj 6 

 animals do not appear to perceive that they are in a medium ygen. 

 different from the ordinary atmosphere. 



12th. The respiration of animals in a medium in which, for the most 

 part, hydrogen replaces the nitrogen of our atmosphere, scarcely differs 

 from existing respiration ; only there is remarked a greater consumption 

 of oxygen, due perhaps to the necessity of compensating for the increased 

 cooling arising from the contact of hydrogen gas. 



The introduction of air into the system is, to a certain extent, auto- 

 matic, and, to a certain extent, dependent on the will. In tranquil res- 

 piration we are wholly unconscious of the motion ; the ex- Nerves in 

 citing impression is made on the pneumogastric nerves, and, voived in re& 

 being conveyed to the respiratory ganglion, the medulla ob- piratlon - 

 longata, is there so reflected that through the agency of the phrenic nerve 

 motion takes place in the diaphragm. The automatic, and therefore un- 

 conscious movement, to a certain extent, occurs in that way. But there 

 is no doubt that the brain also participates in the function. No other 

 evidence of this is required than that we can " hold the breath," and the 

 relative share that the voluntary and automatic mechanisms take is illus- 

 trated by the circumstance that this holding of the breath can only be 

 persisted in for a certain time, when the necessity for respiring becomes 

 altogether uncontrollable. 



It is not, however, to be supposed that so important a condition as 

 that of the introduction of the air is only slenderly provided for. Many 

 other nerves, besides those mentioned, take part in it directly or indi- 

 rectly ; the fifth pair, the nerves of the general surface, and also the great 

 sympathetic, the intercostals, the spinal accessory, which probably gives 

 its motor property to the pneumogastric. Opinion has differed respect- 

 ing the cause which produces the necessary impression on the receiving 

 nerves, some referring it to the presence of venous blood in the capilla- 

 ries of tke lungs, and some to the carbonic acid in the cells. Moreover, 

 there is reason to believe that the presence of an abnormal amount of 

 venous blood in the respiratory ganglions will of itself give rise to res- 

 piratory movements through the proper centrifugal nerves. 



