114 RESPIRATION. 



belief that the arteries, as their name implies, were air-bearing tubes. This has long 

 engaged the attention of physiologists, who have attempted to explain it by various 

 theories. "Without discussing the views on this subject anterior to our knowledge of the 

 great contractile power of the arteries as compared with other vessels, we may cite the 

 following experiment of Magendie as offering a satisfactory explanation. If the artery 

 and vein of a limb be exposed in a living animal and all the other vessels be tied, com- 

 pression of the artery does not immediately arrest the current in the vein, but the blood 

 will continue to flow until the artery is entirely emptied. The artery, when relieved 

 from the distending force of the heart, reacts on its contents by virtue of its contractile 

 coat and completely empties itself of blood. An action similar to this takes place after 

 death throughout the entire arterial system. The vessels react on their contents and 

 gradually force all the blood into and through the capillaries, which are very short, to 

 the veins, which are capacious, distensible, and but slightly contractile. This begins 

 immediately after death, while the irritability 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 cannot 

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

 capillaries of dependent parts of the body. 



CHAPTER IV. 



RESPIRA TION RESPIRA TOR F M VEMENTS. 



General considerations Physiological anatomy of the respiratory organs Eespiratory movements of the larynx- 

 Epiglottis Trachea and bronchial tubes Parenchyma of the lungs Movements of respiration Inspiration- 

 Muscles of inspiration Expiration Influence of the elasticity of the pulmonary structure and walls of the chest 

 upon expiration Muscles of expiration Action of the abdominal muscles in expiration Types of respiration 

 Frequency of the respiratory movements Eelations of inspiration and expiration to each other The respiratory 

 sounds Capacity of the lungs and the quantity of air changed in the respiratory acts Kesidual air Eeserve 

 air Tidal, or breathing air Complemental air Extreme breathing capacity Eelations 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 dif- 

 ferences 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 specimen of true capillary blood. In the capillaries, how- 

 ever, the nutritive fluid, which is identical in all parts of the arterial system, under- 

 goes a remarkable change, which renders it unfit for nutrition. Thus modified it is 

 known as venous blood ; and, as we have seen, 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 materials it has collected 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 cor- 

 responding variations in the composition of the blood throughout the venous system. 



The important principles which 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 

 its corpuscular elements. 



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

 tively-mechanical process of circulation ; but it is no less necessary to the nutritive pro- 

 cesses that carbonic acid, which the blood acquires in the tissues, should be given off. 



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

 organs receive and appropriate oxygen. 



