THE HUMAN BODY. 



lungs, or from the lungs to the heart, that the vessels of the 

 pulmonary circulation owe their names. The pulmonary 

 artery, like the aorta, is provided at its orifice with three 

 valves, called sigmoid or semi-lunar valves. From the right 

 ventricle to the ramifications of the pulmonary artery the 

 blood has but a short distance to pass, and it meets with no 

 resistance at all comparable to that encountered in the sys- 

 temic arterial circulation. The walls of 

 the right ventricle therefore are much 

 thinner than those of the left, and con- 

 sequently have less power. In the ca- 

 pillaries of the lungs the motion of the 

 blood varies in quickness according as 

 the respiration is easy, or retarded by 

 any obstacle, or by the presence of air 

 unfit for the performance of the respi- 

 ratory functions. The capillaries are 

 distributed through the substance of the 

 lungs in such a manner that they cor- 

 respond to the pulmonary cells. (See 

 Respiration, p. 92.) It is in these ulti- 

 mate divisions of the lungs that the oxy- 

 gen of the air combines with the venous 

 blood charged with carbonic acid, and 

 transforms it into arterial blood. The 

 reddish-brown globules of the venous 

 _____ o ____^ _._ blood take a vermilion colour from 

 gram of the course 'of the CO ntact with the oxygen, and charged 



blood in the circulation. . . 1111 



with oxygen, the blood penetrates into 



, ,'. , r -\ 



the radicles of the pulmonary veins, 

 obeying the original impulse, the vis a 

 tergo, as in the general venous system, 

 but with greater quickness. It is now carried back to the 

 left auricle, which immediately transmits it to the ventricle, 

 where its circular course ends, only to commence again 

 immediately. The circulation may be divided into two 

 simultaneous periods, or, as has been already stated, the ima- 

 ginary circle through which the blood passes is composed of 

 two unequal segments described by the fluid column. The 



Fig. 28. Imaginary dia- 

 gram of the course of th 

 ilood in the circulation. 



A. Course of the venous 

 of 



arterial 



