. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 678 UTERUS AND ITS APPENDAGES. covers the placenta, loses by degrees the cha- racter of active vitality, and its tissues are con- verted into molecular fat. Other and corresponding changes, of which an account will be hereafter given (see PLA- CENTA), occur in those structures in which the foetal blood circulates. The profusely de- veloped capillaries which ramify within the villi during the early and middle periods of gestation begin to suffer retrogression as the time of separation approaches, and the fcetal blood flow
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 678 UTERUS AND ITS APPENDAGES. covers the placenta, loses by degrees the cha- racter of active vitality, and its tissues are con- verted into molecular fat. Other and corresponding changes, of which an account will be hereafter given (see PLA- CENTA), occur in those structures in which the foetal blood circulates. The profusely de- veloped capillaries which ramify within the villi during the early and middle periods of gestation begin to suffer retrogression as the time of separation approaches, and the fcetal blood flows in more simple and relatively fewer channels, while, not unfrequently, entire villi become obliterated by calcification. While these changes are proceeding in the temporary structures that serve to connect the foetus with the uterus, structures which begin in part, at least, to become effete, even before the offices for which they are formed have been fully carried out,—the tissues which are to be employed in the process of expulsion are as yet only ripening into full strength, although they also, in turn, are about to suf- fer a like retrogression, but not until the ob- ject of their formation has been accomplished. The contractile fibre, which constitutes the principal portion of the uterine tissue, has gradually, during pregnancy, advanced to that more complete form which is reached com- monly about the sixth month. From this pe- riod probably no new development of muscular fibre takes place,although that which is already formed appears to increase somewhat in size and power. It constitutes now a contractile tissue, capable of exerting great expulsive force. How easily, and in how many ways, the contractile power may be evoked, has been already shown. It is probable that by the series of metamorphoses already enumerated as occurring in the parts which connect the foetus with the uterus, the entire ovum becomes gradually placed in the position of a foreign body within th
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