THE NEEVOUS SYSTEM. 97 



its fibres anatomists have in all ages given the greatest 

 attention. 



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 varolii), and, continued forwards, 

 constitute the crura cerebri. The fibres from these ulti- 

 mately 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 tfye 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 callosum, and anterior and pos- 

 terior commissures, connect the two sides of the brain with 

 each other. 



The posterior columns of the medulla oblongata (restiform 

 bodies) unite with some fibres coming from the neigh- 

 bouring portions of the medulla spinalis, and thus constitute 

 the crura cerebelli, or crura ad medullam oblongatam ; these 

 fibres plunge into the centre of the cerebellar hemispheres, and 

 assist in forming the central medullary mass, which being in- 

 vested with cineritious matter, forms that remarkable assem- 

 blage of laminae to which anatomists give the name of arbor 

 vitae, seen on making a section of either hemisphere of the cere- 

 bellum. Thus the cerebellum receives crura or peduncles 

 from three sources, viz., the pons, the medulla oblongata, 

 and the tubercula quadrigemina. 



191. Nerves. The nerves originating, or as some view 

 it, terminating, in the encephalon, amount to forty -three pairs 

 H 



