672 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



the cells of the grey substance, or reach the encephalon by remaining in the 

 corresponding moiety of the medulla ; for instance, the fibres of the right 

 half of the medulla gain the brain without passing into the left half. 

 Those of the lateral cords decussate, each cord sending to, and receiving 

 from, the other, tubes which cross in the white commissures. The posterior 

 cords contain fibres that extend directly to the brain : these are sensorial ; 

 there are also found transverse fibres that enter the cells of the superior 

 grey cornua, and others that pass into the cells of the inferior or motor 

 cornua. 



Such is, in a few words, the disposition of the nerve-elements in the 

 medulla. The subject is a very long and complicated one, which cannot 

 be dealt with in a more detailed manner in a work on descriptive anatomy. 



Vessels. The medulla receives arteries from the ramifications of the 

 pia mater. The grey is richer in vessels than the white substance ; the 

 latter is penetrated everywhere by a large number of minute arteries ; while 

 the first is traversed by the divisions of an artery that is thrown off by the 

 median spinal, and ascends towards the bottom of the inferior fissure. The 

 veins follow the arteries, and constitute two somewhat voluminous vessels 

 that pass along the grey commissure, to the right and left of the central 

 canal. 



DIFFERENTIAL CHARACTERS IN THE SPINAL CORD OF OTHER THAN SOLIPED ANIMALS. 



In all the species, the white and grey substances affect the disposition above de- 

 scribed; only some slight differences in the reciprocal volume of each have been 

 remarked. As in the Horse, the spinal medulla does not extend beyond the sacral 

 region; its length has no relation to that of the coccygeal region, as certain anatomists 

 would, in principle, establish; in the rabbit, for example, the tail of which is very 

 short, the spinal cord is prolonged into the coccygeal vertebra. 



COMPARISON OF THE SPINAL CORD OF MAN WITH THAT OF ANIMALS. 



The spinal medulla of the adult Man does not reach beyond the first lumbar vertebra, 

 though in the foetus it is in the coccyx.. It is rounder than in the Horse, and the grey 

 substance is, relative to the white, more abundant than in the spinal cord of the domes- 

 ticated animals. The posterior grey cornua are also larger and less elongated than the 

 superior cornua in the Horse ; and the roots of the nerves are also more voluminous. 



CHAPTER III. 



THE ENCEPHALON. 



ARTICLE I. THE ENCEPHALON AS A WHOLE. 



THE encephalon is that portion of the nervous system which is lodged in 

 the cranial cavity. It succeeds, without any line of demarcation, the spinal 

 cord, of which it may be considered, with regard to its figure, as a kind of 

 efflorescence. 



General form and constitution. In shape it is an ovoid mass, elongated 

 from before to behind, and very slightly depressed from above to below. 



When it is viewed on its superior face (Fig. 321), we first see, behind, a 

 white pedicle, the prolongation of the spinal cord, and a single lobe of a 

 grey colour designated the cerebellum. In front of this is remarked two 

 other lobes, separated from the first by a deep transverse fissure, into which 



