316 NERVOUS SYSTEiM. 



alike in structure, but are of corresponding symmetrical forms 

 and dimensions : a remark that not only applies to the brain it- 

 self, but one that holds good, likewise, with regard to the spinal 

 marrow. An animal, therefore, has, to all intents and purposes, 

 tivo brains : and, probably, for the same reason that he has two 

 eyes, two ears, and a double tongue. 



Arteries. — The arteries which supply the brain, are the two 

 vertebrals, besides two other considerable branches from the 

 carotids, called the internal carotids : its blood is returned from 

 the sinuses of the dura mater by the vertebral and jugular veins. 

 It is on the supply from the vertebral arteries, however, that this 

 organ mainly depends for the preservation of that energy essen- 

 tial to the support of life ; for if ligatures be put on these ves- 

 sels, the animal speedily dies ; whereas both the carotids may 

 be tied without occasioning any apparent ill effects. 



We shall now examine the divisions of the brain Cviz. the 

 cerebrum, cerebellum, and medulla oblongata) separately; com- 

 mencing with the 



Cerebrum. 



The largest portion of the brain, and that which presents 

 itself to our view immediately on raising the skull-cap, is the cere- 

 brum. It is lodged in that capacious elliptical chamber which is 

 formed, in front, by the parietal bones; behind, by the ethmoid, 

 sphenoid, and posterior occipital bones ; laterally, by the tem- 

 poral bones ; inferiorly, by the roofs of the frontal sinuses, and 

 by the ethmoidal plates ; and, superiorly, by the tentorium. The 

 visible surface of it (as the cranium is ordinarily sawn open) may 

 be said to represent a convex semi-oval, of which the broader end 

 is turned upward. It is equally divided by a longitudinal fissure 

 along its middle, into which the falx cerebri descends ; and its 

 divisions, which are perfectly symmetrical, both internally and 

 externally, are denominated hemispheres. 



Corpus CaHosum. — In parting the hemispheres, so as to admit 

 light between them (by which a iesw small interpassing vessels 

 and some weak celullar connexions become lacerated), we per- 

 ceive a white longitudinal body, covered by pia mater, uniting 

 the hemispheres at the bottom of this fissure : this is the corpus 

 callosum. Each hemisphere, separately regarded, has an outer 

 surface, waved or convoluted, which is adapted to the undulated 

 concavity of the skull-cap ; an inner perpendicular plane, which 

 is opposed to the one of the opposite hemisphere ; and a base, di- 

 vided into lobes, the description of which I shall postpone until 

 the interior parts have been examined. 



