222 THE PHYSIOLOGY OF THE NATURE 



expel their contents ; the diaphragm relaxes, and the ab- 

 dominal muscles contract, and the contents of the abdomen 

 are forced forward : the intercostal muscles also cease their 

 expanding action, and the capacity of the chest is diminished, 

 by which the air is forced out of it. The necessity there 

 exists for air or breath to support life causes the muscles to 

 act once more ; the lungs again expand, and in this manner 

 respiration is carried on ; both these actions, under ordinary 

 circumstances, being repeated about eight times per minute. 

 There is, indeed, a certain proportion, between the action 

 of the heart and that of respiration ; the pulse throbbing 

 about 4^ times to every breath inhaled and exhaled. If 

 more blood be sent to the heart by the stimulus of exercise, 

 &c., the respiration will be also accelerated : thus, in quick 

 motion, or great exertion, there is panting or quick breath- 

 ing ; the air being suddenly expelled, and as quickly in- 

 spired, that no obstacle may be offered to the passage of the 

 blood. 



We have spoken of the intercostals, the diaphragm, and 

 abdominal muscles, as the respiratory muscles, and un- 

 doubtedly they are the chief agents in breathing. There 

 are, however, auxiliary muscles, w^hich, if they were here 

 recounted, would be but seldom read ; as the mere list of 

 names would necessarily be without interest. Suffice it 

 then to state, that ev^ery muscle, going to or arising from 

 the ribs, is more or less concerned in respiration. 



THE PHYSIOLOGY OF THE NATURE AND PROPERTIES OF THE 



BLOOD. 



In essential characters, blood is the same in all animals : 

 it is invariably red in the higher orders, though the intensity 

 of its hue varies with circumstances ; the principal instance 

 of which is that afforded betw^een the venous and arterial ; 

 while it is, in the former, almost purple, it becomes, in the 

 latter, of a bright scarlet colour. — The specific gravity of the 

 blood is very differently estimated, it being subject to in- 

 crease and decrease at different times. Disease and emacia- 

 tion make it lighter, while in the contrary states, it is found 

 to be heavier. If water be estimated at 1000, the specific 

 gravity of blood may be reckoned at 1050; from which it 

 may, however, be increased to 1120. Venous blood is 



