Empedokles farther laid down many doctrines respecting physiology. He dwelt on the procreation of men and animals, entered upon many details respecting gestation and the foetus, and even tried to explain what it was that determined the birth of male or female offspring. About respiration, alimentation, and sensation, he also proposed theories: his explanation of respiration remains in one of the fragments. He supposed that man breathed, partly through the nose, mouth, and lungs, but partly also through the whole surface of the body, by the pores wherewith it was pierced, and by the internal vessels connected with those pores. Those internal vessels were connected with the blood vessels, and the portion of them near the surface was alternately filled with blood or emptied of blood, by the flow outwards from the centre or the ebb inwards towards the centre. Such was the movement which Empedokles considered as constantly belonging to the blood: alternately a projection outwards from the centre and a recession backwards towards the centre. When the blood thus receded, the extremities of the vessels were left empty, and the air from without entered: when the outward tide of blood returned, the air which had thus entered was expelled.[126] Empedokles conceived this outward tide of blood to be occasioned by the effort of the internal fire to escape and join its analogous element without.[127] [Footnote 126: Emp. Fr. v. 275, seqq. Karst. The comments of Aristotle on this theory of Empedokles are hardly pertinent: they refer to respiration by the nostrils, which was not what Empedokles had in view (Aristot. De Respirat. c. 3).] [Footnote 127: Karsten, De Emp. Philosoph. p. 480. Emp. Fr. v. 307--[Greek: to/ t' e)n mê/nigxin e)ergme/non ô)gu/gion pu=r--pu=r d' e)/xô diathrô=skon], &c. Empedokles illustrates this influx and efflux of air in respiration by the klepsydra, a vessel with one high and narrow neck, but with a broad bottom pierced with many small holes. When the neck was kept closed by the finger or otherwise, the vessel might be plunged into water, but no water would ascend into it through the holes in the bottom, because of the resistance of the air within. As soon as the neck was freed from pressure, and the air within allowed to escape, the water would immediately rush up through the holes in the bottom. This illustration is interesting. It shows that Empedokles was distinctly aware of the pressure of the air as countervailing the ascending movement of the water, and the removal of that pressure as allowing such movement. Vers. 286:-- [Greek: ou)de/ t' e)s a)/ggos d' o)/mbros e)se/rchetai, a)lla/ min ei)/rgei a)e/ros o)/gkos e)/sôthe pesô\n e)pi\ trê/mata pukna/], &c. This dealing with the klepsydra seems to have been a favourite amusement with children.] [Side-note: Doctrine of effluvia and pores--explanation of perceptions--Intercommunication of the elements with the sentient subject--like acting upon like.]