254 THE ADRENAL SYSTEM AND VASOMOTOR FUNCTIONS. 



contractile substance through the latter's own investing mem- 

 brane. Increase of plasma means increase of work: i.e., mus- 

 cular contraction. Waste-materials are as rapidly returned 

 through efferent capillaries to the venules, thus leaving the 

 field clear for continuous function. 



While we have perhaps simplified the processes mentioned, 

 we have complicated another, since we now find it necessary 

 to account for the functions implied by "voluntary," "motor," 

 and "constrictor," all through the operation of one set of 

 nerves. And yet we are now in perfect accord with the ana- 

 tomical side of the question, since there is no evidence that 

 constrictor nerves exist. The only nerve distributed to a 

 voluntary or striped muscle proper, the motor nerve, enters 

 its sheath, breaks into numerous subdivisions, and thus sciul.s 

 one filament occasionally two to each muscular fiber. On 

 the surface of the latter, near the middle, an important ter- 

 minal arrangement prevails: i.e., each nerve-fiber develops its 

 "motor end-plate!" Its white matter of Schwann ceases and 

 its outer covering becomes continuous with that of the muscle, 

 so that its axis-cylinder alone penetrates to the muscular fiber. 

 Here it subdivides into numerous root-like processes, forming 

 a hillock, or motor end-plate, supported by a layer of granular 

 substance which contains a number of large nuclei. It is this 

 end-plate that the impulse first strikes when it reaches the 

 muscular fiber, and it travels from the center of the latter to 

 the two ends. All the elements of the muscle are so disposed 

 as to receive the impulse simultaneously. 



Before analyzing the mechanical result of this impulse or 

 going further into the vasodilator question, a brief allusion to 

 the histology of the arterioles of voluntary muscle must be 

 made. The internal coat is composed, as elsewhere, of endo- 

 thelial cells; when the middle coat is cut transversely, how- 

 ever, it presents a peculiar conformation: its outline festooned. 

 Eanvier 5 divides the middle coat into two layers: an internal 

 elastic lamina and a muscular layer. In his description of 

 these structures he says: "The internal elastic lamina, as is 

 the case with all parts formed of elastic substance, possesses 



6 Ranvier: quoted by H. Berdal, "Histologie Normale," 1894. 



