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PART XI. 

 OF THE NERVES. 



THE nerves are those whitish cords which pass from the 

 brain and spinal marrow to the various parts of the body. 



A general account of their origin is contained in the descrip- 

 tion of the basis of the brain and of the spinal marrow,* which 

 may be considered as introductory to the present subject. 



The nerves, in general, appear to be bundles or fasciculi of 

 small cords, each of which is composed of a series of fibres 

 that are still smaller. These fibres consist of medullary mat- 

 ter, which is derived from the brain and spinal marrow, and 

 is enclosed in a membranous sheath that appears to arise from 

 the pia mater. The smaller the fibre, the more delicate is the 

 membrane which invests it. 



As the nerves proceed from the brain and spinal marrow, 

 through the foramina of the cranium and the spine, they are 

 enclosed in a sheath formed by the dura mater; but when 

 they arrive at the exterior extremities of the foramina in those 

 bones, this coat, derived from the dura mater, appears to sepa- 

 rate into two lamina. The exterior lamen combines with the 

 periosteum, and the interior continues to invest the nerve, but 

 seems to change immediately into cellular substance ; so that 

 the exterior coat of the nerves may be regarded as composed 

 of cellular membrane, which is continued from the sheath 

 derived from the dura mater. 



It has been supposed that the membrane which forms the 

 sheaths for the medullary fibrils of which the nerves are com- 

 posed, is of a peculiar nature ; but it appears to be derived from 



* See volume ii. p. 389. 

 VOL. II. 41 



