CH. XXII.] THE CIRCULATION IN THE BRAIN 313 



to advance the opinion that the quantity of blood in the brain must be the same at 

 all times. This doctrine has been frequently disputed, and many have advanced 

 the theory that increase or diminution of the blood is accompanied with simultane- 

 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 these are unduly dilated, the blood- 

 flow through the brain may be so reduced as to lead to fainting. 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 rills 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 lymphatics, and possibly also into the veins, 

 at any pressure above the cerebral venous pressure ; the tension of this fluid and 

 the pressure in the veins are therefoie always the same. The fluid is secreted by 

 the epithelium covering the vascular fringes of the choroid plexuses in the ven- 

 tricles of the brain (see p. 172), and is absorbed mainly by the lymphatics. 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 reabsorbed. 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 

 by it, and pass into the state which has been termed erection. Such structures are 

 the corpora cavernosa and corpus sponyiosum of the penis in the male, and the 

 clitoris in the female ; and, to a less degree, the nipple of the mammary gland in 



