PULMONARY CIRCULATION 83 



the volume and position of the organ. It is evident that erection de- 

 pends on the peculiar arrangement of the bloodvessels, and is not simply 

 a congestion, such as might occur in any vascular part. During erection 

 there is not a stasis of blood ; but if it continues for a certain time, the 

 quantity passing out by the veins must be equal to that which passes in 

 by the arteries. 



Derivative Circulation. In some parts of the circulatory system, 

 there exists a direct communication between the arteries and the veins, 

 so that all the blood does not necessarily pass through the true capillaries. 

 This peculiarity, which had been noted by Todd and Bowman, Paget and 

 others, has been closely studied by Sucquet. By using a black solidifi- 

 able injection, he found certain parts of the upper and lower extremities 

 and the head, which became colored by the injection while other parts 

 were not penetrated. Following the vessels by dissection, he showed 

 that in the upper extremity, the skin of the fingers and part of the palm 

 of the hand and the skin over the olecranon were provided with vessels of 

 considerable size, which allowed the fluid injected by the axillary artery 

 to pass directly into some of the veins, while in other parts the veins were 

 entirely empty. Extending his researches to the lower extremity, he 

 found analogous communications between the vessels in the knee, toes 

 and parts of the sole of the foot. He also found communications in the 

 nose, cheeks, lips, forehead and tips of the ears, parts that are peculiarly 

 liable to changes in color from congestion of vessels. These observa- 

 tions have been in the main confirmed by the more recent researches of 

 Hoyer. It is evident that under certain conditions a larger quantity of 

 blood than usual may pass through these parts without necessarily pene- 

 trating the capillaries. 



Pulmonary Circulation. The vascular system of the lungs merits 

 the name, which is frequently applied to it, of the lesser circulation. 

 The right side of the heart acts simultaneously with the left, but is 

 anatomically distinct from it, and its muscular walls are less powerful. 

 The pulmonary artery has thinner and more distensible walls than the 

 aorta and distributes its blood to a single system of capillaries, situated 

 but a short distance from the heart. In the lungs, the pulmonary artery is 

 broken up into capillaries, most of them just large enough to allow the pas- 

 sage of blood-corpuscles in a single row. These vessels are provided with 

 a single coat and form a close network surrounding the air-cells. From 

 the capillaries the blood is collected in the pulmonary veins and is carried 

 to the left auricle. There is no great disparity between the arteries and 

 veins of the pulmonary system as regards capacity. The pulmonary 

 veins in the human subject have no valves. 



The blood in its passage through the lungs does not meet with the 



