4 2 4 TEXT-BOOK OF PHYSIOLOGY 



at a temperature of i5C. consumed 38.31 grams of oxygen, while at a tem- 

 perature of 32.8C. the corresponding amount was but 35 grams. Similar 

 results have been obtained by other observers with different animals. The 

 explanation of these facts is to be found in the increased activity of all 

 physiologic mechanisms coincident with a fall, and in the decreased activity, 

 coincident with a rise in temperature. The leaver ^temperatures act as a 

 stimulus to the peripheral terminations of the nerve systeffiy^bdagiAg about 

 reflexly increased activity of the body at large. , The muscles especially are 

 not onfy reflexly but volitionally excited to greater activity. This leads 

 naturally to an increase in the consumption of oxygen and in the production 

 of carbon dioxid and in the evolution of heat. 



In cold-blooded animals the respiratory exchange is influenced in a 

 manner the reverse of that observed in warm-blooded animals. With a 

 rise of external temperature and a corresponding rise of body-temperature 

 the discharge of carbon dioxid steadily increases. Thus a frog in an atmos- 

 phere at oC. with a body-temperature of iC. discharged per kilogram 

 per hour 4.31 c.c. of carbon dioxid; in an atmosphere of 35C. with a body- 

 temperature of 34C. there was discharged 325 c.c. per kilo per hour. 

 Intermediate temperatures were attended by corresponding increases in 

 the amounts of CO 2 discharged. The reason for this difference in the two 

 classes of animals is probably to be found in the cold-blooded animals, in 

 the want, of a self-adjusting heat-regulating mechanism. 



Age. In early youth, as a result partly of the more pronounced activity 

 of the nutritive energies and partly of a cutaneous surface relatively greater, 

 as compared with the mass of the body, than in adult life, the absorption of 

 oxygen and the discharge of carbon dioxid are greater both absolutely and 

 relatively. Thus, in a boy of nine and a half years with a weight of 22 

 kilograms it was found that in twenty-four hours there was a discharge of 

 carbon dioxid amounting to 488 grams, or 0.92 gram per kilo per hour, and 

 in man with a weight of 65.5 kilograms there was a discharge of 804.72 

 grams, or 0.51 gram per kilo per hour. 



THE NERVE MECHANISM OF RESPIRATION 



The nerve mechanism by which the respiratory muscles are excited to 

 and coordinated in activity is extremely complex and involves the action of 

 both afferent and efferent nerves and their related nerve-centers in the 

 central nerve system. For the free introduction of air into the lungs it is 

 essential that the nasal and laryngeal passages and the cavity of the thorax 

 be simultaneously enlarged. The muscles by which these results are ac- 

 complished have already been mentioned and described. Their simultane- 

 ous and coordinate contraction implies the coordinate activity of nerve- 

 centers and their related motor nerves; thus the action of the nasal and 

 laryngeal muscles (the dilatator naris and the posterior crico-arytenoid) 

 involves the activity of the facial and inferior laryngeal nerves respectively, 

 the centers of origin of which lie in the gray matter beneath the floor of 

 the fourth ventricle; the diaphragm and intercostal muscles involve re- 

 spectively the activity of the phrenic and intercostal nerves, the centers 

 of origin of which lie in the anterior horn of the gray matter of the spinal 

 cord at a level, for the phrenic, of the fourth, fifth, and sixth cervical nerves, 



