THE BRAIN, OB ENCEPRALON. 765 



entirety and reciprocal relations in Fig. 424. "We will study them separately and 

 in succession. 



Volume of the brain. — Contrary to what is found in the spinal cord, the 

 dimensions of the brain closely represent those of the cavity containing them : 

 the visceral layer of the arachnoid lying everywhere immediately on the proper 

 envelope of the nervous mass — the pia mater — except at the sub-arachnoid spaces ; 

 and, on the other hand, the arachnoid cavity can scarcely be said to exist while 

 the dura mater is, as it were, glued to the cranial walls, and in reality constitutes 

 their internal periosteum. 



The brain has, therefore, no room to move in its receptacle, but is maintained 

 in it in an almost absolutely immovable condition, which coincides exactly with 

 that of the sutures or cranial articulations. 



Weight. — The total weight of the brain, in average-sized animals, may be 

 inferred from the following figures : Horse, 22 oz. 15 drams ; Ass, 12 oz. 11 

 drams ; Ox, 16 oz. 15 drams ; Sheep and Goat, 4 oz. 9^ drams ; Pig, 5 oz. 10 

 drams ; Dog, 6 oz. 5i drams ; Cat, 1 oz. 1 dram ; Rabbit, 5i drams. 



Cornevin has remarked that the cranial capacity, if not the weight of the 

 brain, is greater in the female than in the male of the domestic species and breeds 

 of animals. 



In comparing these figures with those of the spinal cord, it will be seen that 

 the relative weight of the latter to that of the encephalic mass differs notably in 

 the several animals, being highest in the Dog, and lowest in the Rabbit. The 

 relations in each species, between the two divisions of the nervous system, are the 

 following : Dog, 1 : 5-14 ; Cat, 1 : 3-75 ; Sheep and Goat, 1 : 2'60 ; Ass, 1 : 

 2-40 ; Pig, 1 : 2-30 ; Horse, 1 : 2-27 ; Ox, 1 : 2-18 ; Rabbit, 1 : 2. We give 

 these numbers, as it has always been attempted to establish, in the predominance 

 of the brain, the cause of the development of intelligence, and that the best 

 measure of this predominance is really the relation of the spinal cord to the brain. 

 It has also been attempted to measure this predominance of the brain by compar- 

 ing its weight with that of the entire body ; but it is sufficient to cast one's eye 

 over the tables drawn up with this view in several anatomical and physiological 

 works, to be convinced that this basis does not possess all the value desirable. 



Preparation of the train. — To study the brain, it is necessary to extract it from its bony 

 receptacle ; a result achieved in two ways. The first consists in opening tlie roof of the 

 cranium by hammer and chisel, after removing from its exterior all the parts covering it, or 

 which are in its vicinity. The dura mater is then excised with scissors, and the brain, which is 

 thus directly reached, is completely isolated by raising its posterior extremity, and cutting from 

 behind to before all the nerves passing through the foramina at the base of the cianium, with 

 the pituitary stem, as well as the extremity of the olfactory bulbs. This method is very ex- 

 peditious, but it sacrifices the pituitary gland, which remains firmly embedded in the sella 

 Turcica — an inconvenience we obviate by resorting to the second procedure. In this, the 

 cranium is opened by its base or floor, after separating tiie head from the trunk, cutting away 

 the lower jaw. tongue, and os hyoides. and excising all the soft parts so as to expose the bony 

 surfaces. The head, thus prepared, is held by an assistant, the roof of the cranium resting 

 on a table or block. Armed with a chisel and hammer, the operator first removes the 

 zygomatic arches and the styloid processes of the occipital bone, then the condyles of this bone, 

 the basilar processes, and the sphenoid, palatine, and ethmoid bones, returning to the lateral 

 portions of the cranium, which are chiselled away in succession from the occipital to the eth- 

 moid bones. The brain, being suflBciently exposed, is relieved from its dura mater as in the 

 first method, and raised in the left hand to destroy, by means of scissors held in the right hand, 

 the attachments which yet fix it to the cranial roof, and which are chiefly the veins that open 

 into the sinuses of the dura mater. In afterwards excavating the ethmoidal fossae with the 

 point of a scalpel, the olfactory bulbs are detached and the mass is free. This procedure 

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