82 CIRCULATION OF THE BLOOD 



When the pressure of blood in the arteries going to the brain is in- 

 creased or when there is an obstacle to the return of blood by the veins, 

 more or less congestion takes place, and the blood forces the liquid 

 from the cranial cavity into the spinal canal. The reverse takes place 

 when the supply of blood to the brain is diminished. 



The influence of gravity on the cerebral circulation may be consider- 

 able, as is shown by the following experiment : If an ordinary " hutch " 

 rabbit is held by the ears with the body dependent, the supply of blood 

 to the brain becomes so far reduced that the animal soon becomes un- 

 conscious and will die if kept in this position for half an hour ; but if 

 placed in the horizontal position, it is soon restored to consciousness. 

 This is due to large accumulation of blood in the pendulous abdominal 

 cavity and a consequent deficient supply to the encephalon. 



Circulation in Erectile Tissues. In the organs of generation of both 

 sexes, there exists a tissue that is subject to increase in volume and hard- 

 ness when in a condition of what is called erection. The parts in which 

 the erectile tissue exists are, in the male, the corpora cavernosa of the 

 penis, the corpus spongiosum and the glans penis ; and in the female, 

 the corpora cavernosa of the clitoris, the gland of the clitoris and the 

 bulb of the vestibule. 



The vascular arrangement in erectile organs is peculiar 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 layer, 

 after they have entered the organ, do not simply branch and subdivide, 

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

 which immediately become tortuous and are distributed in the cavernous 

 and spongy bodies in anastomosing vessels, which have but a single thin 

 homogeneous coat. These vessels are larger, even, than the arterioles 

 which supply them with blood, some having a diameter of ^ to -^y of an 

 inch ( i to 1.5 millimeters). The cavernous bodies have an external invest- 

 ment of strong fibrous tissue of considerable elasticity, which sends bands, 

 or trabeculse, into the interior, by which they are divided up into cells. 

 The trabeculae are composed of fibrous tissue mixed with a large number of 

 non-striated muscular fibres. These cells lodge the bloodvessels, 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 trabeculae are 

 more delicate and the cells are smaller. 



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 cav- 

 ernous and spongy bodies. This produces the characteristic change in 



