ANATOMY OF THE NERVOUS SYSTEM. 



177 



gray in the sympathetic trunks and the gray portions of the 

 central organs. 



If an ordinary cerebro-spinal nerve-trunk be examined it 

 will be found to be enveloped in a loose sheath of areolar 

 connective tissue, which forms a packing for it and unites 

 it to neighboring parts. From this sheath, or perineurium, 

 bands of connective tissue penetrate the nerve and divide it 

 up into a number of smaller cords or funiculi, much as a 

 muscle is subdivided into fasciculi; each funiculus has a 

 sheath of its own called the neurilemma, composed of several 



FIG. 78. FIG. 79. 



FIG. 78. White nerve-fibres soon after removal from the Body and when they 

 have acquired their double contour. 



FIG. 79. Diagram illustrating the structure of a white or medullated nerve-fibre. 

 I, 1, primitive sheath; 2, 2, medullary sheath; 3, axis cylinder. 



concentric layers of a delicate membrane, within which the 

 true nerve-fibres lie. These, which would be nearly all of 

 the white kind, consist of extremely delicate threads, on the 

 average, 0.0125 mm. (^oVo- i ncn ) i n diameter, though often 

 considerably smaller, and of a length which is in proportion 

 very great. The core of each nerve-fibre in fact is continuous 

 from a nerve-centre to the organ in which it ends, so that the 

 fibres, e.g., which pass out through the sacral plexus and then 

 run on through the sciatic nerve and its branches to the skin 



