LECT. XVII. THE CIRCULATION. 315 



blood, and leave this orifice free ; hence the blood escapes 

 into the artery in consequence of the construction of the 

 valves, which open from \vithin outwards. 



The arrangement of the valves, in the left cavities of 

 the heart, is similar to that in the right cavities just no- 

 ticed. When the left auricle dilates, the blood from the 

 pulmonary vein enters it ; and at the moment when it con- 

 tracts, which circumstance coincides with that of the 

 dilatation of the ventricle, the blood is propelled into the 

 latter, and from this into the aorta, in consequence of the 

 contraction of the ventricle. 



Two successive sounds reach the ear when applied to 

 the chest, corresponding to the two successive movements 

 of the heart ; the auricles dilate together : and the ventri- 

 cles likewise. 



By contraction of the heart we mean that of the ven- 

 tricles: their contraction is called systole, their dilatation 

 dyastole. 



We have thus sketched the principal features of the me- 

 chanism of the circulation. As for the details, they are 

 not within the limits of the present course, and, there- 

 fore, it is not our province to explain them to you. I 

 would have even suppressed what little I have said on 

 this subject, had I not considered it absolutely necessary 

 to precede the study of the phenomena of the circulation 

 of the blood by an exposition of these elementary anato- 

 mical facts. 



The Blood. The liquid which circulates in the vessels 

 is of a vermilion red colour, in the arteries, and of a dark 

 red in the veins. It is slightly alkaline, has a specific 

 weight of 1-0527 to 1-057, and holds in suspension glo- 

 bules of a greater or less diameter. In most mammals 

 these globules are circular discs ; while in birds, reptiles, 

 and fishes, they are elliptical. This liquid constitutes the 



