CIRCULATION IN SPECIAL PARTS 239 



The Cerebral Circulation. The circulation of the brain is some- 

 what peculiar, since this organ is enclosed in a rigid bony covering. 

 The limbs, glands, and viscera, are enclosed in connective-tissue 

 sheaths, but can expand when the blood-pressure rises ; the expansion 

 of the brain, on the other hand, is confined. The circulation in the 

 marrow of bones resembles that in the brain and spinal cord. 



In 1783, Alexander Monro the younger put forward the view that 

 the quantity of blood within the cranium is almost invariable. " For 

 being enclosed in a case of bone," he writes, " the blood must be 

 continually flowing out of the veins, that room may be given to the 

 blood which is entering by the arteries. For as the substance of the 

 brain, like that of the other solids of our body, is nearly incompressible, 

 the quantity of blood within the head must be the same at all times, 

 whether in health or disease, in life or after death, those cases only 

 excepted in which water or other matter is effused or secreted from 

 the bloodvessels ; for in these a quantity of blood equal in bulk to the 

 effused matter will be pressed out of the cranium." 



These facts are confirmed by experiment. If a glass plate be 

 screwed into a trephine hole, on compressing the innominate and 

 subclavian arteries, the pial arteries can be seen to become less in 

 size. The brain, however, does not collapse or retreat frpm the glass 

 window. If the arteries empty, the veins fill. If, on the other hand, 

 the glass window be faultily placed, and allow leakage into the cranial 

 cavity, air passes within, and the brain collapses under atmospheric 

 pressure. This experiment proves that the brain in the closed 

 cranium can by no means completely empty itself of blood, even 

 though the arterial pressure should fall to zero. 



Similarly, if an animal be placed in the vertical feet-down position, 

 and the skull be trephined, then, on opening the dura mater, the brain, 

 which before was in close apposition with that membrane, can be 

 seen collapsing, as it is emptied of blood by atmospheric pressure. 

 The quantity of cerebro -spinal fluid which moistens the surface of the 

 brain is not large, and the blood-content of the brain can vary suddenly 

 only to a slight degree by displacement of the cerebro-spinal fluid. 



No sure evidence of the condition of the cerebral circulation can 

 be drawn from examination of the brain after death, for in many 

 different ways the relative volume of blood and the serous fluid 

 within the cranium may be altered by post-mortem changes. By 

 slow changes there can come about more blood and less tissue fluid 

 and brain substance in the skull, or less blood and more brain sub- 

 stance and fluid. In inflammatory states brain substance may 

 undergo lysis, and be carried away by the blood -stream, and the 

 bloodvessels dilate and hold more blood. The balance between 

 volume of brain substance and blood must continually vary with the 

 metabolism of this organ. 



The conditions affecting the cerebral circulation may be studied 

 by simultaneously recording (1) the aortic pressure, (2) the vena'cava 

 pressure, (3) the intracranial pressure, (4) the cerebral venous pressure 

 the cranium being, as in the normal condition, a closed cavity. 



