TEE ENCEPHALON. 



673 



the tentorium of the cerebellum passes. Isolated from one another on 

 the middle line by a shallower fissure, these two lobes constitute the brain, 

 and are usually named the cerebral hemispheres. 



In turning over the encephalon 

 to examine its inferior face, we see 

 that the posterior peduncle of the 

 organ — a continuation of the spinal 

 cord — is prolonged beneath the ce- 

 rebellum, which is joined to the 

 lateral parts of its superior face; 

 this portion then enters the cere- 

 bral hemispheres by their inferior 

 face, behind two thiols: white cords — 

 the optic nerves, which mark the 

 anterior limit of this prolongation 

 (Fig. 322). This is the isthmus of 

 the encephalon : a name given to it 

 because it actually forms an inter- 

 mediate bond between the ■ three 

 enlargements which form the prin- 

 cipal mass of the encephalon. 



The cranial portion of the cen- 

 tral nervous mass is, then, com- 

 posed of three apparatus : the 

 isthmus of the encephalon, a pro- 

 longation of the spinal cord ; and 

 the cerebellum and cerebrum, bulbous 

 lobes grafted on the superior face 

 and anterior extremity of this pe- 

 duncle. These three divisions are 

 very well seen in their entirety 

 and reciprocal relations in Figure 

 329. We will study them sepa- 

 rately and in succession. 



Volume of the encephalon. — Con- 

 trary to what is found in the spinal 

 cord, the dimensions of the en- 

 cephalon 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 subarachnoid 

 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 encephalon 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 encephalon, in average-sized animals, 

 m^j 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, 

 6 oz. 10 drams; Dog; 6 oz, 5^ drams; Cat, 1 oz. 1 dram. 



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

 that the relative weight of the medullary axis to that of the encephalic mass 



GENERAL VIEW OF THE EEAIN; TIPPER SUEFAOE. 



1, Medulla oblongata; 2, Middle lobe of the 

 cerebellum ; 3, 3, Lateral lobes of ditto ; 4, 4, 

 Cerebral hemispheres ; 5, Interlobular fissure ; 

 6, 6, Ethmoidal lobules. 



