102 CIECULATION OF THE BLOOD IN THE VESSELS. 



physiological uses of this liquid. When the pressure of blood in the arteries 

 going to the brain is increased or when there is an obstacle to its return by 

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

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

 supply of blood to the brain is diminished. The physiological action of all 

 highly organized and vascular parts seems to require certain variations in the 

 supply of blood ; and there is no good reason to suppose that the brain, in its 

 varied conditions of activity and repose, is an exception to this general rule. 



Physiologists, even before the time of Haller, had noticed alternate move- 

 ments of expansion and contraction in the brain, connected with the acts of 

 respiration. This is observed in children before the fontanels are closed, and 

 in the adult when the brain is exposed by an injury or a surgical operation. 

 The movements are an expansion with the act of respiration, which, in vio- 

 lent efforts, is sometimes so considerable as to produce cerebral hernia, and 

 contraction with inspiration. With the act of expiration the flow of blood in 

 the arteries is favored and the current in the veins is retarded. If the effort 

 be violent, the valve at the opening of the internal jugular may be closed. 

 This act would produce an expansion of the brain, not from reflux by the 

 veins, but from the fact that the flow into the chest is impeded, and the 

 blood, while passing in more freely by the arteries, is momentarily confined. 

 With inspiration the flow into the thorax is materially aided, and the brain 

 is in some degree relieved of this expanding force. 



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

 sexes, there exists a tissue which is subject to increase in volume and rigidity 

 when in a condition of what is called erection. The parts in which the erect- 

 ile tissue exists are, in the male, the corpora cavernosa of the penis, the cor- 

 pus spongiosum and the glans penis ; and in the female, the corpora caver- 

 nos, of the clitoris, the gland of the clitoris and the bulb of the vestibule. 



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

 taken as the type, is peculiar and is not found in any other part of the circu- 

 latory 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 immediately become tortuous 

 and are distributed in the cavernous and spongy bodies in anastomosing ves- 

 sels, 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 ^ to ^ of an inch (1 to 1'5 mm.). 

 The cavernous bodies have an external investment of strong, fibrous tissue 

 of considerable elasticity, which sends bands, or trabeculae, into the inte- 

 rior, by which it is divided up into cells. The trabeculaa are composed of 

 fibrous tissue mixed with a large number of non-striated 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 trabeculae are more delicate and the cells are smaller. 



