332 AN INTRODUCTION TO THE STUDY OF FOSSILS 



each side has three incisors, one canine, two premolars and one 

 molar, the molar being the sectorial tooth here. Sectorial teeth 

 are characteristic of the Carnivora. 



In the elephant the single pair of upper incisors develop into 

 tusks. Mammals that chew the cud, hke the sheep and cow, 

 have no upper incisors in the adult state, though they are present 

 in the embryo, and in their Tertiary precursors they were well 

 developed. 



Absorption and blood circulation. — As the food is absorbed 

 from the digestive canal the (i) emulsified fats and most of the 

 water are carried immediately to the heart and are thence forced 

 to all parts of the body, where the former are stored in the fat- 

 tissue, the (2) sugars, including what when eaten was starch, 

 are taken to the liver, where they are stored until needed; the (3) 

 proteids are taken directly to the liver, where their preparation 

 for use in the body is continued. The energy and heat of the 

 food is set free in the millions of body cells through oxidation, 

 the oxygen being derived from respiration. The end products 

 of (i) and (2) are carbon dioxid and water, in (3) there exist 

 in addition mainly such waste products as urea and uric acid. 



This nutrient food, added to the blood already existing, is 

 pumped by the very muscular heart to all parts of the body 

 through a system of contractile, non-collapsible tubes, — the 

 arteries ; these keep branching until every portion of the body 

 is reached, and are so numerous that the point of a needle 

 cannot penetrate the body without piercing one or more of 

 them. As the terminal microscopic tubes, called capillaries, 

 are exceedingly thin and delicate, the food material and oxygen 

 pass through the walls, partially at least by osmosis, into the 

 cells needing them, while back into the tubes pass the waste 

 products of the cells, such as carbonic acid and nitrogenous 

 compounds. The blood, now laden with waste material, passes 

 backwards towards the heart through a system of collapsible 

 tubes, the veins ; some of the blood passes through the kidneys, 

 there getting rid of the waste products from the breaking down 



