428 Proceedings of Royal Society of Edinhurgh. [jult 9 , 
I hope to be able to show that one mechanism accounts for all. 
At the end of pregnancy the uterus will be found to be divided 
into three parts, viz., the cervix with its canal ^ the lower uterine 
segment, and the uterine musculature above the latter. These parts 
are defined as follows : — 
The cervical canal opens by the os externum into the vagina 
below, and is sharply defined above by the os internum. The 
cervical canal has its anterior and posterior walls in apposition, 
and remains intact until the beginning of labour. Since Stoltz in 
France, and Duncan in this country, insisted that the cervical canal 
during pregnancy took no part in the formation of the uterine 
cavity projDer, and was never encroached on by the foetus, many 
attacks have been made on their view; but all, so far as I can judge, 
have not shaken it. The most valuable section of a full-time 
pregnancy, published recently by Waldeyer of Berlin, confirms this 
doctrine in every particular. 
The lower uterine segment is the segment of the uterine wall lying 
within 2 inches or so of the os internum, and is characterised by 
the loose attachment of the peritoneum to it. Where the peritoneum 
becomes firmly attached, the third portion of the uterus begins. 
The lower uterine segment is bounded below by the os internum, 
while above, there is, in addition to the peritoneal limit already 
mentioned, a part of the uterine wall as its upper boundary 
where the contraction ring and circular vein develop during 
labour. 
The uterine cavity is lined by the placenta and membranes. 
Normally the placenta is placed above the upper limit of the 
lower uterine segment, although rarely part of it dips into it, 
constituting the dangerous condition known as placenta praevia. 
The membranes line the part of the uterine wall unoccupied by 
placenta, and in the great majority of pregnancies, therefore, the 
lower uterine segment is covered on its uterine aspect by membranes. 
The membranes consist of amnion, chorion, decidua reflexa, and 
decidua vera. For our present purpose, we regard them from a 
purely physical aspect. Near the uterine wall v^e find a spongy 
layer, made up of minute trabeculae, and this divides the membranes 
into a compact layer lying next the uterine cavity, and a thin layer 
set on the uterine wall. These spaces are the fundi Qf the uterine 
