THE CIRCULATION OF THE BLOOD. O51 
blood to the nutrition of the tissues; the fact that all the tubes 
that compose the vascular system are made up of living cells; 
that some of these 
cells (in arterioles and 
capillaries) are in a 
semi-fluid condition— Ahi. UW 
in a word, that the 
conditions of the cir- fia Pats fixe 7 
culation as a whole oa Nea 
Fig. 226.—Dicrotic pulse-curve due to hemorrhage. From 
are sut generis, be- carotid of rabbit, with extra-vascular pressure of, in a, 
hei italit 50 mm., b, of 40 mm., c, of 20 mm., and d, of 10 mm. 
cause of their vitali iy mercury. (This and the preceding six tracings from 
—it seems to us amaz- Hosters} 
ing that purely physical explanations, such as would answer 
for a pump and set of rubber tubes, should ever have been 
deemed satisfactory. The whole subject seems to be involved 
in a gross misconception, and should be regarded, we must 
think, from an entirely new standpoint. 
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 witnessed in the veins into which the capillaries open out, 
owing to diminution in the resistance which usually is suffi- 
ciently great to obliterate the pulse-wave. 
Pathological—_tIn 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-flow, 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 over-stepped. 
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 
cardiac physiology—one might almost say revolutionized the 
subject. 
