NERVE CELLS 221 



the fibers themselves. There are numerous lymph spaces around 

 the individual fibers as well as around the funiculi. In situa- 

 tions where the nerves are well protected, as in the cranium, the 

 amount of fibrous tissue in the trunks is small, but where oppo- 

 site conditions prevail, as in muscular substance, this tissue is 

 largely increased in amount as regards both that which surrounds 

 the trunk and that which is sent m between the funiculi and 

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

 as nervi 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. However, as the axis cylinder approaches the seat 

 of its final distribution, it breaks up into several fibrillae, such 

 divisions always taking place at the nodes of Ranvier. 



Nerve Centers. The nerve centers include the gray matter 

 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 sur- 

 rounding 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 elements, blood-vessels and 

 lymphatics. 



Nerve Cells. These are irregular in shape and may be uni- 

 polar, bipolar or multipolar. They also vary much in size. The 

 unipolar cell has a single prolongation which becomes the axis 



