228 THE ANIMAL KINGDOM. 



network over the surface, interwoven with the capillaries, 

 and absorbing the fatty part of our food, whilst the blood- 

 vessels absorb the dissolved part of any kind. The villi 

 are closely crowded in the small intestines, preventing the 

 too quick passage of the food, which here receives the need- 

 ful bile from the liver and pancreatic fluid. 

 A transverse section of 



DUODENUM OF MOUSE 



gives a transparent view of the villi, in which both blood- 

 vessels and lacteals are distinctly visible. 



THE LUNG. 



Human Monkey Bear Puppy Pig Cat Sheep Fowl Goose 

 Turtle Rattlesnake Frog Tortoise. 



The usual sections of lungs of animals present the 'capil- 

 laries of the veins charged with impure blood, woven in 

 close network around these bronchial tubes, which aerate 

 the life-blood, and send it back pure and bright into the 

 heart, to leap forth again throughout the whole living 

 frame. The air we breathe is composed in every one hun- 

 dred volumes of seventy-nine volumes of oxygen. It is the 

 oxygen that gives life and is the essential agent, the 

 nitrogen merely modifies its too energetic action, and when 

 this gas comes into contact with the carbon contained in the 

 blood of these fine capillaries, carbonic acid, deleterious and 

 noisome, is the result, and we breathe it forth at every 

 expiration. Then the sluggish purple blood brightens, and 

 in vermilion streams flows on in a web of capillaries, which, 

 if those of the lungs only were extended, would cover a 

 space of 2642 square feet, and the air-cells themselves 

 number 600,000,000. This lung in a single year will 

 have contracted and dilated 9,000,000 times, will have 

 inhaled 100,000 cubic feet of air, and aerated more than 

 3500 tons of our life-blood. 



If, however, we are looking at slides of other than 

 human lungs the reptile, or the bird we shall see modi- 

 fications wisely arranged for the habits of the creature. In 

 the lung of the Fowl or Pheasant, an immense number of 



