ESSENTIAL CHARACTERS OF BIRDS 



77 



Thus prepared, the food is passed into the intestines (Model 

 I. E. 35-41), where all that is nutritious is absorbed, either by 

 the blood or special vessels forming what is known as the " lac- 

 teal " system ; the undigestible residue, passing into the " large 

 intestine " or " rectum " is finally expelled. 



Birds, it should be noted, which feed on fish have very long 

 intestines, and the walls thereof 

 of great thickness, thereby they 

 are protected from injury by broken 

 fishbones. Grain-eating birds have 

 similarly long intestines ; in fruit- 

 and in insect-eating birds the intes- 

 tines are very short and thin walled. 



Into the hinder end of the large 

 intestine the ducts of the kidneys 

 open, and the ducts of the generative 

 organs conveying the sperms, or 

 eggs, as the case may be. 



The Lungs and Air-Sacs. The 

 lungs of birds do not hang freely 

 in the cavity of the thorax, but 

 are attached to its roof (Model D. 

 9). They are further peculiar in 

 that they give off diverticula, or 

 pouches, known as air-sacs, which 

 contain large stores of respiratory 

 air. They are to be found on each 

 side of the body cavity, and in 

 between the " merry- thought. " If 

 chromic or picric acid be forced 

 with a syringe into the windpipe, 

 these sacs can be filled, when, after 

 a few hours, their walls will be hard- 

 ened so that on dissection these 

 cavities can be readily studied. The 



air from these air-sacs finds its way all over the skeleton in many 

 birds, the bones being hollow. In some species, however, certain 

 bones are filled with marrow, as in the humerus of the swallow. 



swan, to show the great thickness 

 of the muscular walls of the gizzard, 

 the cavity of which is shown by the 

 deep shading. G, gullet ; P, the 

 region known as the "proventric- 

 ulus." The walls of this region are 

 thick, and supplied with numerous 

 digestive glands. Mg, muscular 

 wall of gizzard. 





