THE NERVE FIBER 187 



164). Part of the cell body is sometimes free of tigroid bodies and is of 

 the same texture as the nerve fiber of which it is a direct continuation. 

 This is called the implantation cone in the vertebrates, where it is 

 roughly cone-shaped (see Figs. 157, 158, 160, and 161). In the arthro- 

 pods it forms a long curved area which reaches around the nucleus 

 (Fig. 165). 



Technic. The study of the nerve-cell bodies requires only carefully 

 prepared paraffin sections, as long as this study does not extend to the 

 processes. Staining is an important factor, and several staining methods 

 have been evolved for the purpose of learning more about the structure 

 of these objects. See LEE'S " Microscopist's Vade Mecum." 



LITERATURE 



MANN, G. "The Histology of Nerve Cells," Report of the 68th Meeting of the British 

 Association for the Advancement of Science. 



ROHDE, E. "Die Ganglienzelle," Zeits.f. Wiss. Zool., Band LXIV. 



McCLURE, C. F. W. " On the Finer Structure of the Nerve Cells of Invertebrates: i Gas- 

 teropoda," in Zool. Jahrb. Abt. Morph., Band XI, 1897. 



THE NERVE FIBER 



The nerve fiber is that part of a neuron which is specialized for con- 

 duction. It is an integral part of the nerve cell; an evagination of its 

 cytoplasm. Its growth, or the process of its evagination from the cell, 

 has been traced in the living embryonic development of the cell and in the 

 regeneration of nerve tissue. As proved by experiment, it dies when 

 separated, in situ, from the cell body, which then develops a new and 

 similar process to take its place. 



Looked upon in this light, we can see that its structure must be a 

 modification of that of the cell body. This occurs in two degrees, forming 

 two kinds of fibers, those that retain the tigroid bodies or neurochon- 

 dria of the cell body and those which do not. Some dendritic processes 

 are the only ones that retain the neurochondria, and they do this for only 

 a part of their course. Otherwise, the fiber is everywhere the same, a 

 continuation of the cytoplasm of the nerve cell, containing a bundle of 

 fibrils that are either continuations of those in the cell body or similar to 

 them. The fibrils are parallel in a general way, and as has been stated 

 in discussing the nerve cell, they are probably structures by whose agency 

 the nerve impulse is forwarded through the neuron. That the fiber can 

 forward the impulse from perceptory surface to discharging surface 

 without the direct aid of the cell body, or even in the absence of the cell 

 body, is proved by the experiments of Bethe on certain of the nerve cells 

 of a crab. These cells were selected for the experiments because of the 



