362 FUNCTIONS OF THE NERVOUS SYSTEM. 



that of all the rest of the Cerebro-spinal mass, more than eight times that of 

 the Cerebellum, thirteen times that of the Medulla Oblongata, &c., and twenty- 

 four times that of the Spinal Cord. The average weight of the whole En- 

 cephalon, in proportion to that of the body, in Man, taking the average of a 

 great number of observations, is about 1 to 36. This is a much larger propor- 

 tion than that which obtains in most other animals; thus the average of Mam- 

 malia 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 Me- 

 dulla 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 Ce- 

 rebrum is larger in proportion; in fact, it is probably. not nearly so large ; for 

 in Birds and Rodentia, the sensory ganglia form a very considerable propor- 

 tion 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. 



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

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

 proportion which it receives being, according to the estimate of Haller, as 

 much as one-fifth of the whole. The manner in which this blood is con- 

 veyed to the Brain, and the conditions of its distribution, offer some pecu- 

 liarities worthy of notice. The two Vertebral and two Carotid arteries, by 

 which the blood enters the cavity of the cranium, have a more free communi- 

 cation 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 dis- 

 tributed, the cessation of the circulation through the nervous matter being 

 immediately productive (as formerly shown, 290) 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; and it has been supposed that the 

 total amount of blood circulating through it must consequently be invariable, 

 any disturbance of the circulation being due to an undue turgid ity of the 

 arteries and corresponding emptiness of the veins, or vice versa. But this is 

 by no means the case; for, independently of the fact that varying states of 

 functional activity will doubtless produce a considerable variation in the entire 

 bulk of the nervous mass, we find a special provision for equalizing the bulk 

 of the contents of the cranial cavity, and for counterbalancing the results of 

 differences in the functional activity of the brain and in its supply of blood. 

 This is the existence of a fluid, which is found beneath the arachnoid, 

 wherever pia mater exists in connection with the brain and spinal cord; 

 whether on the surfaces of these organs, or in the ventricles of the latter. 

 The amount of this fluid seems to average about two ounces; but in cases of 

 atrophy of the brain, as much as twelve ounces of fluid may sometimes be 

 obtained from the crauio-spinal cavity; whilst in all instances, in which the 

 bulk of the brain has undergone an increase 1 , whether from the production of 

 additional nervous tissue, or from undue turgescence of the vessels, there is 



