The diameter of the placenta is about six inches, and its thickness about one inch and a half. The length of the umbilical cord is from eighteen to twenty-four inches, its diameter about half an inch. These dimensions are, however, subject to great variation. Instances are mentioned of the cord being five feet long, and as thick as the child's arm. I have seen one myself four feet long. Sometimes it will be very short, not more than eight or ten inches. It is composed of one artery and two veins, twisted together like the strands of a cable, and of a sheath surrounding them composed of the chorion and amnion. Between the sheath and the vessels is a thick gelatinous fluid called the Gelatine of Wharton. This explanation, it must be remembered, is in fact merely hypothetical. The direct passage of the blood through the Placenta, from the mother's vessels into those of the cord, is denied by many physiologists, who contend that there is an intermediate set of vessels in the Placenta, in which it first undergoes important changes. They also contend that the impure blood does not pass through into the mother's veins at all, but is purified in the Placenta, and immediately returned. Some have even averred that the Placenta is not required at all, to supply nourishment, but is merely a purifying organ. It is now known, however, that it is not absolutely essential to either process, for children have been born alive, and perfectly formed, which merely floated loosely in the amniotic liquor, having neither Placenta nor cord, nor any other connection with the mother. How they were nourished we cannot tell. These, however, must be regarded merely as curious exceptions, there being little doubt but that fœtal nutrition is ordinarily effected through the Placenta and cord, by means of the mother's blood, somewhat in the manner we have described. PECULIARITIES OF THE FŒTAL CIRCULATION. From the circumstance of the fœtus not using its heart and lungs, like the adult, its circulation has several modifications.