THE BRAIN. 763 



The veins are proportionately large. Two are lodged in the grey commissm-e ; 

 and a third, which is voluminous, lies in the middle superior fissure. 



All the vessels arc accompanied by the slender nerves disposed in plexuses 

 issuing with the communicating veins, or the superior radicles which they follow 

 into the substance of the cord. 



In the cord there are perivascular lymph spaces^ which probably open into the 

 deep layer of the pia mater. 



Differential Chaeacters in the Spinal Cord of the otoee Animals. 



In all the species, the white aud grey substances affect the disposition above described ; 

 only some slight differences in the reciprocal volume of each have been remarked. As in the 

 Horse, the spinal cord 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 

 Tertebrse. 



Comparison of the Spinal Cord of Man with that op Animals. 



The spinal cord of achilt 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 domesticated 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 than in that animal. 



No nerve-cells are found in the posterior coiuua, and Clarke's column is limited to the 

 dorsal region; so that the sensitive nerves of the back and loins arise in the dorsal region. 

 The nerves of the cervical region arise from a series of nuclei arranged in the medulla 

 oblongata. 



CHAPTER III. 

 THE BRAIN, OR ENCEPHALON. 



Article I. — The Brain as a Whole. 



The hrain 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 composition. — In shape it is an ovoid mass, elongated from 

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



When it is viewed on its superior face (Fig. 423), 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 the tentorium cerebelli 

 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 brain to examined its inferior face, we see that the posterior 

 peduncle of the organ — a continuation of the spinal cord — is prolonged beneath 

 the cerebellum, which is joined to the lateral parts of its superior face ; this por- 

 tion then enters the cerebral hemispheres by their inferior face, behind two thick 

 white cords — the optic nei-ves — which mark the anterior limit of this prolongation 

 (Fig. 424). This is the isthmus of the brain — ^a name given to it because it 



