STRUCTURE OF THE NEURONE 



765 



in plate-form, and lie in contact with the substance of the muscle fibre. In man and all mammals, 

 the area covered is usually somewhat oval and is marked by a granular differentiation of the 

 muscle substance. This with the telodendria is known as a motor end-plate. The telodendria 

 of sympathetic axones ending upon cardiac and smooth muscle fibres are fewer and simpler than 

 those of cranio-spinal axones upon skeletal muscle. They consist of a few fine fibrils, with very 

 small varicosities along them and at their ultimate terminations, which run longitudinally along 

 the muscle fibre in close relation with its substance. Those upon gland cells are similar in 

 character except that they often form a loose pericellular plexus about and upon the cell. The 

 varicosities of telodendria are sometimes called end-feet and closer study of them has shown that 

 they themselves consist of fine plexuses of the neuro-fibrils described below as contained in the 

 cell-bod.v of the neurone and extending throughout all its processes. Quite recently Boek has 

 found that a sympathetic axone may sometimes accompany a cranio-spinal axone to an end 

 plate on a skeletal muscle fibre. 



Synapses. — Every functionally complete nerve pathway consists of two or more neurones 

 arranged in series. Very often, the series consists of many more than two, the impulses iDeing 

 transmitted from neurone to neurone. The axone, bearing the impulse away from the cell-body 

 of one neurone, gives off terminal branches, each of which loses its sheath and breaks up into 

 telodendria which twine themselves upon the dendrites or cell-body of another neurone. The 

 mpulse is transferred from one neurone to another by means of contact rather than by direct 

 anatomical continuity of the parts of the two neurones. Such terminations of axones are known 

 as synapses. 



Fig. 608. — Drawings Illustrating the Abundance and General Arrangement of the 



Tigroid Masses in Cell-bodies of Neurones in Resting Condition. 

 A, cell-body from spinal ganglion. B, large cell-body from ventral horn of spinal cord, a, 



axone. d, dendrites. 



Capsule 





Q ■ 



A B 



In the terminal arrangement of the telodendria, synapses assume forms varying from com- 

 pact "pericellular baskets" and "climbing fibres" to the more open arborisations composed of 

 fewer twigs in simpler arrangements, "end-brushes." In case of the spinal ganglion type of 

 neurone, the cell-body of the majority of which has no dendritic processes, the telodendria of the 

 visiting axone form an anastomosing pericellular plexus inclosing the entire cell-body. This and 

 the simple end-brush form of synapses are illustrated in fig. 606. It should be mentioned that, 

 contrary to the general belief that impulses are transmitted by simple contact of the neurones in 

 the series, it has been claimed that the ultimate twigs of the telodendria frequently penetrate the 

 substance of the receiving ceU-body and are fused in continuity. If during the processes of 

 growth this becomes true, instead of being an appearance produced by the technique employed, 

 it is better considered as merely an exception to the general rule. 



Internal structure of the neurone. — The cell-body of the neurone consists of a large, spherical, 

 vesicular nucleus and a cytoplasm continuous into its axone and dendritic outgrowths. Its 

 nucleus is further characterized by having most usually but one nucleolus, large, spherical and 

 densely staining, situated in a karyoplasm containing otherwise a remarkably small amount of 

 chromatin. Of the cytoplasm, the two most interesting structures are its fibrillar and its gran- 

 ular components. 



The fibrillar structure, known as the neuro-fibrilloe, represents a growth and elaboration of 

 the spongioplasmic reticulum of the original, embryonal cell. The filaments increase in thick- 

 ness during the development of the neurone, and, in the sending out of its processes, the meshes 

 of the original reticulum become so drawn out in the processes as to give the appearance of a more 

 or less parallel arrangement of threads. The reticular or net-like arrangement is usually more 

 nearly retained in the cytoplasm immediately about the nucleus, since here the stress of the out- 

 growing processes is less directly applied. In the ceU-body of the spinal ganglion type of neu- 

 rone, when no dendrites are given off, the net-like arrangement is apparent throughout the cyto- 

 plasm except in that region giving rise to the axone. On the other hand, in the typical so-called 

 "pjTamidal cell" of the cerebral cortex, from which two chief processes, the axone and the apical 



