116 THE PHYSIOLOGY OF NEEVE 



plates for its point of attack, while nicotin paralyzes the dendritic 

 processes of the cell-body and atropin the terminals of the axon. 

 Secondly, it is a well-known fact that the time which an impulse requires 

 for its passage through a neuron, is largely taken up by its journey 

 through the cell-body and the end-plate. In the latter, for example, 

 the delay is appreciable, amounting to more than one thousandth of a 

 second. Thirdly, it has been demonstrated that the fatigue of muscle, 

 resulting from excessive indirect stimulation, makes itself felt first 

 of all in the end-plate and not in the nerve-fiber nor in the muscle 

 tissue. These and other facts unmistakably point toward the 

 presence of a third substance which, strictly speaking, is neither nerve 

 tissue nor muscle tissue but a modification of the former. It is usually 

 designated as the intermediary or receptor substance. It is conceivable 

 that the constituents of this substance arrange themselves as electro- 

 lytes in a way to permit of the passage of the excitation in only one 

 direction. This is true of the end-plate as well as of the synapse. 



The Function of the Different Parts of the Nerve. — The trans- 

 mission of the wave of excitation is effected by the axis-cylinder, or 

 rather, by the neurofibrils of which it is composed. The latter, as 

 has previously been shown, ramify throughout the cytoplasm and 

 form connections between the different poles of the cell-body and its 

 processes. 



The myelin sheath is said to possess a protective, insulating and 

 nutritive function. The first assertion finds substantiation in the 

 fact that the medullary sheath is composed of a spongy network con- 

 taining a soft fatty material. Thus, if a nerve-fiber is torn, droplets 

 of a substance will be seen to ooze out which exhibit a double outline 

 similar to that of the nerve-fiber itself. If subjected to osmic acid, 

 these globules stain black, owing to the reduction of the osmium. 

 Moreover, the cross-section of a fiber invariably appears as a heavy 

 dark ring surrounding a light, faintly stained central area. It is also 

 a well-known fact that ether and other solvents are capable of removing 

 this fat at least in part so that the fiber assumes the appearance of a 

 round tubular space surrounding the axis-cylinder. The latter may 

 then be stained with carmin and other dyes to render it more conspi- 

 cuous. It seems, however, that the contention that the myelin sheath 

 supports and protects the axis-cylinder in a mechanical way, cannot be 

 emphasized especially, because the non-medullated axons of the 

 sympathetic system show perfect conduction. Moreover, axons are 

 never medullated throughout their entire extent but lose their sheath 

 near the cell-body as well as near the end-organ. In the third place, 

 while the cerebrospinal nerves are ordinarily in possession of such a 

 covering, they do not attain it simultaneously but at different periods 

 of embryonic life. In fact, in some animals, such as the rat, this sheath 

 is not developed until several days after birth. Meanwhile the new- 

 born animal shows perfectly coordinated movements.^ 



1 Donaldson, Jour, of Comp. Neurology, xx, 1910, 119; and Ambronn and Held, 

 Arch, fur Anat. und Physiol., 1896, 208. 



