206 



Or THE BLOOD AND CIBCULATION. 



lateral slits, which are protected by valves. The simplicity 

 of the circulating organs in insects forms a striking contrast 



Fig. 223.-Circulation of insects. 



to the preceding classes ; but we shall see, when treating of 

 the function of respiration, that in insects the air is so com- 

 pletely conveyed to all parts of their bodies, that a simple 

 arrangement suffices for the perfect seration of their blood. 



[371. We have seen that the arteries terminate in the veins 

 in the periphery of all the organs ; these two divisions of the 

 vascular system are connected by the capillary vessels. A view 

 of these vessels can only be obtained by successful minute 

 injections, and the aid of the microscope ; size injections of 

 the skin, and the mucous membranes of the lungs and 

 intestinal canal, exhibit the peripheral capillary system in 

 great variety. The web of the frog's foot, the fishes' tail, and 

 the branchiae of the tadpoles* of frogs, and salamanders, shew 

 the splendid spectacle of the vascular system in action. T. W.] 



[ 3/2. However different the more minute capillary reticu- 

 lations in the various organs appear, they may nevertheless be 



* Every season of the year is not alike favourable for making observa- 

 tions on the circulation. It is only in the spring that tadpoles are to be 

 had, but they are excellent subjects. They should be rolled up in moist 



