THE CIRCULATION OF THE BLOOD. 981 
course of a single hour are of the most complex and variable 
character. 
Though special attention has been drawn to such rhyth- 
mical variations as may be witnessed in the rabbit’s ear, bat’s 
wing, etc., there can be little doubt that changes as marked, 
though possibly less distinctly rhythmical, are constantly tak- 
ing place in the vertebrate body, and especially in that of man, 
with his complex emotional nature and the many vicissitudes 
of modern civilized life. The frequent changes in color in the 
faces of certain people are in this connection suggestive, though 
we hope we have made it clear that these vascular modifica- 
tions are dependent chiefly on centripetal influences from every 
quarter, though actually brought about by centrifugal im- 
pulses. Whether there is a rhythm obscured by minor rhythms, 
owing to an independent or automatic action of the vaso-motor 
center, though not improbable, must be regarded as undeter- 
mined as yet. 
The question of the distribution of vaso-motor nerves to 
veins is also one to which a definite answer can not be given. 
THE CAPILLARIES. 
The cells of which the capillaries are composed have a con- 
tractility of their own, and hence the caliber of the capillaries 
is not determined merely by the arterial pressure or any similar 
mechanical effect. 
Certain abnormal conditions, induced in these vessels by 
the application of irritants, cause changes in the blood-flow, 
which can not be explained apart from the vitality of the ves- 
sels themselves. 
Watched through the microscope under such circumstances, 
the blood-corpuscles no longer pursue their usual course in the 
mid-stream, but seem to be generally distributed and to hug the 
walls, one result of which is a slowing of the stream, wholly 
independent of events taking place in other vessels. It is thus 
seen that in this condition (stasis) the capillaries have an in- 
dependent influence essentially vital. We say independent, for 
it is still an open question whether nerves are distributed to 
capillaries or not. That inflammation, in which also the walls 
undergo such serious changes that white and even red blood- 
cells may pass through them (diapedesis), is not uninfluenced 
by the nervous system, possibly induced through it in certain 
cases, if not all, seems more than probable. 
