38 ZOOLOGY. 



powerful organ. The food, after leaving the gizzard, 

 passes into the small intestine, where it is acted upon by 

 the bile and pancreatic juice. The large intestine, which 

 is very short, passes into a bladder-like cavity called 

 the cloaca (Lat. cloaca, a sink), which also receives the 

 secretions of the kidneys and the genital organs. There 

 is a similar arrangement in reptiles. 



36. Circulation. The mean temperature of the blood of 

 birds is 110, while that of mammals is about 100. The 

 higher temperature is partly due to the great activity of 

 these animals, and partly to the " double respiration," by 

 which the blood is aerated in the air sacs of the body, as 

 well as in the lungs. The covering of feathers, which' 

 are bad conductors of heat, assists in keeping up the high 

 temperature. 



The red corpuscles of the blood are oval in shape, and 

 contain a central solid particle or nucleus. 



The structure of the heart and the nature of the 

 circulation in birds and mammals are very much alike. 

 The heart has four chambers, two auricles and two 

 ventricles; and the right and left sides have no com- 

 munication with each other. The venous blood is received 

 from the body into the right auricle. It then passes 

 into the right ventricle, which sends it to the lungs, 

 through the pulmonary artery. Having been aerated in 

 these organs, and changed into arterial blood, it is con- 

 veyed by the pulmonary veins to the left auricle, thence 

 to the left ventricle, which drives it through the aorta 

 and its branches to all parts of the body. 



The valve which separates the right auricle from the 

 right ventricle, instead of being "tricuspid" as in mam- 

 mals, is of a triangular form. The action of the heart is 

 more rapid than in mammals, and its walls and valves 

 are relatively stronger. 



37. Eespiration. Birds have two lungs of a bright red 

 colour, and spongy structure. They are placed at the 

 back of the chest, and are not suspended in a pleura! 

 membrane, as in mammals, 



