A subaquatic existence calls for but slight reduction in pressure in 

 order to compel movement ; accordingly, we have the blood rushing in and 

 out of the heart as a result of the rhythmical expansions and contractions in this 

 organ ; while for increasing this action, the vessels are combined in the 

 mechanics by means of the vaso-motor apparatus, and which, undoubtedly, con- 

 nects with the action in the placenta. The right ventricle is thicker and stronger 

 than the left, as adjustment for this mechanics, since the diastoles should 

 aspirate the blood in the two cavse, while the two systoles would produce high 

 pressure in the arterial system for increasing circulation in the capillaries. 



'By means of this combined action in the placenta, heart and vessels, a rapid 

 circulation is readily effected under the high pressure that obtains in intra- 

 uterine life ; but anything which should interfere with this mechanics by 

 reducing pressure, e.g., escape of the amniotic fluid, would promptly destroy 

 life. How, otherwise, account for this circumstance, since the vascular connec- 

 tions are uninjured? Moreover, the lividity of the skin, which occurs in these 

 cases, proves conclusively the existence of venous stasis in the systemic capil- 

 laries, and insufficiency of the heart's action to carry on circulation in the 

 absence of the normal pressure upon the embryo. The same circumstance 

 occurs to the air-breather when carried to too great an altitude, nothwithstand- 

 ing the extensive arrangements for changing pressure in the chest, which exist 

 in the latter. This circumstance may be especially noticed in balloonists, and 

 also in persons ascending mountain ranges, respiration and circulation 

 becoming more and more embarrassed as the journey is proceeded with, 

 and venous stasis more and more conspicuous, till the limit of endurance 

 is reached or life itself is terminated, as occurred in the celebrated case at 

 Paris, in which the voyage was made by two balloonists, one losing his life, the 

 other being unconscious when the balloon descended. 



THE ACTION IN MEDUSA CONTRASTED WITH THE ACTION IN THE PLACENTA. 



The action in medusce feeding in the juices in the sea, piimping them in and 

 out of the internal compartments by means of rhythmical expansions and con- 

 tractions for changing pressiire upon them, may be taken to illustrate the 

 mechanical action in the placenta, which feeds in the juices of the womb, 

 pumping them in and out itself from the maternal sinuses. Thus the peduncles 

 (Fig. 2, a, a, a) answer to the tufts while the mantle represents the body of the 

 placenta. But to make the analogy complete, a tubing should project from the 

 convex surface of the latter to represent the umbilical vein and arteries, and 

 which, for obvious reasons, are not present. The analogy is more striking, 

 however, than would appear on the surface, since from the periphery of the 

 stomach a system of radiating canals extends to the edge of the mantle, dividing, 

 subdividing and anastomosing to form a continuous and complicated capillary 

 plexus around the free margin of the mantle, inclusive of the colored lobes, to 

 form the respiratory apparatus. This may be taken to represent the circulation 

 on the distal side of the placenta, and though, for obvious reasons, the force is 

 increased in the embryo, still the mechanical principle is the same for both cir- 

 calations. 



