THE CEREBRAL CIRCULATION. 143 



horizontally and the membrane is flat, is marked on the glass tube. On 

 screwing in the apparatus, the air index, which is at first forced out, is again, 

 by raising the pressure bottle, brought to the normal position. That is to say, 

 the brain, which is slightly bulged outwards by the intracranial pressure, 

 is forced into that flattened position, which it naturally must assume against 

 the closed cranial wall. The elevation of the manometer, consequent on this 

 procedure, gives the normal intracranial tension. The pressure of the cranial 

 contents against the skull wall is exactly balanced, and the skull becomes once 

 more a closed cavity. The air index in the " cerebral pressure gauge " exhibits 

 cardiac and respiratory undulations of intracranial pressure. 



The brain pulsates in the direction where the resistance is least. Thus, 

 when the cranium is trephined, and the dura opened, the pulse can scarcely be 

 seen in the occipito-atlantal membrane, for the trephine hole has become the 

 seat of least resistance. Similarly, when the trepbine hole is closed by a glass 

 window, the occipito-atlantal membrane becomes the seat of least resistance, 

 and the pulsation appears there. 



The diastole and systole of the brain are made possible in the unyielding 

 box of the cranium in two ways — • 



1. By the ebb and flow of the cerebrospinal fluid. The occipito-atlantal 

 and other vertebral ligaments extend in cerebral diastole, and allow the fluid 

 to escape from the cranial cavity, while in systole, through the elasticity of these 

 ligaments coming into play, it is driven back. 



2. By expression of the blood in the cerebral veins another mechanism is 

 provided. The cerebral venous pressure can be studied in the following way : — 

 In the dog, the superior longitudinal sinus, with its tributaries, opens into 

 a large venous cavity within the occipital protuberance. This bony cavity is 

 the torcular Herophili. The transverse sinuses lie within the osseous part of 

 the tentorium cerebelli, and lead out of this cavity. Part of the blood finds 

 its exit by the post-glenoid foramen, and reaches the external jugular vein, 

 but a large portion passes into the large sinuses which run down the vertebral 

 canal. It is quite simple to drill a hole into the torcular, and then immediately 

 to screw into the hole a brass tube filled with magnesium sulphate solution. 

 This tube is connected with a manometer containing MgS0 4 solution. 



The manometer can be directly observed, or, if connected with a delicate 

 tambour, its movements can be recorded. In this way a continuous record of 

 cerebral venous pressure can be obtained with little dissection and considerable 

 ease. The manometer exhibits all the cardiac and respiratory pulsations, the 

 latter in a marked way. 



The brain is an organ which pulsates with every stroke of the 

 heart, and with every respiratory movement, and, owing to its peculiar 

 conditions, enclosed as it is in the unyielding cranium, the cardiac pulse 

 is not entirely spent in distending the arterioles and capillaries, but is 

 transmitted to the venous sinuses. The brain is lifted up by the stroke 

 of the arteries at its base, and is thrown against the pial veins and the 

 venous sinuses. By rise of pressure in the thoracic veins in expiration, 

 the blood is dammed back into the sinuses, and the brain must then 

 expand more, at the expense of the cerebro-spinal fluid. The contrary 

 occurs in inspiration ; the veins are emptied by the suction action of 

 the thorax, the brain collapses, and the cerebro-spinal fluid re-enters the 

 cranial cavity. Normally, the brain is more affected by the expiratory 

 rise of general venous pressure ; this is in contrast with such an organ 

 as the kidney, which follows the inspiratory rise of general arterial 

 pressure. If the inspiratory arterial rise of cerebral capillary tension 

 becomes in any way increased, so as to be greater than the expiratory 

 venous rise, then the respiratory curve of the brain is reversed. The 



