412 MINOT. [Vot. II. 
again take a more horizontal course; the branches then curl 
over downward, and, after a second short descent toward the 
decidua, again send out horizontal branches. The result of this 
arrangement is a terrace-like appearance in the course of the 
vessels ; they approach the uterine side of the placenta in this 
very characteristic manner. The number of terraces is variable ; 
usually there are two or three, but sometimes there is only one, 
or they may number four or even five. Arrived at the end of 
its terraces, the main vessel takes a more nearly perpendicular 
course, and rapidly subdi- 
vides. Immediately after 
entering the villi, the ar- 
teries and veins give off 
but few capillaries, but 
after a short course in the 
main stalk of the villus, 
the vessels give rise to 
many branchlets, and grad- 
ually the character of the 
circulation changes, until 
in the smallest villous 
twigs there are capilla- 
ries only, ‘Gat’ 27-7 ine 
, oe , vascular trunks have 
Cut 27.— Portion of an injected villus 
from a placenta of about five months; magni- a marked tendency to di- 
fied 210 diams. chotomous division, which 
is maintained within the 
villi to a certain extent; the arterioles and veinlets in the 
mature placenta go from their trunks at wide angles for the 
most part, and subdivide in the same manner, so that they 
spread out through the whole substance of the placenta. The 
vessels next the decidua take a more horizontal trend, like the 
top branches of a wind-swept tree. As the vessels run in the 
villi, of course the way in which the latter branch out deter- 
mines the paths of the former ; hence by following the distri- 
bution of the vessels we inform ourselves as to the ramifica- 
tions of the villi. Thus the horizontal course of the vessels 
on the uterine side of the placenta corresponds to the well- 
known fact that the ends of the villi attached to the uterus 
become bent and adhere by their sides to the decidual surface.” 
