30-i THE CIRCULATION. 



the capillary system of the head and neck, to return by the jugulars, 

 will require a longer interval. That, again, which descends by the 

 abdominal aorta and its divisions to the lower extremities, and 

 which, after circulating through the tissues of the leg and foot, 

 mounts upward through the whole course of the saphena, femoral, 

 iliac and abdominal veins, must be still longer on its way ; while 

 that which circulates through the abdominal digestive organs and 

 is then collected by the portal .system, to be again dispersed through 

 the glandular tissue of the liver, requires undoubtedly a longer 

 period still to perform its double capillary circulation. The blood, 

 therefore, arrives at the right side of the heart, from different parts 

 of the body, at successive intervals; and may pass several times 

 through one organ while performing a single circulation through 

 another. 



Furthermore, the chemical phenomena taking place in the blood 

 and the tissues vary to a similar extent in different organs. The 

 actions of transformation and decomposition, of nutrition and secre- 

 tion, of endosmosis and exosmosis, which go on simultaneously 

 throughout the body, are yet extremely varied in their character, 

 and produce a similar variation in the phenomena of the circula- 

 tion. In one organ the blood loses more fluid than it absorbs ; in 

 another it absorbs more than it loses. The venous blood, conse- 

 quently, has a different composition as it returns from different 

 organs. In the brain and spinal cord it gives up those ingredients 

 necessary for the nutrition of the nervous matter, and absorbs cho- 

 lesterine and other materials resulting from its waste ; in the muscles 

 it loses those substances necessary for the supply of the muscular 

 tissue, and in the bones those which are requisite for the osseous 

 system. In the parotid gland it yields the ingredients of the saliva ; 

 in the kidneys, those of the urine. In the intestine it absorbs in 

 large quantity the nutritious elements of the digested food ; and in 

 the liver, gives up substances destined finally to produce the bile, 

 at the same time that it absorbs sugar, which has been produced 

 in the hepatic tissue. In the lungs, again, it is the elimination of 

 carbonic acid and the absorption of oxygen that constitute its prin- 

 cipal changes. It has been already remarked that the temperature 

 of the blood varies in different veins, according to the peculiar 

 chemical and nutritive changes going on in the organs from which 

 they originate. Its color, even, which is also dependent on the 

 chemical and nutritive actions taking place in the capillaries, varies 

 in a similar manner. In the lungs it changes from blue to red ; 



