216 INNERVATION. [CHAP. Vlll. 



sheath of the same membrane which surrounds the elementary 

 fibre of striped muscle. From its deep surface thin layers of 

 areolar tissue pass, forming so many partitions between the smaller 

 bundles, or the individual fibres, of which the nervous trunk is 

 composed. The office of this structure is evidently to give protec- 

 tion to the delicate nerve-tubes, and to support the plexus of minute 

 capillary vessels from which they derive their nutriment. 



The neurilemma is composed of fibres of white fibrous tissue, 

 and presents to the naked eye the silvery aspect of that texture. 

 Some persistent cell-nuclei are scattered throughout it. That por- 

 tion of it which forms the partition between the fibres contains a 

 little yellow fibrous tissue of the finest description. 



The blood-vessels are distributed upon the external investing 

 sheath, and upon the septa. They are disposed similarly to those 

 of muscles, and run parallel to the fibres of the nerve. The capil- 

 laries are among the smallest in the body : they form oblong meshes 

 of considerable length, completed at long intervals by vessels which 

 cross the fibres of the nerve more or less transversely. These 

 blood-vessels are generally derived from neighbouring arterial 

 branches ; sometimes a special vessel accompanies a nervous trunk, 

 and even perforates it, passing along its axis, as in the great 

 sciatic and the optic nerves. 



The composition of a cerebro-spinal nerve may be shown by 

 removing the neurilemma, and separating the fibres by needles. 

 These fibres are chiefly of the tubular kind. In diameter they 

 vary considerably, but do not exceed the 15 1 00 of an inch in man 

 and the mammalia. They lie within the sheath in simple juxta- 

 position, and parallel to each other ; excepting where a branch is 

 about to separate, when a bundle of nerve-tubes gradually deviates 

 from its previous course, and forms a very acute angle with the 

 truck, still, however, preserving the parallelism of its constituent 

 fibres. 



Nerves are said to arise or have their origin in the nervous centre 

 to which they are on the one hand attached, and to terminate or be 

 distributed among the elements of the various textures on the other 

 hand. It is best to continue the use of so definite and simple a 

 meaning to these terms. Attempts to alter their signification in 

 accordance with opinions of the functions of the constituent fibres 

 can at present do little but confuse descriptions. We call a nerve 

 cerebral or encephalic, if it be connected at its origin with some part 

 of the nervous mass within the cranium ; and spinal, if its apparent 

 origin be from the spinal cord. 



