THE CIECULATION OP THE BLOOD. 273 



somewhere, usually in the heart, but that in all the vertebrates 

 the best blood is invariably that which passes to the head and 

 upper regions of the body. The deficiencies in the heart, owing 

 to the imperfections of valves, septa, etc., are in part counter- 

 acted in some groups by pressure relations, the blood always 

 flowing in the direction of least resistance, so that the above- 

 mentioned result is achieved. 



Capillaries are wanting in most of the invertebrates, the 

 blood flowing from the arteries into spaces (sinuses) in the tis- 

 sues. It is to be noted that a modifted blood (lymph) is also 

 found in the interspaces of the cells of organs. Indeed, the 

 circulatory system of lower forms is in many respects analogous 

 to the lymphatic system of higher ones. 



18 



