CENTRAL NERVOUS SYSTEM. 287 



supply to the pial vessels in man probable. Nevertheless the efficiency of 

 this vaso-motor mechanism does not appear to be great, since various authors 

 fail to find physiological evidence for a local control of the arterioles. 



The reactions of the central vessels are broadly those of a system of elastic 

 tubes in a closed cavity. As a result, it is found that the quantity of blood 

 in the central system is subject to very slight variations only. A rise in the 

 arterial pressure causes a more rapid flow of the blood through the encepha- 

 lon. It also causes a rise in the venous pressure, and with this a correspond- 

 ing rise in the intracranial pressure, the last two varying in the same sense 

 and to the same extent. 1 



The flow through the central system is subject to the influence of gravity. 

 and takes place the more readily the more the resistance is diminished. 2 

 The principal controlling mechanism is in the splanchnic area. According 

 to the condition of the vessels in this area, the intracranial blood-pressure 

 varies. 



It is to be noted in passing that when a person lying on a table is bal- 

 anced on a transverse axis, this axis is about 8.77 em. to the cephalic side of 

 the line which joins the heads of the femurs. 3 This leaves, of course, the 

 splanchnic area mainly on the cephalic side of this axis, and hence any inflow 

 of blood from the extremities would tend to make the head end of the person 

 thus balanced dip down. This dip will occur even when the splanchnic area 

 alone is filled, and hence the dipping as such would not necessarily indicate 

 an increase in the quantity of blood in the encephalon. 



In the adult the cranial cavity is almost rigidly closed. There is an 

 opportunity for the escape of a small quantity of cerebro-spinal fluid through 

 the foramen magnum into the vertebral canal. When, as the result of in- 

 creased arterial pressure, the brain has distended so as to drive out the sub- 

 dural fluid, the brain is forced against the walls of the cranium and blocks 

 the outflow into the spinal canal. On the other hand, it has been found 

 that if a mass displacing from 2 to 3 c.c. be introduced into the subdural 

 space of a dog, the brain will adjust itself without rise of intracranial pres- 

 sure. If in this case the volume of the mass introduced is increased, there 

 follows a rise of intracranial pressure, and this rise in every instance tends to 

 impede the circulation through the brain. While the lbntanelles are open 

 the brain normally pulsates, and we recognize in its variations in volume :ill 

 the different variations in blood-pressure with which we are familiar. The 

 pulsation of the brain is doubtless an important aid to the movements «>f the 

 fluids within, and hence tends to facilitate nutrition during the earlier Btages 

 of growth. 



In pathological eases where the cranial wall has been destroyed there is 

 a similiar variation in volume to be observed in the adult, and it is possible 

 that the beneficial effects which in so many instances follow trephining of 



1 Howell: American Journal of I'livsinh^n/, lS'JS, i. \ () . 1. 



1 Hill : Journal >>f Physiology, 1895, v<>l. xviii. 



3 W. und Ed. Weber: Mechanikder memchlichen Qehwerkzeuge, 183G. 



