DEVELOPMENT OF THE F(ETUS. 43 



the beginning of the second month, it nearly equals the half of the 

 whole body. In the fifth week, the extremities become visible, the 

 upper generally appearing first, in the form of small blunt promi- 

 nences. As they develope themselves, the distant points appear first, 

 that is, first the hands, which seem to be fixed to the shoulders, then 

 the fore-arm, then the arm. The same is true of the lower extremi- 

 ties. The external organs of generation appear after the develop- 

 ment of the extremities, as also do the nose, ears, and mouth. Ossi- 

 fication commences in the base of the cranium, and the bones under 

 the scalp are those in which the process is last completed. The 

 length of a full-grown foetus is about eighteen inches, the weight 

 between six and eight pounds as the average. 



Duration of pregnancy. — On this subject very little can be posi- 

 tively said, the average duration being about ten lunar months, or 

 280 days. There can be no doubt that many females fall short of 

 this computation, as well as that there are many who go beyond it. 

 Many well-authenticated instances are on record, under the authority 

 of Drs. Merriman, Blundell, Montgomery, Dewees, Meigs, and others, 

 where both these deviations have occurred. The same is true of the 

 lower animals. Dr. Rigby supposes that the duration of pregnancy 

 is much influenced by the time in the intermenstrual period at which 

 conception took place ; that there is at every menstrual period an 

 irritability about the uterus, even when gravid, which disposes it to 

 take on contraction. Hence, when impregnation has occurred im- 

 mediately after an appearance of the menses, the uterus will have 

 attained such a dilatation and weight of contents by the time that 

 the ninth period has arrived, that it will not be able to pass through 

 this state of catamenial excitement without contracting, or in other 

 words, labour coming on : hence it is that we find a considerable 

 number of labours fall short of the usual time, so much so, that 

 some authors have even considered the natural term of human ges- 

 tation to be 273 days, or 39 weeks. 



If, on the other hand, impregnation has occurred just before a 

 menstrual period, the uterus may not have attained such a development 

 as to prevent its passing the ninth period without expelling its con- 

 tents, but may even go on to the next without this process taking 

 place. Under this view of the subject, he further remarks, " the 

 duration of time between each menstrual period should also be taken 

 into account, some women menstruating at very short, others at very 

 long intervals ; but although this will affect the number of periods 

 during which the pregnancy will last, it will not influence the actual 

 duration of time, as this will more immediately depend upon the 

 size and weight of contents which the uterus has attained." 



The method of calculation, is to compute from a fortnight after 

 the last appearance of the menses, and the period so fixed is usually 



