312 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



ous diminution or increase of the cerebro-spinal fluid, so that the contents of the 

 cranium are kept uniform in volume. But the recent work of Leonard Hill has 

 shown that the Monro-Kellie doctrine is in the main true. Histological evidence 

 has recently been obtained of the existence of nerve plexuses round the pial 

 arteries. The arteries are muscular, and the nerves therefore are most probably 

 vaso-motor in function. Experimental evidence so far, however, has not estab- 

 lished that the action of these nerves is a marked one* ; the cerebral circulation 

 passively follows the slightest changes in aortic and, more especially, vena cava 

 pressure, and no active vaso-motor change has been conclusively proved. The 

 velocity of blood-flow through the brain is thus influenced markedly by the con- 

 dition of the vessels of the splanchnic area. If the tone of the skeletal muscles and 

 that of the vessels be suddenly inhibited by fear, or temporarily destroyed by shock 

 the blood will drop owing to its weight into the dilated and supported vessels in the 

 most dependent parts of the body. The flow of blood through the brain will, under 

 these conditions, cease, that is to say, so long as the body is in the erect posture. 

 Thus, to restore a fainting person the head must be lowered between the knees. 

 Muscular exercise, by returning blood to the heart from the veins of the lower parts 

 of the body, conduces to the maintenance of an efficient cerebral circulation. 



It is not the volume of the blood but the velocity of flow which is altered in 

 the brain by changes in the general circulation. The brain with its circulating 

 blood almost entirely fills the cranial cavity in the living animal ; that is, there is 

 no more cerebro-spinal fluid there than is sufficient to moisten the membranes. 

 Cerebro-spinal fluid escapes into the veins at any pressure above the cerebral 

 venous pressure ; the tension of this fluid and the pressure in the veins are therefore 

 always the same. The fluid probably transudes from the vascular fringes of the 

 choroid plexuses in the ventricles of the brain, and is absorbed by the pial veins. 

 There is not enough of this absorbable fluid present to allow of more than a slight 

 increase of the volume of blood in the brain. If the aortic pressure rises and the 

 vena cava pressure remains constant the conditions in the brain are as follows : 



More blood in the arteries, less in the veins, increased velocity of flow. 



While if the aortic pressure remains constant and the vena cava pressure rises, 

 the conditions are : 



Less blood in the arteries, more in the veins, diminished velocity of flow. 



The brain presses against the cranial wall with a pressure equal to that in the 

 cerebral capillaries. A foreign body introduced within the cranium, such as a 

 blood-clot or depressed bone, produces local anaemia of the brain, by occupying the 

 room of the blood. So soon as the capillaries are thus obliterated the pressure is 

 raised to arterial pressure. This local increase of cerebral tension cannot be trans- 

 mitted by the cerebro-spinal fluid, because this fluid can never be retained in the 

 meningeal spaces at a tension higher than that of the cerebral veins, but is 

 immediately re-absorbed. The anatomical arrangements of the tentorium cerebelli 

 and the falciform ligaments are such as to largely prevent the transmission through 

 the brain-substance of a local increase of pressure. There is complete pressure 

 discontinuity between the cranial and vertebral cavities. The serious results that 

 follow cerebral compression are primarily due to obliteration of the blood-vessels, 

 and consequent anaemia of the brain. A very small foreign body will, if situated 

 in the region of the bulb, produce the gravest symptoms. For the centres which 

 control the vascular and respiratory systems are rendered anaemic thereby. The 

 cerebral hemispheres may, on the other hand, be compressed to a large extent 

 without causing a fatal result. The major symptoms of compression arise as soon 

 as any local increase of pressure is transmitted to the bulb and causes anaemia 

 there. 



In Erectile Structures. The instances of greatest variation in the quantity of 

 blood contained, at different times, in the same organs, are found in certain 

 structures which, under ordinary circumstances, are soft and flaccid, but, at certain 

 times, receive an unusually large quantity of blood, become distended and swollen 



* The only experimental evidence yet adduced as to the functional activity of these nerves is con- 

 tained in the work of Ferrier and Brodie. They perfused detibrinated blood through a recently excised 

 brain, and found that the addition of adrenalin to the blood always produced constriction of the vessels 

 and a lessened blood flow. 



