CLASS MAMMALIA. IO3 



The food is converted by the action of the digestive 

 organs into a white, milky fluid, which is then absorbed 

 from the intestine by a series of special vessels, and 

 poured into the blood. The blood is thus composed of 

 the elaborated products of digestion, and, like that of aU 

 quadrupeds, it is warm, having an average temperature of 

 98". The blood is distributed to all parts of the body by 

 means of a system of closed tubes — the blood-vessels, — the 

 propelling force being derived from a central contractile 

 organ — the heart. The heart is four-chambered, and is so 

 constructed that the impure blood which has circulated 

 through the body, does not mix with the pure blood com- 

 ing from the iungs. On the contrary, the blood which 

 has become impure by contact with the tissues is always 

 submitted to the action of the oxygen of the air in the 

 breathing-organs, before it is again allowed to be driven 

 through the body. 



The breathing-organs of the Dog are in the form of two 

 lungs placed within the cavity of the chest, the animal 

 being strictly an air-breather, and never having gills at any 

 time of its life. The air is admitted to the lungs by a tube 

 Avhich opens into the throat, and is known as the " wind- 

 pipe," and the lungs never communicate with air-recep- 

 tacles as in Birds. 



The main masses of the nervous system of the Dog con- 

 sist of the brain, protected within the skull, and the spinal 

 cord, protected by the backbone. The senses are well 

 developed, with the exception of that of touch, in the de- 

 licacy of which the Dog is far exceeded by man. 



The innumerable breeds of the domestic Dog, in spite 

 of the enormous differences between them, seem to be 

 descended from no more than three or four wild types, of 

 which the wolf and jackal are the most important. Some 

 of the most striking varieties of the Dog, such as the grey- 

 hound, appear to be exceedingly ancient, and it is not 

 possible to say positively from what wild stock they have 

 descended. The chief characters which distinguish the 

 more typical breeds of Dog, such as the sheep-dog, from the 

 wolf, are the possession by the former of a recurved tail 



