

THE CEREBRUM, AND ITS FUNCTIONS. 747 



include an amount of nervous matter, which is four times that of all the rest 

 of the Cranio-Spinal mass, more than eight times that of the, Cerebellum, thir- 

 teen times that of the Medulla Oblongata, &c., and tiventy-four times that of 

 the Spinal Cord. The average weight of the whole Encephalon, in proportion 

 to that of the body, in Man, taking the average of a great number of observa- 

 tions, is about 1 to 36. This is a much larger proportion than that which ob- 

 tains in most other animals ; thus the average of Mammalia is stated by M. 

 Leuret to be 1 to 186, that of Birds 1 to 212, that of Reptiles 1 to 1321, and 

 that of Fishes 1 to 5668. It is interesting to remark, in reference to these 

 estimates, that the Encephalic prolongation of the Medulla Oblongata in Man 

 (being about one-sixteenth of the weight of the whole Encephalon) is alone 

 more than twice as heavy in proportion to his body, as the entire Encephalon of 

 Reptiles, and ten times as heavy as that of Fish. But there are some animals 

 in which the weight of the Encephalon bears a higher proportion to that of the 

 body than it does in Man ; thus in the Blue-headed Tit, the proportion is as 1 

 to 12, in the Goldfinch as 1 to 24, and in the Field-Mouse as 1 to 31. It does 

 not hence follow, however, that the Cerebrum is larger in proportion ; in fact, 

 it is probably not nearly so large ; for in Birds and Rodent Mammals, the Sen- 

 sory Ganglia form a very considerable proportion of the entire Encephalon. 

 The importance of distinguishing between the several parts of this mass, which 

 are marked out as distinct, alike by their structure and connections, as by the 

 history of their development, has not been by any means sufficiently attended to. 

 778. The Encephalon altogether receives a supply of Blood, the amount of 

 which is very remarkable, when its comparative bulk is considered ; the propor- 

 tion which goes to it being, according to the estimate of Haller, as much as one- 

 fifth of the whole. The manner in which this blood is conveyed to the brain, 

 and the conditions of its distribution, offer some peculiarities worthy of notice. 

 The two Vertebral and two Carotid arteries, by which the blood enters the 

 cavity of the cranium, have a more free communication by anastomosis than 

 any similar set of arteries elsewhere ; and this is obviously destined to prevent 

 an obstruction in one trunk from interrupting the supply of blood to the parts 

 through which its branches are chiefly distributed the cessation of the circu- 

 lation through the nervous matter being immediately productive (as formerly 

 shown, 355) of suspension of its functional activity. Not only must there 

 be a sufficient supply of blood, but it must make a regulated pressure on the 

 walls of the vessels. Now the Encephalon is differently circumstanced from 

 other vascular organs, in being inclosed within an unyielding bony case ( 533) ; 

 and we find a special provision for equalizing the bulk of the contents of this 

 cavity, and for counterbalancing the results of differences in the functional ac- 

 tivity of the brain and in its supply of blood, in the existence of a fluid which 

 is found beneath the arachnoid, both on the surface of the brain and spinal cord, 

 and in the ventricles of the former. The amount of this " cerebro-spinal fluid" 

 seems to average about two ounces j but in cases of atrophy of the brain, as 

 much as twelve ounces of fluid may sometimes be obtained from the cranio-spi- 

 nal cavity ; whilst in all instances in which the bulk of the brain has under- 

 gone an increase, whether from the production of additional nervous tissue, or 

 from undue turgescence of the vessels, there is either a diminution or a total 

 absence of this fluid. It appears from the experiments of Magendie (to whom 

 our knowledge of the importance of this fluid is chiefly due), that its with- 

 drawal in living animals causes great disturbance of the cerebral functions, pro- 

 bably by allowing undue distension of the bloodvessels ; it is, however, capable 

 of being very rapidly regenerated ; and its reproduction restores the nervous 

 centres to their natural state. As the " cerebro-spinal fluid" can readily find 

 its way from the subarachnoid spaces of the cranial cavity into those of the 



