CENTRAL NERVOUS SYSTEM. 735 



histological, is against the existence of vaso-motor nerves in the vessels of the 

 pia or of the encephalon and cord (see Circulation). 1 



The circulation in these regions, therefore, is not modified by any reflex varia- 

 tions in the calibre of the vessels. The authors just cited do not find any evi- 

 dence for a local control of the arterioles whereby the products of nerve-cell 

 activity cause an increase in the diameter of the vessels affected by these sub- 

 stances. 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 

 encephalon. It also causes a rise in the venous pressure, and with this a 

 corresponding rise in the intracranial pressure, the last two varying in the 

 same sense and to the same extent. 



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 balanced 

 on a transverse axis, this axis is about 8.77 centimeters 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 oppor- 

 tunity for the escape of a small quantity of fluid through the foramen magnum 

 into the vertebral canal. When, as the result of increased arterial pressure, 

 the brain has increased so as to drive out the subdural fluid, the brain is forced 

 against the walls of the cranium and blocks the outflow into the spinal canal. 

 In the same way it has been found that if a mass displacing from 2-3 cu.cm. 

 be introduced into the subdural space of a dog the brain will adjust itself with- 

 out rise of intracranial pressure. If in this case the volume of the mass intro- 

 duced 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 

 fontanelles are open the brain normally pulsates, and we recognize in its varia- 

 tions in volume all the different variations in blood-pressure with which we are 

 familiar. The pulsation of the brain is doubtless an important aid to the 

 movements of the fluids within and hence tends to facilitate nutrition during 

 the earlier periods of growth. 



In pathological cases where the cranial wall has been destroyed, there is 

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



1 Bayliss, Hill, and Gulland: Journal of Physiology, 1895, vol. xviii. 



2 Hill : Journal of Physiology, 1895, vol. xviii. 



3 W. und Ed. Weber: Mechanik der menschlichen Gehwerkzeuge, 1836. 



