310 CIRCULATION OF BLOOD. L.ECT. XVII. 



being confined within the narrow limits which I feel obliged 

 to impose on myself in these lectures. I shall, therefore, 

 restrict myself to an exposition of those experimental results 

 which are best established, and which are necessary for the 

 theory of this function. 



Circulatory Apparatus. The apparatus for the circula- 

 tion of the blood may, in its simplest form, be reduced to 

 a system of canals or tubes, forming a kind of complete 

 circle, invested at some point with a muscular substance to 

 develope the force necessary for putting the blood into 

 motion. This apparatus becomes necessarily more com- 

 plicated in proportion as we ascend in the scale of beings ; 

 and, whilst, in the lower animals, the nutritive liquid fills 

 a vast system of lacunae, which constitutes the whole of 

 their organization, and is endowed with a slow and irregular 

 motion ; in the higher animals, on the contrary, the blood 

 circulates in a system of canals of a peculiar organization, 

 in a fixed direction, and with a constant, but more or less 

 considerable rapidity. The apparatus is complete when, 

 for the execution of this function, two orders of vessels are 

 employed whose structure is very different, and which com- 

 municate with each other after having divided into a great 

 number of ramifications of a constantly decreasing diameter. 

 In one order of vessels the blood travels from the great 

 trunks towards the smaller ones; and in the other it moves 

 in an opposite direction. At the point where these two 

 systems arise there is a peculiar organ called the heart. 

 The small tubes which altogether form the extremities of 

 the arteries and the veins, are called capillary vessels. And 

 as these two orders of tubes terminate by the opposed ex- 

 tremity in the cavities of the heart, the vascular apparatus 

 may be correctly said to form a complete circle, which the 

 blood traverses, returning incessantly to its starting point. 



I cannot pass over in silence various particulars relating 



