DISEASES OF THE FETUS AND ITS MEMBRANES. 



The fetus is contained in the hermetically sealed uterine 

 cavity of the mother and is further protected by the fetal en- 

 velopes and fluids which surround and protect it from trauma- 

 tic injuries and from thermic changes, so that it is well guarded 

 against dangers from without. The maternal system furnishes, 

 through the placenta, abundant nourishment and removes by the 

 same avenue, its excretions. When describing the placenta we 

 noted that its structure is such that, while freely permitting the 

 passage of nutrient substances, from the mother to the fetus, and 

 waste products from fetus to mother, it generally serves as an 

 efficient barrier against the passage of disease-producing micro- 

 organisms. The fetus is thus protected upon every hand in an 

 unusual degree against disease or accident. 



Nevertheless the fetus is subject to many forms of disease and 

 accident which may imperil its life or bring about changes 

 which render it of little value when born. 



In the preceding pages, in tracing the development of the 

 fetus, we have from time to time mentioned some aberrations in 

 the development of the fetus, such as monstrosities and defects 

 which more or less destroy the value of the young animal. 



Though some of the.se aberrations in development render the 

 fetus incapable of living at the time of birth, and others have 

 little danger to the life of the young animal but simply affect its 

 value, none of them, as a rule, lead to death of the fetus during 

 its intra-uterine life. 



On the other hand, various di.sea.ses of the fetus or of its mem- 

 branes lead more or less frequently to its death and expulsion. 



Death of the Fetus. 



Death of the fetus occurs in all species of animals and at any 

 stage of intra-uterine life from the beginning to the end. In the 

 majority of cases, when death of the fetus occurs, it is promptly 

 expelled. This we know as abortion. 



In other instances, which are far more rare, the dead fetus is 

 not expelled but is retained within the uterus for a more or le.ss 

 indefinite period. The retained dead fetus may pursue a variable 

 course, according to circumstances. It may undergo aseptic 

 wasting or mummification. There may occur a maceration or 

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