NERVE CELLS 221 



penetrate it and reach the fibers themselves. There are nu- 

 merous lymph spaces around the individual fibers as well 

 as around the funiculi. In situations where the nerves are 

 well protected, as in the cranium, the amount of fibrous 

 tissue in the trunks is small, but where opposite conditions 

 prevail, as in muscular substance, this tissue is largely in- 

 creased in amount as regards both that which surrounds the 

 trunk and that which is sent" in between the funiculi and 

 fiber. This tissue has ramifying in it a network of fibers 

 known as neri nervorum. The blood supply is not large. 



Individuality of Nerve Fibers. It is to be remembered 

 that so far as can be determined every nerve fiber, having 

 entered a trunk, proceeds without interruption to the part to 

 which it is finally distributed, whether that part be the skin, 

 or a viscus, or a muscle, or a gland, or some organ of special 

 sense, or another nerve cell, or what not. Collections of 

 fibers forming bundles run together in the same trunk, may 

 leave that trunk together, may send out part of their fibers to 

 another bundle or trunk, or may receive other fibers from 

 other funiculi ; but everywhere the relation of the primitive 

 fibers to each other is simply one of contiguity. Hbwever, 

 as the axis cylinder approaches the seat of its final distribu- 

 tion, it breaks up into several fibrillse, such divisions always 

 taking place at the nodes of Ranvier. 



Nerve Centers. The nerve centers include the gray mat- 

 ter of the brain and cord and the ganglia in both the cerebro- 

 spinal and sympathetic systems. These centers have a gray 

 color due to the presence of a pigmentary substance in the 

 cells and surrounding tissue. The ganglionic centers are 

 simple collections of nerve cells with their usual accessory 

 elements myelocytes, intercellular granular matter, delicate 

 membranes covering some of the cells, connective tissue ele- 

 ments, blood-vessels and lymphatics. 



Nerve Cells. These are irregular in shape and may be 

 unipolar, bipolar or multipolar. They also vary much in 

 size. The unipolar cell has a single prolongation which be- 



