RESPIRATION. 415 



time were for oxygen 13.9 grams and for carbon dioxid 13.12 grams. Lavoi- 

 sier and Sequin, having reference only to the oxygen, found that a man 

 at a temperature of 15 C. consumed 38.31 grams of oxygen, while at a tem- 

 perature of 32.8 C. 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 lower temperatures act as a 

 stimulus to the peripheral terminations of the nerve system, bringing about 

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

 not only 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 atmo- 

 sphere at o C. with a body-temperature of i C. discharged per kilogram 

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

 temperature of 34 C. there was ^a 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 

 action is extremely complex and involves the action of both afferent and effer- 

 ent 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 accomplished have already been 

 mentioned and described. Their simultaneous 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 respectively the activity of the phrenic and 

 intercostal nerves, the centers of origin of which lie in the anterior horn of 



