82 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



mechanical excitation of one part of it might lead to a contraction of the 



whole. Similarly, a partly dried-up frog may be seen, if mechanically 



excited, to make movements simulating life. The cause of these movements, 



also, is not understood. Drying of the muscle in its early stages greatly 



increases its irritability because of the concentration of the salts, but that 



does not account for the loss of insulation. 



Transmission of Excitation by Means of End-organs. — In spite of the 



rapid advances which have hem made in the histology and physiology of the 



nervous system during the past few years, we are still in doubt as to the exact 



way that the axone, the exciting branch of the neurone, stimulates the cell 



to which it is distributed. In many 



cases, at least, the axone terminates in an 



end-organ which is physiologically dif- 



j ferent from the rest of the cell, and this 



end-organ is the exciting agent. The 



relation of the protoplasm of the end- 



organ to the protoplasm of the cell which 



it stimulates, whether one of continuity 



or contiguity, is not certain, but most 



histological and physiological observa- 



Fig. 31. Nerve-termination in voluntary tions are distinctly in favor of the latter 

 muscle of the rabbit, stained in methylen-blue 



vitam), fixed, sectioned, and counter- View, 

 stained in alum carmin. A, surface view; B, The physiology of the end-organs of 



longitudinal section through nerve tormina- .. .. ' 



tion and muscle-fibre; C, cross-section; S, motor axones distributed to striated 



sarcolemma; n. I., neurilemma. (From Text- muscles is best known 

 book oj Histology, Bohm and Davidoff, revised 

 by '■ < Huber W. B. Saunders, Philadel- Fig. 31 sllOWS a surface View and 



pina, 1900). a l on git U( lj na l an( ] cross-section of the 



end-organ of an axone supplying a voluntary muscle of a rabbit. The axis- 

 cylinder loses its medullary sheath shortly before reaching the fibre, and the 

 neurilemma becomes continuous with the sarcolemma, so that the axis-cylin- 

 der on penetrating the sarcolemma comes into direct contact with the sarco- 

 plasma of the muscle. The sarcoplasroa is heaped together at this place, 

 making a little mound, and the axis-cylinder, after dividing into a number 

 of fine terminal twigs, vmh in the midst of this mass of sarcoplasma. Evi- 

 dently the nerve and muscle protoplasm come into very close relation. On 

 the other hand, nerve and muscle protoplasm retain each its peculiar reaction 

 to staining-fluids, and as far as these chemical reactions can show each main- 

 tain- it- peculiar chemical and histological structure. Moreover, the results 

 of physiological experimentation have shown that, although no definite histo- 

 logical boundary has been found between the axone and its terminal organ, 

 the exciting organ must be considered to be a specially differentiated struct- 

 ure, differing widely from the rest of the neurone. 



The motor end-organ uses up more time in the excitation of the muscle 

 than would be required for transmission of the excitation through a like 

 amount of nerve- or muscle-substance. It is found by experiment that a 



