8lO PROTECTIVE AND NUTRITIVE APPARATUS OF THE BRAIN. 



in the calvarium, the large amount of blood thrown into the arteries with every 

 systole causes the expulsion of an equal amount of blood from the veins. The 

 act of breathing causes, besides, a respiratory movement of the brain, which is 

 elevated on expiration and falls on inspiration. This movement is due in part 

 to the respiratory fluctuation in pulse and in part to variations in the amount 

 of blood in the veins of the cranial cavity. Finally, there is to be recognized a 

 movement of vascular elevation and depression, occurring from twice to six times 

 in a minute, corresponding to the periodic-regulatory dilatation and contraction 

 of the vessels. This movement is influenced by emotional disturbances. It occurs 

 most regularly during sleep. 



The movements of the brain are apparent especially where its membranes 

 offer slight resistance, therefore, for example, at the fontanels in children and in 

 artificial trephine-openings. The presence of the cerebrospinal fluid is, however, 

 exceedingly important with respect to this movement, probably because it prop- 

 agates the pressure uniformly and, thus, concentrates every systolic and ex- 

 piratory vascular dilatation upon the portion on the calvarium that does not 

 offer resistance. If the fluid is drained away the movement becomes small to 

 the point of disappearance. 



As the arteries within the rigid calvarium undergo change in volume with the 

 movement of the pulse a pulsatory variation in the volume of the veins (sinuses) 

 is constantly observed, the opposite of that in the arteries. Emotional disturb- 

 ances increase the pulsation of the brain. At the moment of awaking the amount 

 of blood in the brain diminishes, while sensorial irritations during sleep, without 

 awakening the subject, increase the amount of blood. In slight degree the brain 

 may undergo passive movement within the cranial cavity on change in the position 

 of the head. 



The vessels of the pia are naturally in part under the influence of the vaso- 

 motor nerves accompanying them; in part their size may be influenced from 

 remote parts of the body. If a trephine-opening be closed by means of a small 

 glass window, the effects upon the lumen of the vessels can be observed with the 

 aid of a microscope. Irritation of the sympathetic affects only the vessels of the 

 same side, but does not alter the blood-pressure upon the other side (through 

 the circle of Willis). Paralysis of the vasomotor nerves, also by means of nar- 

 cotics, causes dilatation of the vessels. The vessels contract strongly in death. 

 They are dilated in connection with cerebral activity, as well as during sleep. 

 Transitory anemia of the cerebral arteries is followed by their secondary dilata- 

 tion and hyperemia. Irritation of the vasomotor center, for example by 

 asphyxia or strychnin or reflexly, causes the presence of an increased amount of 

 blood in the arteries of the central nervous system in consequence of collateral 

 hyperemia. These arteries, therefore, do not take part in the contraction of all 

 of the remaining arteries. Excessive elevation of pressure in the cerebrospinal 

 cavity in consequence of hyperemia is offset by the escape of cerebrospinal fluid 

 into the lymph-sheaths of the cerebrospinal nerves. Cerebral irritation that 

 excites epileptic attacks cause an increased supply of blood independently of the 

 blood-pressure. Sudden ligation of all of the cerebral arteries causes immediate 

 loss of the sensonum, and later on marked irritation of the medulla oblongata 

 and its centers and finally rapid death with convulsions. 



As a result of the free anastomoses at the base, the individual portions of the 



cerebrum are protected against anemia on compression or ligature of one or another 



vessel. Within the cerebrum the arteries are distributed as terminal arteries, 



that is in the area of their terminal distribution they do not form anastomoses 



with neighboring arterial branches. On the other hand, the peripheral arteries 



on the outer surface of the brain, the arteries of the corpus callosum, of the 



lossa of Sylvius, and the deep cerebral, form free anastomoses. The sudden 



assumption of the erect posture by persons who have occupied the recumbent 



position for a long time and are at the same time anemic is not rarely attended 



ith cerebral anemia from hydrostatic causes, associated with loss of conscious- 



s and obscuration of the senses. Alterations in the position of the body have 



herwise no. effect upon the pressure in the cerebral vessels. Death occurs in 



some animals after vertical elevation of the trephined skull and even more quickly 



they are placed upon the centrifuge. Exceedingly severe muscular exertion 



LS well as marked activity on the part of other organs greatly reduce the pressure 



in the carotid arteries. 



Cerebral Pressure. Enclosed within the unyielding calvarium there is on the 

 ; nand the brain together with the nutritive fluid (lymph) that permeates it, 



