244 



COMPARATIVE PHYSIOLOGY. 



curve. 2. A downward curve, rendered irregular by the occur- 

 rence of peaks or crests and notches. The first of these are 



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Fig, 208.— Pulse tracing from carotid artery of healthy man (after Moens). x, com- 

 mencement of expansion of artery; A, summit of first rise; C, dicrotic secondary 

 wave; ■ B, prcdicrotic secondary wave; w, notch preceding this; D, succeeding sec- 

 ondary wave. Carve above is that made by a tuning-fork with ten doable vibra- 

 tions m a second. 



termed the predicrotic notch and crest, and the succeeding ones 

 the dicrotic notch and crest. The latter seem to be the more 

 constant. 



Venous Pulse. — Apart from the variations in the caliber of 

 the great veins near the heart, constituting a sort of pulse, 

 though due to variations in intra-cardiac pressure, a venous 

 pulse proper is rare as a normal feature. One of the best-known 

 examples of such occurs in the salivary gland. When, during 

 secretion, the arterioles are greatly dilated, a pulse may be wit- 

 nessed in the veins into which the capillaries open out, owing 

 to diminution in the resistance which usually is sufficiently 

 great to obliterate the pulse-wave. 



Pathological. — In severe cases of heart-disease, owing to 

 cardiac dilatation or other conditions, giving rise to incompe- 

 tency of the tricuspid valves, there may be with each ventricu- 

 lar systole a back-ilow, visible in the veins of the neck. 



A venous pulse is a phenomenon, it will be evident, that 

 always demands special investigation. It means that the usual 

 bounds of nature are for some good reason being overstepped. 



Comparative, — Before entering on the consideration of phe- 

 nomena that all are agreed are purely vital, we call attention to 

 the circulation in forms lower than the mammal, in order to 

 give breadth to the student's views and prepare him for the 

 special investigations, which must be referred to in subsequent 

 chapters; and which, owing to the previous narrow limits (re- 

 searches upon the frog and a few well-known mammals) having 

 at last been overleaped, have opened up entirely new aspects of ■ 



