TUBERCULO-INFECTION OF MAN. 
555 
tions, exceed 150 mm. per second for the whole of its path. This 
would correspond to rather more than one-third of a mile per 
hour. In the arch of the aorta the average speed may be twice 
as great. Yet a red corpuscle, even if it continued to move with 
the velocity with which it set out through the aorta, would only 
cover about fifteen miles in twenty-four hours, and would re¬ 
quire five years to go around the world.” (Stewart’s Manual of 
Physiology.) Although the average diameter of a capillary is 
only about 10 u (5 to 20 u in different parts of the body), the 
number of branches is so prodigious that the total cross section of 
the systemic capillary tract has been estimated at 500 to 700 times 
that of the aorta. The total cross-section of the vascular chan¬ 
nel gradually widens as it passes away from the left ventricle. 
In the capillary region it undergoes a great and sudden increase. 
At the venous end of this region the cross-section is, however, 
again somewhat abruptly contracted, and then gradually lessens 
as the right side of the heart is approached; but the united sec¬ 
tional area of the large thoracic veins is greater than that of the 
aorta. (Stewart, page 105.) In man, Stewart adds, the pulmo¬ 
nary circulation-time is probably usually not much less than 
twelve seconds nor much more than fifteen seconds—a compara¬ 
tively long time. (Stewart, page 111.) 
Anatomical Considerations .—These are of two kinds—such 
as refer to ordinary normal anatomical conditions, and such as 
are abnormal. 
A word about the latter: It is especially to the arrangements 
of the blood-vessels that I have reference. As is well known, 
they are subject to numerous variations. These may consist of a 
deviation either from the usual size of the channels or from their 
usual position and their connections with other vessels. Many 
of these varieties, if less efficient, are, nevertheless, not only com¬ 
patible with life, but cause no disturbance whatever in the per¬ 
formance of the ordinary functions of the body. Others, again, 
are of such a nature as to be compatible only with the conditions 
of the circulation subsisting during intra-uterine life, and there¬ 
fore prove fatal at birth. Some are of considerable interest from 
