CIRCULATION IX THE CRANIAL CAVITY. 107 



they contain. The question then arises as to what is displaced to make room for 

 the blood in congestion, and what supplies the place of the blood in anaemia. An 

 anatomical peculiarity, which has not yet been considered, offers an explanation of 

 these phenomena. Magendie has shown, by observations on living animals, confirmed 

 by dissections of the human body, that between the pia mater and the arachnoid of the 

 brain and spinal cord there exists a liquid, the cephalo-rachidian fluid, which is capable 

 of passing from the surface of the brain to the spinal canal and communicates with the 

 fluid in the ventricles. This he has conclusively demonstrated to be situated, not 

 between the layers of the arachnoid, as was supposed by Bichat, but between the inner 

 layer of this membrane and the pia mater. The communication between the cranial 

 cavity and the spinal canal is very free. This has been demonstrated by exposing the dura 

 mater of the brain and of the cord, making an opening in the membranes of the cord so 

 as to allow the liquid to escape (which it does in quite a forcible jet), when pressure on the 

 membranes of the brain not only accelerated the flow but pressed out a quantity of the liquid 

 after all that would escape spontaneously had been evacuated. It is easy to see one of 

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

 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 taking place when the supply of blood to the brain is diminished. 

 The functions of all highly-organized and vascular parts seem to require certain varia- 

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

 varied conditions of activity and repose, is any exception to this general rule, although 

 the physiological conditions of its vascularity are not easily studied. 



Physiologists, even before the time of Haller, had noticed alternate movements 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 expiration, which, in violent efforts, is sometimes so considerable as to produce 

 protrusion, and contraction with inspiration. Magendie also studied these movements, 

 which he explained in the following way : 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 mate- 

 rially aided, and the brain is in some degree relieved of this expanding force. 



Robin, His, and others have noted a peculiarity in the small vessels of the brain, 

 spinal cord, and pia mater, which is curious, but the physiological significance of which is 

 not yet apparent. These vessels are surrounded by a thin, amorphous sheath, which has 

 a diameter of from y^j- to -^^ of an inch greater than that of the vessel itself. Be- 

 tween this and the blood-vessel is a transparent liquid. This structure, which has been 

 observed in no other part of the circulatory system, is regarded by Robin as the com- 

 mencement of the lymphatics of the nervous centres. What effect this disposition of 

 the vessels may have upon the facility with which they may become dilated or contracted, 

 it is difficult to determine. 



Circulation in Erectile Tissues. In the organs of generation of both sexes, there 

 exists a tissue which is subject to great increase in volume and rigidity when in a state 

 of what is called erection. The parts in which the erectile tissue exists are, in the male, 

 the corpora cavernosa of the penis, the corpora spongiosa, with the glans penis; and, in 

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

 of the vestibule. In addition, Rouget has lately demonstrated the presence of a struct- 

 ure analogous to erectile tissue in the body of the uterus and in a bulb annexed to the 



