NERVES. 



467 



as " nucleated enlargements of the axial cylinders." The cells 

 are termed unipolar, bipolar, and multipolar, according to the 

 number of processes they possess. 



The ganglia are composed of nerve cells and gelatinous fibres, 

 invested by a thin envelope, continuous with the neurilemma of 

 the nerves, which sends minute processes inwards. 



NERYES. 



A nerve consists of a bundle of tubular fibres, held together 

 by areolar tissue, enclosed in a membranous sheath, and divided 

 into a number of fasciculi, each being invested by an inflection 

 of the sheath ; they have also an external 

 covering of cellular tissue, known as the 

 vagina cellulosa or cellular sheath. 



During their course, the nerves have 

 frequent intercommunication, either by con- 

 necting branches or in a more intimate 

 and complex manner in what is termed 

 a plexus. When the nerves divide into 

 branches, there is never any splitting of 

 the ultimate fibres, nor yet is there ever 

 any coalescing of them they retain tlt'ir 

 individuality from their source to their 

 termination. 



Nerves ma}- be traced from the surface of 

 the brain and spinal cord into the substance 

 of these organs, the place where they leave the 

 surface being termed the apparent, and the 

 ultimate origin the deep origin or root of the 

 nerve. In some cases the root is single, but in 

 others the fibres spring from different parts of 

 the nerve centre ; such nerves are said to have two or more 

 roots. These roots may have different functions, as in the spinal 

 nerves, where the superior root is sensory and the inferior motor. 

 Whenever this admixture of functionally distinct fibres is found, 

 a ganglion is placed at or near the apparent root of the nerve. 



Nerves terminate in a variety of ways by loops, as in the 

 voluntary muscles, and in some of the soft structures, as the dental 

 pulp ; by terminal plexuses, and bulbs, in mucous membranes. In 

 many of the sensory nerves, there are small, oval masses, the 

 Pacininn bodies, at or near their peripheral terminations; these 



FIG. 175. 



Pacinian body from the 

 mesentery of a cat. a. 

 Stem of the body ; 6, Layer 

 of connective tissue ; c, 

 Cavity of the body; d, 

 Nerve fibre. 



