78 



HAND-BOOK OF PHYSIOLOGY. 



the nucleus containing a nucleolus. Besides varying much in shape, partly 

 in consequence of mutual pressure, they present such other varieties as 

 make it probable either that there are two different kinds, or that, in the 

 stages of their development, they pass through very different forms. Some 

 of them are small, generally spherical or ovoid, and have a regular uninter- 

 rupted outline. These simple nerve-corpuscles are most numerous in the 

 sympathetic ganglia; each is enclosed in a nucleated sheath. Others, 



which are called caudate or stellate nerve-cor- 

 puscles (Fig. 313), are larger, and have one, 

 two, or more long processes issuing from 

 them, the cells being called respectively uni- 

 polar, bipolar, or multipolar; which pro- 

 cesses often divide and subdivide, and appear 

 tubular, and filled with the same kind of 

 granular material that is contained within the 

 corpuscle. Of these processes some appear 

 to taper to a point and terminate at a greater 

 or less distance from the corpuscle; some ap- 

 pear to anastomose with similar offsets from 

 other corpuscles; while others are continuous 

 with nerve -fibres, the prolongation from the 

 cell by degrees assuming the characters of the 

 nerve-fibre with which it is continuous. 



Ganglion-cells are each enclosed in a trans- 

 parent membranous capsule similar in appear- 

 ance to the nucleated sheath of Schwann in 



FIG. 313. Ganglion nerve-corpus- rn *j_i J.T i i j? 



cies of different shapes (Klein and nerve- fibres: within this capsule is a layer of 



small flattened cells. 



That process of a nerve-cell which becomes continuous with a nerve- 

 fibre is always unbranched, as it leaves the cell. It at first has all the 

 characters of an axis-cylinder, but soon acquires a medullary sheath, and 

 then may be termed a nerve-fibre. This continuity of nerve-cells and 

 fibres may be readily traced out in the anterior cornua of the grey matter 

 of the spinal cord. In many large branched nerve-cells a distinctly fibril- 

 lated appearance is observable; the fibrillae are probably continuous with 

 those of the axis-cylinder of a nerve. 



THE FUNCTIONS OF NERVE FIBRES. 



It will be evident from the account of nervous action previously given 

 (p. 45 et seq., Vol. II.) that nerve-fibres are stimulated to act by anything 

 which increases their irritability, but that they are incapable of originating 

 of themselves the condition necessary for the manifestation of their own 

 functions. When a cerebro-spinal nerve-fibre is irritated in the living 



