IN reviewing the pathological specimens listed in our obstetrical and gynacological museum at McGill University my interest was drawn to the relative frequency of damage to the lower uterine segment during labour. I proceeded to examine these specimens and study the anatomi-cal changes in the lower uterine segment during pregnancy and labour and to correlate the incidence of traumatic defects in this section of the uterus.

The necessity of a threefold division of the uterus into corpus, isthmus and cervix must be accepted. Since the lower uterine segment takes in the cervix canal plus the isthmus, we must clearly define what the isthmus is in order to separate its relative anatomical alterations from those of the cervix canal. The isthmus, as described by 0. Frankl, is the hollow space in the cranial part of the cervix, the mucous membrane of which resembles that of the corpus without being identical with it morphologically or physiologically. It is not the circular border line between the cavity of the cervix, as Langer called it long ago. Aschoff denominates the" orificium uteri internum ana-tomicum" as the border line between the corpus and the cranial end of the isthmus, andnames the caudal aspect of the isthmus as the" orificium uteri internum histologicum". This de-scription, although practically speaking correct, is subject to alteration because the orificium isthmi anatomicum does not always coincide