146 PHILOSOPHY OF ZOOLOGY. 



enveloped by the same integuments which we have already 

 noticed as belonging to the brain itself. It differs from the 

 brain, however, in the arrangement of its component parts, 

 the medullary matter here occupying the surface, while the 

 cineritious is disposed towards the centre in a cross-like 

 form. It is divided longitudinally into two equal halves, 

 by a channel which is very obvious on the dorsal aspect, 

 and on each side there is likewise a groove. By some ana- 

 tomists, it is considered as consisting of two medullary cords 

 only, divided in the direction of the mesial line; while 

 others consider it as consisting of four cords, two anterior 

 and two posterior. Considerable support seems to be af- 

 forded to the last opinion, by the circumstance of its origi- 

 nating from four medullary cords, two from the cerebrum, 

 and two from the cerebellum. The two former may be 

 considered as forming the anterior, and the two latter the 

 posterior portions. These cords, however, are brought in- 

 to intimate union by numerous filaments, which pass from 

 the one to the other. Mr SEWELL, of the Veterinary Col- 

 lege, London, has ascertained the existence of a canal in 

 the centre of the spinal marro%v in the horse, bullock, sheep, 

 hog, and dog. It extends, uninterruptedly, from the cala- 

 mus scriptorius to the cauda equina ; is lined by a mem- 

 brane, resembling the tunica arachnoidea, and contains a 

 transparent colourless fluid, like that which is contained in 

 the ventricles of the brain *. 



3. The NERVES may be regarded either as originating 

 from, or terminating in, the brain or spinal marrow. We 

 shall consider them, for the present, in the former point of 

 view. The integuments of the nerves resemble, in appear- 

 ance, those of the brain, closely invest them on all sides, and 



* " A Letter on a Canal in the Medulla Spinalis of some Quadrupeds." 

 Phil. Trans. 1809, p. 146. 



