674 Till- CENTS A L AXIS OF THE NERVOUS SYSTEM. 



differs notably in the several animals, being highest in the Dog. The 

 relations in each species, between the two divisions, 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. Wo give these numbers, as it has 

 always been attempted to establish in the predominance of the encephalon 

 the cause of the development of intelligence, and that the best measure of this 

 predominance is really the relation of the spinal axis to the encephalic mass. 

 It has also been attempted to measure this predominance of the encephalon by 

 comparing its weight with that of the entire body ; but it is sufficient to cast 

 one's eye over the tables drawn up with this viesv in several anatomical and 

 physiological works, to be convinced that this basis does not possess all the 

 value desirable. 



Preparation of the encephalon. To study the encephalon, it is necessary to extract it 

 from its bony receptacle ; a result achieved in two ways. The first consists in opining 

 the 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 linn excised -with 

 scissors, and the encephalon, which is thus directly reached, is completely isolate! by 

 raising its posterior extremity, and cutting from behind to before all the nerves }<. 

 through the foramina at the base of the cranium, with the pituitary stalk, as well as the 

 extremity of the olfactory lobes. This method is very expeditious, but it sacrifices the 

 pituitary gland, which remains firmly inerusted 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 the 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 styloid processes of the occipital bone, then the condyles of this 

 bone, the basilar processes, and the sphenoid, palate, and ethmoid bones, returning to the 

 lateral portions of the cranium, which are chiselled away in succession from th < ceipital to 

 the ethmoid bones. The encephalon being sufficiently expoted, 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 ex- 

 cavating the ethmoidal fosste with the point of a scalpel, the olfactory lobes are detached, 

 and the nervous mass is free. This procedure is more difficult than the first, but 

 possesses several advantages over it; for not only do we preserve the pituitary gtauid, but 

 have the ethmoidal lobes more intact, and may also have, if desired, the ganglia of the 

 cranial nerves, with a more or less considerable portion of the nerves themseh 



After indicating the methods for extracting the encephalon from its bony case, we 

 ought to say some words aa to the course to be pursued in order to study it successfully. 

 To do this it is advantageous to have two brains; one of these should be hardened by 

 steeping it for some weeks in alcohol (or methylated spirit), or in water to which has 

 been added a tenth part of nitric acid. This hardening contracts the nervous substance, 

 and causes the cavities and reliefs to appear more manifest. (It is a good plan to place 

 the brain, base uppermost, in a suitable vessel, and if a piece of cloth be spread beneutli 

 it, its removal therefrom will be greatly facilitated.) 



We commence by examining rapidly the whole apparatus, and pass immediately to 

 the study of the isthmus, of which it is necessary to have at first a well-deli nrd idea. 

 We therefore take a hardened specimen, and isolate this portion of the enoopbalon in 

 the manner represented in figure 323 ; to do this, it suffices to cut through the peduncles 

 of the cerebellum, and excise the cerebral hemispheres upwards and backwards ; the 

 remains of these and the cerebellum should be preserved for an analysis of their 

 structure. The isthmus thus isolated is fitted for an examination of its external confor- 

 mation and its internal cavities : the ventricle of the optic layers and the aqueduct of 

 Sylvius, into which we may penetrate by a superior longitudinal incision. 



After the isthmus, the cerebellum is to Itc studied : in its external conformation, on an 

 intact specimen ; and in its internal conformation and structure on the incised piece. 



We terminate with the cerebrum, whose superficies is soon examined, and whose 

 interior should be studied in the following manner : It is necessary to begin by demon- 

 strating the existence of ventricles in the olfactory lobes, and their communication 

 with all the other internal cavities of the brain, which can easily be done by the in- 

 flation of one of these organs by means of a straw (or dissecting-case tube) \\hii-h 



