AVES. 



325 



of the fertile districts of a tropical country, ve- 

 getable food of a more easily digestible nature 

 may be selected, and it need not be detained un- 

 necessarily long, where a fresh supply can be 

 so readily procured. But in the Ostrich, which 

 dwells amidst arid sands and barren deserts, 

 every contrivance has been adopted in the struc- 

 ture of the digestive apparatus to extract the 

 whole of the nutritious matter of the food which 

 is swallowed. 



In the Grallutores, where no material dif- 

 ferences of locomotive powers or means of 

 obtaining food exist, the co2ca present in their 

 development a direct relation to the nature of the 

 food, and are most developed in the Gruidte. 

 The same holds good in the Natatores. 



Why the increased extent of intestinal sur- 

 face in the above different cases should be 

 chiefly obtained by the elongation of the co2ca, 

 will appear from the following considerations. 

 In consequence of the stones and other foreign 

 bodies which birds swallow, it is necessary that 

 there should be a free passage for these through 

 the intestinal canal, which is therefore generally 

 short and of pretty uniform diameter. In the 

 Omnivorous birds of the tropics,as the Hornbills, 

 Toucans, Touraoos, and Parrots, which dwell 

 among ever-bearing fruit-trees, the rapid pas- 

 sage of the food is not inconsistent with the 

 extraction of a due supply of nourishment, but 

 is compensated by the unfailing abundance of 

 the supply. But where a greater quantity of the 

 chyle is to be extracted from the food, and where, 

 from the nature of the latter, a greater proportion 

 of foreign substances is required for its tritura- 

 tion, while the advantages of a short intestinal 

 tract are obtained, the chyme is at the same 

 time prevented from being prematurely expelled 

 by the superaddition of the two ccecal bags 

 which communicate with the intestines by 

 orifices that are too small to admit pebbles or 

 undigested seeds, but which allow the chyme to 

 pass in. Here, therefore, it is detained, and 

 chylification assisted by the secretion of the 

 coecal parietes, and the due proportion of nutri- 

 ment extracted. 



The large intestine is seldom more than a 

 tenth part of the length of the body, and, 

 except in the Ostrich and Bustard, is continued 

 straight from the cceca to the cloaca ; it may 

 therefore be termed the rectum rather than the 

 colon. It is usually wider than the small in- 

 testine, and its villi are coarser, shorter, and 

 less numerous. The rectum (a, Jig. 164) 

 terminates by a valvular circular orifice (6), 

 in a more or less dilated cavity, which is the 

 remains of the allantois, and now forms a 

 rudimental urinary bladder, (c d). The ureters 

 (A A), and efferent parts of the generative ap- 

 paratus (f, g,) open into a transverse groove 

 at the lower part of the urinary dilatation, 

 and beyond this is the external cavity which 

 lodges, as in the Reptiles and Marsupial and 

 Monotrematous Quadrupeds, the anal glands 

 and the exciting organs of generation. The anal 

 follicles in Birds are lodged in a conical glan- 

 dular cavity, which communicates with the pos- 

 terior part of the outer compartment of the cloaca, 

 and has obtained from its discoverer the name of 



Bursa Fubricii (k). Berthold considers this 

 part as a subdivision of the urinary bladder in 

 Birds, and Geoffrey St. Hilaire as the analogue 

 of Cowper's glands. 



Fig. 



164 



Cloaca of the Condor. 



Digestive glands. The liver is large in 

 Birds, and proportionally larger in the Aquatic 

 species than in Birds of Prey. In the former 



Fig. 165. 



Posterior mew of the biliary and pancreatic due 

 in the Hornbill. 



