24 FIELD ZOOLOGY. 



the blood which is then drawn in, hence the blood must 

 be aerated elsewhere. The respiratory system is made 

 to conform to the needs of this rudimentary vascular 

 system. The tracheas, penetrating as they do the intra- 

 muscular spaces of the body, provide the means for 

 bringing the air into contact with the blood. The blood 

 bathes all the tissues of the body, fills all spaces not 

 filled by the organs, and even bathes the cells of those 

 organs; hence, wherever a trachea empties out its air, 

 the blood there present is oxygenated. The general blood 

 movement is, then, forward through the tissues, till it 

 finally works its way around through the general body 

 spaces and passes backward, following the body contour 

 lines, to re-enter the heart again at its rear or sides. 



Alimentation. 



In man, the alimentary canal, beginning at the 

 mouth, is modified into mouth, oesophagus, stomach, 

 small intestine, large intestine, with the familiar three 

 modifications of the last-named, the unassimilated 

 residue finding exit at the anus. In different parts of 

 this long canal are secreted and discharged into the food 

 there present, the various fluids whose function is the 

 reduction of the solid foods taken, to liquid form, suitable 

 for the building of new body cells or the rejuvenation 

 of over-worked cells. This process of reduction is diges- 

 tion, and the various fluids may be named: as saliva, 

 whose active principle is ptyalin; pepsin, trypsin, pan- 

 creatin, steapsin, etc. 



In insects the simplest alimentary canal is that of 

 the primitive insects, in which it is merely a nearly 

 straight tube, constricted at either end, and enlarged 

 in the middle into a main digestive cavity with muscular 



