THE STRUCTURE OF THE ELEMENTARY TISSUES. 



cylinder breaks up into its elementary fibrillee, to end in various ways to 

 be described later. 



Plexuses. At certain parts of their course, nerves f orm 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 connec- 

 tion with the spinal cord than it would have if it proceeded to its desti- 

 nation without such communication with other nerves. Thus, each 

 nerve by the wideness of its con- 

 nections 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 dependent upon the 

 integrity of any one nerve. 



Nerve-Cells. 



The nerve-cell is the nodal and 

 important part of the neuron, and 

 from it are given off thedendrites 

 and axis-cylinder process or nenr- 

 axon. It consists of a mass of 



protoplasm, of Varying shape and 



size, containing within it a nu- 

 cleus and nucleolus. All nerve- 

 cells give off a number of proc- 



esses which branch out in various directions, dividing and sub- 

 dividing like the branches of a tree, but never anastomosing with each 

 other or with other cells. These branches are what have already been 

 referred to as the dendrites of the cell. They were formerly called the^ro- 

 toplasmic processes (figs. 103A, 104). It is thns seen that the neuron or 

 nerve-unit consists of a number of subdivisions, namely, the cell-body) 

 with its nucleus and nucleolus, the dendrites, or protoplasmic processes,* 



103 A -Nerve-cell with short axis-cylinder 



