PHYSIOLOGY. 159 



latation of these cavities, which alternately happen. Thus, 

 while the left ventricle of the heart is in a state of contraction, 

 the blood passes out of it into the aorta ; but at the same time 

 no blood passes into it from the left auricle, which is then di- 

 lated and filled by the blood flowing into it from the pulmonary 

 vein. It is only when the ventricle is emptied by contraction, 

 and consequently relaxed, that the blood passes into it from the 

 auricle, urged by the contractions of the auricle and adjoining 

 sinus venosus, which succeed immediately to that of the ven- 

 tricle. During this contraction of the auricle and filling of the 

 ventricle, no blood passes from the ventricle into the aorta, nor 

 till a contraction of the ventricle succeeds in consequence of its 

 being filled. The same circumstances take place with regard 

 to the right ventricle and auricle of the heart, and precisely at 

 the same times ; for it appears, that the two ventricles of the 

 heart are contracted and relaxed at the same time ; and, in like 

 manner, the two auricles. 



CL. That the course of the blood, as described (CXLVIII. 

 and CXLIX.), is its usual and constant course, appears from 

 the inspection of the heart in living animals ; from the situa- 

 tion of the valves of the heart ; from the situation of the valves 

 at the orifices of the aorta and pulmonary artery ; from the situ- 

 ation of the valves of the veins ; from the effects of ligatures 

 made upon the arteries and veins ; from the effects of haemorr- 

 hagies of the veins ; from observations with the microscope ; 

 and, lastly, from experiments of infusion and transfusion in liv- 

 ing, and of injection in dead animals. 



CLI. This course of the blood is, however, in some parts, 

 and upon some occasions, a little changed. 1. In the penis, 

 and some other parts, the blood does not pass from the extreme 

 arteries immediately into continuous veins, but is poured out 

 into an intermediate cellular texture, from which it is afterwards 

 received by the extreme veins. 2. In the small branches of 

 the aorta, the blood does not move constantly in a direction 

 from the heart towards the extremities, but, in certain portions 

 of them, is sometimes retrograde to that course. In this, it is 

 favoured by the anastomoses frequent between the small vessels, 

 which, at the same time, prevent this deviation from being con- 

 siderable or durable, while the action of the heart continues. 



