OF THE NERVOUS SYSTEM. 



103 



callosum, and anterior and posterior 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 neighbouring 

 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 invested with cineritious 

 matter, forms that remarkable assemblage 

 of laminae to which anatomists give the 

 name of arbor vitse, 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 

 (see Fig. 64 and Fig. 66) ; they are reckoned numerically 

 from before backwards. The first twelve pairs arise within 

 the cranium, and leave the cavity by apertures in the cranial 

 bones ; the thirty- one pairs which follow arise from the spinal 

 marrow, and leave the osseous canal by openings called 

 intervertebral, as being placed between the vertebrae. 



Each of these pairs of nerves is formed of a great number 

 of filaments, enclosed in a neurilemma. These elementary 

 fibres are extremely fine ; they do not unite with each other, 

 but pass from the central extremity of the nerve to the peri- 



of the neck and thorax belonging to the sympathetic system of nerves. 

 1, pneumogastric nerve, the principal branches of which form plexuses with 



10, 11, cardiac branches proceeding to the heart ; 13, pulmonary plexus ; 

 14, lingual nerve ; 15, terminal portion of the great hypoglossal ; 16, glosso- 

 pharyngeal nerve ; 17, spinal accessory of Willis ; 18, cervical nerve of the 

 second pair of spinal nerves ; 19, third cervical pair ; 23, 26, 27, 28, pairs of 

 cervical nerves, uniting with the first dorsal to form the brachial plexus ; 

 24, superior cervical ganglion of the great sympathetic ; 25, middle cervical 

 ganglion ; 26, inferior cervical ganglion ; 27 to 30, dorsal ganglions. 



* A portion of the spinal marrow. a, medulla spinalis ; b, posterior root 

 of one of the nerves ; c, ganglion situated on this root ; d, anterior root of 

 the same nerve, about to unite with the posterior root after this latter has 

 passed the ganglion ; e, common trunk formed by both roots ; f, small 

 branch proceeding to anastomose with the great sympathetic. 



