THE CIllOULATIOX OF THE BLOOD. 237 



An exceeding] y interesting fact with regard to the passage of the 

 wave in any direction has been made out by partial division of the mus- 

 cular fibres at any point, whereby one part of the wall of the heart is 

 left connected with the other parts by a small portion of undivided 

 muscular tissue, and the wave of contraction then being only able to 

 pass to the next portion of the wall every second or third beat. Thus 

 division of the muscle has much the same effect as partial clamping it 

 in the same position, or of a ligature similarly applied, but not tied 

 tightly. It may, therefore, be suggested that Stannius' ligature acts as 

 it partial or complete block, and prevents the stimulus of the sinus-beat 

 from passing further down the heart, but that the parts below the liga- 

 ture may be made to contract by stimuli applied to them directly. 

 Nearly all the information to be obtained as to the phenomena of the 

 contraction of heart-muscle apart from the rhythmic action of the 

 organ itself, may be obtained from a heart to which a Stannius' ligature 

 has been applied; indeed, the effect of minimal stimuli, the effect of 

 rapidly repeated shocks, and the refractory period of heart-muscle may 

 all be studied from a heart in this condition. 



The velocity of the wave of contraction in frog's heart-muscle has 

 been shown to be f to f inch, or 10-15 mm. a second. 



In pointing out the differences between the phenomena of contrac- 

 tion in skeletal and heart-muscle, the similarities between the two are 

 not to be overlooked; thus it has been shown that the effect of cold, 

 heat, fatigue,' and other influences have very much the same effect in 

 both cases. 



2. The influence of the central nervous system. 



The heart is capable of automatic rhythmic movement, as hns been 

 clearly shown by its behavior when removed from the body, and it has 

 been shown further that there is reason for believing that the power 

 resides in the inherent property of its muscle fibres themselves. While 

 in the body, however, the heart's beats are under control of the central 

 nervous system. To this nervous control, we must next direct our at- 

 tention. The influence which is exerted by the central nervous system 

 appears to be of two kinds, firstly, in the direction of slowing or inhibit- 

 ing the beats, and secondly, in the direction of accelerating or augment- 

 ing the beats. The influence of the first kind is brought to bear upon 

 the heart through the fibres of the vagi nerves, and that of the second 

 kind through the sympathetic fibres. 



Influence of the Vagi. It has long been known, indeed ever since 

 the experiments of the Bros. Weber in 1845, that stimulation of one or 

 both vagi produces slowing of the beats of the heart. It has since been 

 shown in all of the vertebrate animals experimented with, that this is 

 the normal action of vagus stimulation. Moreover, section of one nerve, 



