lo Oct., 1908]. Elements of Animal Physiology. 639 



activity of oxidation in the animal's tissues. The following table gives 

 an idea of the respiratory exchange in a state of rest of a number of 

 animals for 24 hours : — 



But in the same animal the amounts may be greatly increased by exerci.se. 

 Thus, a horse weighing 450kg. has been found to give the following 

 figures : — 



Oxygen absorbed per minute. Carbon-dioxide given out per minute. 



Kest 17 litres ... ... ... i ■ 6 litres 



Ordinary work 15 7 /> ... ... ••• i3-7 " 



Heavy work 293 /' ... ... ... 28 n 



Further, when the animal is exposed to cold greater than can be combatted 

 bv cutting down the heat loss, the proper temperature is maintained by an 

 increa.sed oxidation in the body and thus again the respiratory exchange 

 is increased. The respiratory exchange is also to .some extent dependent 

 on the amount of food. An increase in nutritive material absorbed is 

 always accompanied by increased oxidation and production of carbon- 

 dioxide. 



THE REGULATION OF RESPIRATION. 



The muscles concerned in respiration receive rhythmic impulses from 

 their respective nerves which latter arise from the central nervous system 

 and chieflv from the spinal cord. Thus if the phremc nerve in the neck 

 be cut, the diaphragm is thrown out of action and inspiration must be 

 carried out by the thorax alone. It has been found by experimental in- 

 vestigation that a collection of nerve cells in the medulla oblongata is 

 specially connected with the nerves to the respiratory muscles and is also 

 in communication with a number of afferent or sensory paths and 

 particularlv tho.se in the vagus (loth cranial) nerve. This collection of 

 nerve cells is concerned with the regulation of respiration and it has been 

 called the respiratory centre. When these cells are excited, impulses 

 pass down the appropriate nerves and the respiration increases in rate or 

 depth or what is more common in both together and the ventilation of the 

 lungs is herebv increased. The respiratory centre can be excited in a 

 number of wa\s — • 



I. Bv carbon- dioxide brought by the blood. As has been shown 

 the red blood leaving the lungs still contains carbon-dioxide and it has 

 been found that this amount, when the animal is at rest, is just sufficient 

 to excite the centre to produce quiet breathing. If a man takes .several 

 deep breaths then the breathing may be stopped for a longer period than 

 could normally be borne. This condition is due to the fact that the deep 

 breathing removes more carbon-dioxide than usual and not until this gas has 

 mounted up to a certain concentration will the centre act. When an 

 animal is vigorously exercLsed the breathing gets laboured. This again is 



* kg = Kilogramme = i'2 pounds. 



