vi HISTOLOGY OF THE FROG 133 



lively late period. The cells forming the sheath of a nerve 

 are of mesodermic origin ; the nerve fiber being therefore a 

 structure derived from two germ layers. 



The regeneration of nerve fibers which have been cut in 

 two shows an intimate dependence of the axis cylinder upon 

 the ganglion cell from which it arises. The portion of the 

 axis cylinder distal to the cut, and consequently no longer 

 connected with the nerve cell, degenerates, and becomes 

 replaced by an outgrowth from the proximal part which fol- 

 lows the track of the degenerating fiber until the structure 

 of the whole nerve is restored. This phenomenon is but a 

 special case of the general principle that a portion of a cell cut 

 away from the part containing the nucleus invariably dies. 



The nerve or ganglion cells are found in those parts 

 which are spoken of as the nerve centers ; viz. the brain, 

 spinal cord, spinal ganglia, and the various ganglionic 

 masses of the sympathetic system. These centers are made 

 up of ganglion cells and their fibers, together with the con- 

 nective tissue which binds them together and the vessels 

 which supply them with nutriment and carry away their 

 waste products. Ganglion cells are generally irregular in 

 outline, with a nucleus near the center. Their cytoplasm is 

 granular and under proper treatment shows a network, the 

 strands of which are connected with the fibrillae of the nerve 

 fiber and other processes of the cell. Two kinds of pro- 

 cesses are commonly distinguished : the axis cylinder pro- 

 cess, which acquires a sheath and forms a part of a nerve 

 fiber ; and the protoplasmic processes, often several in 

 number, which are shorter than the former and generally 

 branched. Nerve cells are designated as unipolar, bipolar, 

 or multipolar, according as they possess one, two, or three or 

 more processes. Unipolar ganglion cells are found in the 

 sympathetic ganglia. 



