CIRCULATION. 59 



is propagated along the fibres at a rate which agrees 

 with that of the propagation of the wave of contraction, 

 so that the latter is preceded in its progress by the 

 former. In nerved muscles, which are excited through 

 their nerves, the excitatory waves are similarly propa- 

 gated, but originate from nerve endings. In injured 

 muscle the electrical difference between injured and 

 uninjured surfaces is diminished during excitation. 

 This excitatory effect was therefore called by du Bois- 

 Reymond the "negative variation." 



The excitatory disturbance is, in all cases, followed by 

 other changes, which correspond in time to the contrac- 

 tion, but have not yet been fully investigated. 



All of the phenomena above described are comprised by du Bois-Reymond 

 in a theory, according to which every portion of living muscular fibre con- 

 tains electromotive particles, each of which has ends (poles), which are 

 directed towards the ends of the fibre, and are negative to the zonal surface, 

 which corresponds to the surface of the fibre. The effect of excitation is 

 to produce a momentary diminution of the electromotive force of these 

 particles. The particles being called by du Bois-Reymond * molecules, ' the 

 theory is known as the molecular theory of the muscle current. 



In involuntary muscle the process of contraction is 

 similar, but much slower. It is attended with electrical 

 changes of the same nature as those observed in volun- 

 tary muscle. 



CIRCULATION. 



The Arterial Circulation. The arterial system is an 

 elastic receptacle for blood, the form of which is dendritic. 

 At the ends of the ramifications, blood flows by in- 

 numerable capillary channels into the venous system. 

 Into the trunk, blood is injected at intervals by the 

 heart, each injection lasting from three to four-tenths of 

 a second. In the aorta the blood is squeezed by the 

 arterial wall with pressures which probably vary from 



