THE NEURON AND ITS CONDUCTING PATHS 115 



The lipoids are found chiefly in the myelin sheath, but as non-medullated fibers 

 also contain them, they are not restricted to this particular part of the fiber. 

 Medullated fibers, moreover, contain a much larger quantity of cerebrosids than 

 the non-medullated, while the latter exceed in the lipoids, such as lecithin, kephalin 

 and cholesterin. Ordinary fat is found in the epineurium, and gelatin in the con- 

 nective tissue throughout the nerve. Very small amounts of creatin, xanthin, 

 lactic acid, uric acid and urea have also been detected. The quantity of inorganic 

 salts is small, amounting to only about 1.0 per cent, of the total solids. Potassium 

 which is most abundant, 1 is said to play an important part in conduction. 2 



The Function of Nerves. In the lower forms in which nervous 

 elements are not present, the wave of excitation is propagated to other 

 parts of the relatively small organism in a direct way, because proto- 

 plasm possesses not only the power of irritability but also that of con- 

 ductivity. In a measure this is also true of the higher animals, but 

 the conduction of the waves of excitation must here assume a some- 

 what different character, owing to the minute subdivision of the body 

 into many colonies of cells which are frequently widely separated from 

 one another. Previous to the disco veiy of the nerves it was believed 

 that these impulses pursue a direct course in all directions through the 

 different tissues, but we now know that long-distance conduction is 

 effected solely with the help of nervous tissue which is especially 

 suited for this function on account of its unusual irritability and con- 

 ductile power. Conduction, therefore, presents itself first of all as an 

 intracellular propagation of the wave of irritability and secondly, as a 

 transfer of this wave to other 'colonies of cells elsewhere in the body. 

 The result of this transmission of an excitation depends of course upon 

 the character of the end-organ with which the nerve is connected, as 

 well as upon the functional qualities of its center. Inasmuch as it is 

 the function of the nerve to conduct impulses, the character of the 

 energy evolved in consequence of it, must therefore be wholly dependent 

 upon the effector with which it is functionally connected. 



Irreciprocal Conduction. The preceding discussion has brought 

 out the important fact that the conduction in neurons is irreciprocal, 

 i.e., it takes place in only one direction. Thus, an impulse passes 

 with greatest ease across the end-plate into the muscle, but not from 

 the muscle into the axon and the cell-body. The same conditions 

 prevail in the synapse, the conduction being from the axon of one 

 neuron into the dendrites and cell-body of the next. This "Law 

 of Forward Direction, " as it has been called by Sherrington, 5 possesses 

 a physico-chemical basis, inasmuch as it has been shown that the 

 different parts of the neuron are not built up of the same chemical 

 substances. That this is so may be gathered from the fact that 

 such agents as curare, nicotin, atropin and adrenalin do not affect the 

 neuron uniformly throughout its substance but only in particular 

 places. Curare, as has been pointed out previously, selects the motor- 



1 Macallum, Ergebn. der Physiol., vii, 1908. 



2 Macdonald, Proc. Royal Soc., Ixxvi, 1904-05, 322. 

 * Proc. Royal Society, London, lii, and following. 



