VERTEBRATA. 75 



tendons. The gallinaceous birds, in which the gizzard is most 

 powerful, swallow pebbles and other hard bodies, which serve the 

 purpose of reducing their food, like the gastric teeth of crustacea, 

 insects, and gasteropods ; but in the carnivorous birds, with a thin 

 membranous gizzard, no such substances are required, all the neces- 

 sary changes being effected by the activity of the gastric secretions. 

 The parietes of this organ are subservient in a remarkable manner 

 to a known law, to which the whole muscular system yields, that 

 of increasing its growth in proportion to the functions imposed on 

 it; this was strikingly illustrated in the case of a sea-gull, which 

 Mr. Hunter kept for a year, living, contrary to its nature, upon grain. 

 At the end of that period he contrasted its gizzard with that of 

 another sea gull, which had been living on fish, and found that the 

 digastric muscles of the former had acquired nearly three times the 

 development of the latter. He accomplished similar phenomena by- 

 changing the food of an eagle and of a tame kite, the former throve 

 very well on bread, but that it was dissatisfied with its fare, is to be 

 inferred from its seizing the earliest opportunity of breaking its 

 chain, and effecting its escape. These facts show in a clear manner 

 the provision of nature for the preservation of life under a variety 

 of circumstances. 



The intestine is shorter in birds than in mammalia, its different 

 divisions are better marked than in the lower classes, and in the young 

 bird a remnant of the entrance of the vesicula umbilicalis may be 

 seen on the interior part of the small intestine in the form of a 

 small ccecal appendage, and in many gallinaceous and some aquatic 

 birds it remains through life. The course of the small intestine 

 varies much in the different orders of birds; the duodenum always 

 makes a long fold, which embraces the long, bilobate pancreas in 

 its concavity. The large intestine is about a tenth part of the 

 length of the body, and usually has two coeca at its commencement, 

 except in the bustard and ostrich, it runs a straight course from 

 the cceca to the cloaca ; it is generally a little larger than the small 

 intestine, and its villi are shorter, coarser, and fewer. The coeca 

 coli are of great size in the gallinaceous and other granivorous 

 birds, where they arise by two narrow canals, and enlarge into 

 wide sacs, often several times the size of the intestine, as in the 

 turkey. In the ostrich they have the mucous membrane disposed 

 in the form of a spiral fold. They are least developed in the gral- 

 latores and the nocturnal rapacious birds. In the herons and several 

 other birds there is but one, as in the invertebrate and lower ver- 

 tebrate animals, and as in the plantigrade carnivora, they are alto- 

 gether wanting in the zygodactylous birds. 



The rectum terminates by a round valvular orifice in a dilatable 

 cavity, the upper part of the cloaca, which is the remains of the 

 allantois, and now forms a rudimental urinary bladder, and in the 

 ostrich it serves for the retention of the urine as in the higher 

 viviparous animals. At the lower and back part of this urinal por- 



