430 Proceedings of Boyal Society of EdinhurgJi. [july 9 , 
during retraction diminishes the uterine area it hounds. The 
changes there taking place are prohahly as follows : — 
As the result of the uterine pains in the first and second stages of 
labour, and in the beginning of the third stage, we get a diminution 
in area of the placenta, i.e., in its long axes, and an increase in its 
thickness. No separation of the placenta takes place until some 
time after the child is horn. It is further known that the foetal 
heart is slowed during the pains, and also, as we can note when the 
child’s head is horn, that the face becomes congested during a pain, 
the congestion passing off as the pain dies away. 
To understand all this, we must consider briefly the blood supply 
of the placenta and uterus. 
The placenta has blood poured into it from two sides. On the 
amniotic side the umbilical artery gives it a most abundant supply, 
and one that rapidly pours into it. On its attached side the curling 
arteries pour blood into the intervillous spaces, the two blood 
supplies thus interdigitating with one another. The foetal blood 
passes in by two arteries and returns by one vein. The venous 
supply of the uterus is, so far as Hyrtl’s injections show, much more 
abundant than the arterial. 
The maternal blood pours by the curling arteries directly into the 
intervillous spaces, and returns by veins to the uterine wall. The 
result of a uterine pain is to compress the curling arteries, and 
prevent blood passing into the intervillous spaces. It does the 
same to the uterine veins, hut as these are so abundant, the blood 
in the intervillous spaces drains off, and we get no congestion nf 
them. I have examined microscopical sections of a parturient six 
months’ uterus, and found no blood in the intervillous spaces. The 
abundant foetal blood supply of the placenta is well shown in them 
(PI. XYIII. fig. 4). The same holds good in a case of Porro uterus, 
an inverted uterus with placenta attached, and in the separated full- 
time placenta; in all, the intervillous spaces are empty (figs. I, 2, 
and 6). They are indeed practically obliterated, and the villi are in 
close apposition (PL XYIII.). This is quite different in the pregnant 
and non-parturient uterus. In a four months’ pregnancy examined 
(PI. XYIII. fig. 3), the intervillous spaces are wide and the villi far 
apart. During a pain, therefore, the diminution in area of the placenta 
is compensated for by its thickening, and there is prohahly an actual 
