LENGTH OF GESTATION. 467 



little more than six months ; and in which, by careful treatment, the 

 infant was reared in a condition 'of health and vigour. And there is 

 reason to believe, that infants have lived for some time, and might pro- 

 bably have been reared under better management, that were born as 

 early as the 24th or 25th week. 



827. The act of Parturition, by which the foetus is expelled from the 

 Uterus, is accomplished in part by the contractile power of the Uterus 

 itself; and in part by the combined operation of the various mus- 

 cles, which press upon the abdominal cavity, and which effect the expul- 

 sion of the faeces and urine. No definite account can be given of the 

 reasons, why this change should take place at the period which has been 

 mentioned as its usual date ; but we are as much in the dark in regard 

 to other periodic phenomena of Animal life ; and we must probably 

 look for its source in the maturation of the placental structure, which 

 prepares it for detachment (like the dropping-off of a ripe fruit), and in 

 the complete evolution of the contractile tissue of the uterus, the con- 

 tractions of which may be considered to commence spontaneously when 

 it has attained a certain epoch in its growth, just as do those of the 

 heart in the embryo ( 814). For some days previously to the com- 

 mencement of labour, there is usually a slow contraction of the fibres 

 of the fundus and body of the uterus, and a yielding of those of the 

 cervix; so that the child lies- lower, and the size of the abdomen dimi- 

 nishes. This slow contraction is probably not dependent upon any act 

 of the nervous system ; but upon the direct excitement of the contrac- 

 tility of the muscular substance of the uterus. When labour properly 

 commences, however, the Spinal system of nerves comes into play, and 

 the uterine contractions are of a reflex nature. As before, however, 

 the act of contraction is confined to the fundus and body of the uterus ; 

 the fibres of the cervix uteri, and of the vagina, being in a state of re- 

 laxation, which allows them to yield to the pressure of the child's head. 

 In the first stage of labour, the Uterine contractions appear to be alone 

 concerned ; and it is not until the head of the child is passing through 

 the os uteri, and is entering the vagina, that the assistance of the ab- 

 dominal muscles is called in. These act, in the first instance, as in 

 ordinary expiration ; but their power is much increased by the voluntary 

 retention of the breath, so that the whole of their contractile force may 

 be applied to the expulsion of the foetus. In a later stage of labour, 

 this retention of the breath becomes involuntary, during the accession of 

 the "pains;" and the expulsion of the foetus is commonly effected with 

 considerable force, especially if the previous resistance has been con*- 

 siderable. 



828. The same action which expels the foetus, usually detaches the 

 placenta ; and if the uterus contract with sufficient force, after this has 

 been thrown off, the orifices of the vessels which communicated with it 

 are so effectually closed, that little or no hemorrhage from that source 

 takes place. When efficient contractions do not occur, they may fre- 

 quently be excited by pressure upon the uterus itself ; by the applica- 

 tion of cold to the abdominal surface, to the extremities, and (in severe 

 hemorrhage) to the entire body ; or by the application of the child to 

 the nipple, which will frequently at once succeed in producing the de- 



