PRESENTATIONS AND POSITIONS OF THE FETUS. 



-In a general way, we have already outlined the physiology of 

 labor and have indicated the means by which the fetus is expelled 

 from the uterus when it has completed its intra-uterine develop- 

 ment or when it has been thrown out because of its death or 

 disease in case of abortion or premature birth. 



It is essential that we should thoroughly understand that, in 

 order for these physiologic processes to be carried out in a normal 

 manner, it is necessary for the attitude of the fetus to be such 

 that it will be practicable for it to pass through the birth canal. 

 There are certain attitudes of the fetus which make its passage 

 possible, while others render it virtually impossible. The possi- 

 bility of a fetus being born alive and without assistance depends 

 fundamentally upon which parts of the fetal body present at the 

 inlet, and secondarily upon the relations of the parts which 

 present to the circumference of the pelvis. In dealing with the 

 mechanism of parturition, we recognize two fundamental ele- 

 ments in reference to the attitude of the fetus — presentation and 

 position. 



Presentation, mechanically expressed, is the relation existing 

 between the spinal axes of the mother and fetus. The term in- 

 dicates that portion or general region of the fetus which offers 

 at the pelvic inlet at the time of partui'ition. 



Position expresses the relation of the presenting portion of the 

 fetus to the circumference of the pelvic inlet. It is the relation 

 of the presenting part to the sacro-pubic or the bis-iliac diame- 

 ter of the maternal pelvis. 



Presentations of the Fetus. 



The uteri of domestic animals are more or less tubular in their 

 general outline, and the fetus represents an elongated oval, the 

 long axis of which normally corresponds with the long axis of 

 the uterine cavity. The transverse diameter of the body of the 

 fetus approximately equals the dimensions of the dilated birth 

 canal, through which it must necessarily pass in being born. 

 The fetal chest has an even greater diameter than that of the 

 bony canal through which it must pass. Not only is it necessary 

 that the long axis of the fetus should be parallel to that of the 

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