THE NERVOUS SYSTEM. 455 



Bundles of fibres run together in the nerve-trunk, but merely lie in 

 apposition with each other; they do not unite: even when they anasto- 

 mose, there is no union of fibres, but only an interchange of fibres be- 

 tween the anastomosing funiculi. Although each nerve-fibre is thus 

 single and undivided through nearly its whole course, yet as it approaches 

 the region in which it terminates, individual fibres break up into several 

 subdivions (Fig. 315) before their final ending. 



Plexuses. At certain parts of their course, nerves form plexuses, in 

 which they anastomose with each other, as in the case of the brachial 

 and lumbar plexuses. The objects of such interchange of fibres are, (a) 

 to give to each nerve passing off from the plexus, a wider connection 

 with the spinal cord than it would have if it proceeded to its destination 

 without such communication with other nerves. Thus, each nerve by 

 the wideness of its connections, is less dependent on the integrity of 

 any single portion, whether of nerve-centre or of nerve-trunk, from 

 which it may spring, (b) Each part supplied from a plexus has wider 

 relations with the nerve-centres, and more extensive sympathies; and, by 

 means of the same arrangement, groups of muscles may be co-ordinated, 

 every member of the group receiving motor filaments from the same 

 parts of the nerve-centre, (c) Any given part, say a limb, is less depen- 

 dent upon the integrity of any one nerve. 



Nerve terminations. As medullated nerve-fibres approach their ter- 

 minations they lose their medullary sheath, and consist then merely of 

 axis-cylinder and primitive sheath. They then lose also the latter, and 

 only the axis-cylinder is left with here and there a nerve-corpuscle partly 

 rolled around it. Finally, even this investment ceases, and the axis- 

 cylinder breaks up into its elementary fibrillae. 



B. Nerve-Cells or Corpuscles. 



Nerve-cells comprise the second principal element of nervous tissue. 

 They are not generally present in nerve-trunks, but are found in all col- 

 lections of nervous tissue called ganglia. They vary considerably in 

 shape, size, and structure in different ganglia. 



a. Some of them are small, generally spherical or ovoid, and have a 

 regular uninterrupted outline. These simple nerve-cells are most nume- 

 rous in the sympathetic ganglia; each is inclosed in a nucleated sheath. 

 b. Others, which are called caudate or stellate nerve-cells (Fig. 317), are 

 larger, and have one, two, or more long processes issuing from them, the 

 cells being called respectively unipolar, bipolar, or multipolar: which 

 processes often divide and subdivide, and appear tubular, and filled with 

 the same kind of granular material that is contained within the cell. 

 Of these processes some appear to taper to a point and terminate at a 

 greater or less distance from the cell; some appear to anastomose with 

 similar offsets from other cells; while others are continuous with nerve- 



