8 



membranes ; hence, to be very active it would call for considerable force for 

 producing this. 



FIG. 5. EXTREMITY OF FCETAL TUFT, from human placenta at term, in its recent condition. 

 A, a, capillary bloodvessels. Magnified 135 diameters. (After Dalton.) 



During attachment, when the sinuses are filled with the maternal blood they 

 are greatly larger than after detachment, when the blood is forced out, and the 

 tufts are compressed against each other in the maternal passages, reducing 

 the placenta fully one-half its size. At the time of birth, the necks of the 

 uterine sinuses, where they join the placenta, are readily torn through (Fig. 4, 

 o, c, c, c), and, peeling off easily with the decidua, the organ is expelled, unless, 

 forsooth, abnormal nutritive changes in the parts have made the union more 

 intimate. 



The problem in the mechanics concerns the manner by which the blood is 

 speeded in and out thesf sinuses commensurate with the increasing wants of the 

 growing embryo, and how this in turn is connected with the arterial and venous 

 systems of the mother for compelling correspondence in them, the whole forming 

 & connected movement a movement within a movement, so to speak. 



We have seen that change of pressure is the law of the animal circulation., ;md 

 it now remains to make rigid application of this principle in mechanics to 

 the special phenomena, .anatomical and physiological, appertaining to this inter- 

 esting region, in order to make them also intelligible, which otherwise are utterly 

 inexplicable. 



It follows, that for increasing circulation in the placenta we must provide 

 for rhythmical changes in pressure, since no other force applies for producing 

 it, and we must connect the action in the womb (which expands and contracts 

 regularly and rhythmically) with the action in the placenta, so that when the 

 latter [expands for producing low pressure, the former contracts for producing 

 the opposite, and vice versa. 



In this manner an active circulation in and out the placental sinuses could be 

 readily produced. 



The following facts may be given in support of this opinion : 



1. The womb must expand and contract regularly and rythmically in order to 



