OF THE NERVOUS SYSTEM. 101 



spinal canal, lumbar, sacral, and coccygeal (where it is quite 

 imperfect), is occupied with the membranes and by the nerves 

 proceeding from the spinal marrow. This assemblage of nerves 

 within the canal was called by the older anatomists the cauda 

 equina, in the centre of which will be found the terminating 

 filum or thread of the organ itself. 



The spinal marrow is composed, like the brain and cere- 

 bellum, of a medullary and cineritious portion ; but here the 

 grey matter is central and the medullary external. A fluid, 

 clear and limpid, discovered by Cotunni, a distinguished 

 Neapolitan physician, fills up the space intervening between 

 the arachnoid membrane and the pia mater, by means of 

 which, no doubt, this extremely delicate organ is protected 

 from shocks and other injuries. 



190. Structure of the Encephalon. The organ is 

 composed of two distinct substances, the cineritious and 

 medullary ; this latter is fibrous, and to the tracing its 

 fibres anatomists have in all ages given the greatest atten- 

 tion. 



By commencing with the spinal marrow, and tracing its 

 fibres upwards, it will be found that it is composed of two 

 halves, united to each other by transverse medullary fibres. 

 These halves are each composed of six medullary bands or 

 columns, of which four occupy the anterior surface, two the 

 posterior. The four anterior ones may be traced upwards 

 into the corresponding pyramidal and olivary bodies, pro- 

 ceeding from thence into the brain itself; the two posterior 

 proceed, we shall find, to the cerebellum. To return to the 

 olivary and pyramidal fasciculi: we observe, first, that a 

 portion of those on the right side cross to the left, and those 

 from the left to the right column, thus decussating with each 

 other. After this decussation, they plunge into the annular 

 protuberance (pons Yarolii), and, continued forwards, con- 

 stitute the crura cerebri. The fibres from these ultimately 

 expand in the convolutions of the anterior and middle lobes 

 of the brain. The. longitudinal fibres of the olivary fasciculi 

 ascend like those of the pyramids, and passing through the 

 pons form the inner and posterior parts of the peduncles of 

 the brain ; during their course they traverse several masses 

 of the cineritious substance, increase in size, and finally pass 

 into or form various parts of the brain, as the thalami optici 

 and corpora striata; finally, they expand into the cerebral 

 convolutions. Several transverse commissures, as the corpus 



