GENERAL CHARACTERS OF MAMMALIA. 21 



3. Circulation and respiration are carried on in the same man- 

 ner as in man. In all the mammalia the blood is warm, and the 

 heart has four distinct cavities ; two ventricles and two auricles. 

 The lungs are always composed of a great number of very small 

 cells, and do not permit the air to pass from them into different 

 parts of the body, as is the case in birds. 



4. In general, it is easy to distinguish at first sight, one of the 

 mammalia from a bird, a reptile, a fish, or any other animal, by 

 simply considering its external form, and the nature of its integu- 

 ments ; the mammalia are in fact, the only animals whose bodies 

 are covered with hair ; ordinarily their general form does not 

 widely differ from that of the quadrupeds which are constantly 

 before our eyes, and which we naturally take as the type of the 

 group; but sometimes they are not recognised by so superficial 

 an examination ; for there are some whose skirts are completely 

 bare, and whose bodies, instead of resembling that of a horse, 

 a dog, or any other one of the ordinary mammalia, possess forms 

 proper to fishes : the dolphin and whale are examples of this 

 kind. (Plate 6, fig. 6, 7, and 9.) 



5. The hairs of the mammalia are very analogous to the 

 feathers of birds, and serve in the same manner to protect the 

 skin, and preserve the heat developed in the interior of the body; 

 in reptiles, fishes and other cold blooded animals which do not 

 manifestly produce internal heat, there does not exist any similar 

 envelope. 



6. The hairs are produced like the teeth, by small secreting 

 organs, lodged in the thickness of the derma, or immediately be- 

 neath it. Each hair is formed in a little pouch or bulb, which 

 communicates externally by a narrow opening. They grow, 

 like the teeth, at the base, or new matter is added beneath that 

 which is already formed. 



7. On examining the hairs with a microscope, we sometimes 

 perceive, very distinctly, that they are formed of a number of 

 little horns received one into the other, (like a nest of boxes,) but 

 in general, they have the appearance of a simple horny tube, the 



3. How is the circulation of the blood carried on in the class mammalia? 

 Is their blood warm or cold ? How many cavities has the heart? What is 

 the character of the lungs of these animals ? Does the air pass from the 

 lungs to different parts of the body ? 



4. What general characters enable us to distinguish animals of the class 

 mammalia from those of any other class ? Do all animals of the class 

 mammalia bear a general resemblance to common quadrupeds? 



5. What is the use of the hairs on the bodies of the mammalia ? 



6. How are the hairs produced ? How do they grow? 



7. What is the structure of hairs ? 



