

22 CLASS I. MAMMALIA. 



which has only one bone, long and firm, expending to the el- 

 bow ; the fore-arm, which has two long bones, parallel to 

 each other, extending from the elbow to the wrist ; the wrist, 

 having eight small and irregular bones ; and the hand, on 

 which there are four fingers, each with four bones, and the 

 thumb with three. These bones are united together, so as to 

 form movable joints of various degrees of flexibility and 

 power, by means of firm substances called ligaments. The 

 surfaces which move upon one another, are covered by a 

 smooth polished substance that renders all their motions easy 

 and free from impediment. 



The lower extremities are constructed in a similar manner. 

 The thigh-bone, the largest and strongest bone in the body, 

 is connected above with one of the bones of the pelvis, by 

 means of a large, round head, which is received into a socket 

 of corresponding size, arid thus forms the hip-joint. Its lower 

 end, together' with the knee-pan and one of the two bones 

 of the leg, contributes to form the knee-joint. These last are 

 parallel to each other, and extend from the knee to the ankle. 

 The ankle is composed, like the wrist, of a number of small 

 bones, of which there are seven, one of them projecting be- 

 hind to form the heel. The toes have the same number of 

 bones as the fingers and thumbs, but are shorter and less 

 capable of free and extensive motions. 



These different bones are covered by muscles, fat, and skin, 

 which constitute the principal soft parts of the body The 

 muscles are fibrous organs, attached to the bones generally 

 by tendons, whose contractions put the bones in motion, and 

 thus originate all the movements of which we are capable. 

 They act, in fact, like cords attached to levers, and operate 

 according to strict mechanical principles. 



The organs, by whose operation the digestion of food, the 

 circulation of the blood, and the other important functions are 

 performed, are contained in the three cavities of the cranium, 

 the thorax, and the abdomen, which have been already curso- 

 rily described. We proceed to a consideration of these sev- 

 eral functions, beginning with that of digestion. 



The food is in the first place taken into the mouth, mixed 

 with the saliva, and ground into a kind of paste, by the ac- 

 tion of the jaws and teeth. It is then swallowed through a 

 long muscular canal, the oesophagus or gullet, which passes 

 through the thorax behind the heart and lungs, near the back- 

 bone, and is conveyed, through its upper or cardiac orifice, 

 into the stomach. This is an irregularly-shaped muscular 



