1(5 



poured into the right auricle, that from the upper cava passing at once in the 

 right ventricle, while that in the lower cava (which includes the blood from the 

 umbilical vein) passes through the right into the left auricle, with which it 

 directly communicates, guided by the Eustachian valve, but also pushed over 

 and deflected in this direction by the weight of the descending current from 

 the upper cava ; but if the head be downward then by its own weight the blood 

 would gravitate in this direction, the influx of blood from the upper cava also 

 compelling it. And 'with the absence of blood as a counter-force in the left 

 auricle this blood is necessarily compelled in the latter, thence in the left ven- 

 tricle and aorta, while that in the right ventricle passes in the arterial system 

 at the aortic arch by way of the pulmonary artery and ductus arteriosus. After 

 birth, however, the pumping action in the lungs reverses all this in manner as 

 above described. The pulmonary artery in the embryo, in place of dis- 

 charging through the lungs, left auricle and ventricle, empties its blood at 

 once into the aorta as it passes under the arch, and which is also in the direction 

 of least resistance, since it is impossible for this blood to thread its way 

 through the capillary meshes of the unexpanded alveoli. This circumstance has 

 forcible illustration, even in the air-breather, and when the alveoli are filled 

 with residual air, by simply inhibiting inspiration by closing the mouth and 

 nose so as to prevent expansion in the lungs, the blood, in consequence, rapidly 

 accumulating in the right side of the heart and venous system. Tn the space 

 of a minute there is lividity of the lips and whole cutaneous surface from 

 venous stasis in the systemic capillaries. If longer than this, an appalling 

 venous suffusion pervades the surface ; in the face most, for this is the most vas- 

 cular portion, with the large venous trunks in close proximity to the heart. 

 Even the eyes are forced outward, becoming prominent from distension of the 

 iutra-orbital veins caused by obstruction in the cavernous and lateral sinuses. 

 But the instant the obstruction is removed and the lungs are permitted to 

 expand the dammed-up blood surges through the alveoli, and all runs on as be- 

 fore. In other words, the heart and vessels are unable to carry on circulation 

 in the absence of the pumping action in the lungs, for which afflux and efflux 

 of air is essential. But all this has been sufficiently explained. 



INCUBATION. CIRCULATION IN THE EGG. 



Why should there be an air-chamber to the egg (Figs. 8 and 9) ? We 

 are now prepared to furnish a scientific explanation to this physiological problem, 

 otherwise inexplicable, viz., the contents of the egg for developing the chick is 

 enclosed by a firm unyielding wall of living marble, and since the animal circu- 

 lation is dependent upon rapid rhythmical changes in pressure, it follows that 

 provision should be made within the shell for effecting this, otherwise the action 

 in the heart and vessels could not take place. This air-chamber (g), together 

 with the important relations it sustains to circulation and elaboration in the 

 growing chick, organilogically, therefore, must be regarded as one of the most 

 essential and important elements in egg-structure, the underlying principle to all 

 the nutritive changes which are ushered in under the action of external temper- 



