HISTOLOGY OF NERVES. 173 



eye they are commonly grayer in color than the cerebro- 

 spinal nerves. 



The Sporadic Ganglia. These, for the most part very 

 minute, nerve-centres are found scattered in nearly all 

 parts of the Body. They are especially abundant in the 

 neighborhood of secretory tissues and about blood-vessels, 

 while a very important set is found in the heart. Nerves 

 unite them with the cerebro-spinal and sympathetic cen- 

 tres, and probably many oH them belong properly to the 

 .sympathetic system. 



The Histology of Nerve-Fibres. The microscope shows 

 that in addition to connective tissue and other accessory 

 parts, such as blood-vessels, the nervous organs contain tis- 

 .sues peculiar to themselves and known as nerve-fibres and 

 nerve-cells. The cells are found in the centres only; while 

 the fibres, of which there are two main varieties known as 

 the white and i\\Qgray, are found in both trunks and cen- 

 tres; the white variety predominating in the cerebro-spinal 

 nerves and in the white substance of the centres, and the 

 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 pcrineurium, 

 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 funiculns 

 has a sheath of its own called the neurilemma, composed 

 of several concentric layers of a delicate membrane, with- 

 in which the true nerve-fibres lie. These, which would be 

 nearly all of the white kind, consist of extremely delicate 

 threads, about 0.0125 millm. (-3-5^7 inch) in diameter, but 

 frequently of a length which is in proportion very great. 

 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 of the toes, are 

 three to four feet long. If a perfectly fresh nerve-fibre 



