282 Elementary Biology. 



salivary glands. These latter are absent, however, from 

 the Amphibia. 



By the action of these various secretions the contents of 

 the alimentary canal are thus rendered capable of absorption. 

 That process takes place through the walls of the small 

 intestine chiefly, where there are very many special vessels 

 known as lacteals and blood-capillaries in readiness to re- 

 ceive the soluble products. The lacteals are extremely 

 delicate ducts, whose walls are composed of a single layer 

 of thin plate-like cells (squames). They abound in the villi 

 of the small intestine and in the wall of the alimentary canal 

 generally. These lacteals communicate with larger trunks, 

 which ultimately pour their contents into the general blood 

 circulation. The food matters also enter the capillaries 

 and so pass directly into the circulation. 



The circulatory organs are in the frog, and, indeed, in all 

 the Vertebrata, very highly differentiated. It will not be 

 possible to give more than a brief outline of the plan of these 

 organs. Further details must be obtained from zoological 

 treatises. 



Generally speaking, the circulatory system of any one of 

 the higher animals, and of the frog in particular, consists 

 fundamentally of a pumping organ or heart and two sets of 

 vessels, one passing from the heart (arteries) and one passing 

 to the heart (veins). Blood, the contents of these various 

 organs", is, as we have previously seen, a fluid which has 

 two all-important duties to perform, viz. (i) to convey to the 

 various tissues of the body the nutriment absorbed from 

 the alimentary canal, and (2) to act as the medium for the 

 carriage of a supply of oxygen to the tissues whereby that 

 disintegration of organic compounds may be effected which 

 we have seen to be a sine qua non in the manifestation of 

 life, and incidentally to act as the medium for the con- 

 veyance of the products of disintegration from the tissues to 

 the exterior. 



The blood of the frog is a red, slightly viscid fluid, which 



