108 CIRCULATION OF THE BLOOD. 



ovary of the human female. He has shown by injections that the uterus is capable of 

 erection like the penis. In some other parts, such as the nipple and the mucous mem- 

 brane of the vagina, which are sometimes described as erectile, the peculiar vascular 

 arrangement which is characteristic of true erectile tissues is not found. In the nipple, 

 the hardness which follows gentle stimulation is simply the result of contraction of the 

 smooth muscular fibres with which this part is largely supplied, and it is analogous to the 

 elevation of the follicles of the skin from the same cause, in what is called goose-flesh. 

 In the vagina, congestion may occur, as in other mucous membranes, but there is no 

 proper erection. 



The vascular arrangement in erectile organs, of which the penis may be taken as the 

 type, is peculiar to them and is not found in any other part of the circulatory system. 

 Taking the penis as an example, the arteries, which have an unusually-thick muscular 

 coat, after they have entered the organ, do not simply branch and divide dichotomously, 

 as in most other parts, but send off large numbers of arborescent branches, which imme- 

 diately become tortuous and are distributed in the cavernous and spongy bodies in nu- 

 merous anastomosing vessels, with but a single thin, homogeneous coat, like the true 

 capillaries. These vessels are larger, even, than the arterioles which supply them with 

 blood, some having a diameter of from oV to T 4 T of an inch. The cavernous bodies have 

 an external investment of strong fibrous tissue of considerable elasticity, which sends 

 bands, or trabeculse, into the interior, by which it is divided up into cells. The trabeculse 

 are composed of fibrous tissue mixed with a large number of smooth muscular fibres. 

 These cells lodge the blood-vessels, which ramify in the tortuous manner already indicated 

 and finally terminate in the veins. The anatomy of the corpora spongiosa is essentially 

 the same, the only difference being that the fibrous envelope and the trabeculse are more 

 delicate and the cells are of smaller size. 



Without going fully into the mechanism of erection, which comes more properly 

 under the head of generation, 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 pf 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 di- 

 rect 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 capilla- 

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

 has been closely studied by M. Sucquet, who was first led to investigate the subject by 

 noticing that by injecting a very small quantity of fluid, entirely insufficient to fill all 

 the capillaries of a member, it was returned by certain of the veins. On using a black, 

 solidifiable injection, he found that there were certain parts of the upper and lower ex- 

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

 not penetrated. Following this out by dissection, he showed that, in the upper extrem- 

 ity, the skin of the fingers and part of the palm of the hand and the skin over the ole- 

 cranon are 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 analo- 

 gous 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. 



