108 RESPIRATION RESPIRATORY MOVEMENTS. 



short, to the veins, which are capacious, distensible and but slightly con- 

 tractile. This begins immediately after death while the contractility of the 

 muscular coat of the arteries remains, and is seconded by the subsequent 

 cadaveric rigidity, which affects all the involuntary as well as the voluntary 

 muscular fibres. Once in the venous system, the blood can not return on 

 account of the valves. Thus, after death, the blood is found in the veins and 

 capillaries of dependent parts of the body. 



CHAPTEE IV. 



RESPIRA TION RESPIRA TORY MO VEMENTS. 



General considerations Physiological anatomy of the respiratory organs Movements of respiration Inspi- 

 rationMuscles of inspiration Expiration Muscles of expiration Types of respiration Frequency 

 of the respiratory movements Relations of inspiration and expiration to each other Respiratory 

 sounds^-Capacity of the lungs and the quantity of air changed in the respiratory acts Residual air 

 Reserve air Tidal, or breathing air Complemental air Extreme breathing capacity Relations in 

 volume of the expired to the inspired air Diffusion of air in the lungs. 



THE characters of the blood are by no means identical in the three great 

 divisions of the vascular system ; but physiologists have thus far been able to 

 investigate only the differences which exist between arterial and venous blood, 

 for the capillaries are so short, communicating directly with the arteries on 

 the one side and the veins on the other, that it is impossible to obtain a speci- 

 men of true capillary blood. In the capillaries, however, the nutritive fluid, 

 which is identical in all parts of the arterial system, undergoes changes which 

 render it unfit for nutrition. Thus modified it is known as venous blood ; 

 and the only office of the veins is to carry it back to the right side of the 

 heart, to be sent to the lungs, where it loses the vitiating substances it has col- 

 lected in the tissues, takes in a fresh supply of oxygen and goes to the left, 

 or systemic heart, again prepared for nutrition. As the processes of nutrition 

 vary in different parts of the organism, there are of necessity corresponding 

 variations in the composition of the blood in different veins. 



The important substances that are given off by the lungs are exhaled 

 from the blood ; and the gas which disappears from the air is absorbed by 

 the blood, mainly by the red corpuscles. 



A proper supply of oxygen is indispensable to nutrition and even to the 

 comparatively mechanical process of circulation ; but it is no less necessary 

 to the nutritive processes that carbon dioxide, which the blood acquires in 

 the tissues, should be removed. 



Respiration may be defined strictly as the process by which the various 

 tissues and organs receive and appropriate oxygen. 



As it is almost exclusively through the blood that the tissues and organs 

 are supplied with oxygen, and as the blood receives and exhales most of the 

 carbon dioxide, the respiratory process in the lungs may be said to consist 



