PULMONARY CIRCULATION. 103 



Without going fully into the mechanism of erection, it may be stated in 

 general terms that during sexual excitement, or when erection occurs from 

 any cause, the thick, muscular walls of the arteries of supply relax and allow 

 the arterial pressure to distend the capacious vessels lodged in the cells of the 

 cavernous and spongy bodies. This produces the characteristic change in the 

 volume and position of the organ. It is evident that erection depends upon 

 the peculiar arrangement of the blood-vessels, and is not simply a congestion, 

 such as could occur in any vascular part. During erection there is not a 

 stasis of blood ; but if it continue for any length of time, the quantity which 

 passes out of the part 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 minute vessels which have 

 been described as true capillaries. This peculiarity, which had been noted 

 by Todd anJ Bowman, Paget and others, has been studied by Sucquet. By 

 using a black, solidifiable injection, he found that there were 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 are provided 

 with vessels of considerable size, which allowed the fluid injected by the axil- 

 lary artery to pass directly into some of the veins, while ki 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 ends of the ears, parts which are particularly 

 liable to changes in color from congestion of vessels. These observations 

 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 penetrating the true 

 capillaries and thus exerting a modifying influence upon nutrition. 



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 entirely distinct 

 from it, and its muscular walls are very much less powerful. The pulmonary 

 artery has thinner and more distensible coats than the aorta and distributes 

 its blood to a single system of capillaries, situated very near the heart. In 

 the substance of the lungs, the pulmonary artery is broken up into capilla- 

 ries, most of them just large enough to allow the passage of the blood-cor- 

 puscles in a single row. These vessels are provided with a single coat and 

 form a very close net- work surrounding the air-cells. From the capillaries 

 the blood is collected by the pulmonary veins and conveyed to the left auri- 

 cle. There is no great disparity between the arteries and veins of the pul- 

 monary system as regards capacity. The pulmonary veins in the human 

 subject are not provided with valves. 



