THE CROP AND GIZZARD OF BIRDS. 587 



is bounded by two flat surfaces, covered by a very thick, cuticular, horny, or 

 even tuberculated lining membrane, supported on a dense, fibrous, submucous 

 coat. The lining membrane is hardest in the granivorous birds, especially in 

 those species in which the food is most solid ; its density increases at the points 

 where the pressure and friction are greatest. In the petrel, it presents a layer 

 of small square tubercles, suggesting a likeness to the gastric denticles of cer- 

 tain Gasteropods. Hunter observed that in a sea-gull fed on barley, the mus- 

 cles and cuticle of the gizzard became thicker than natural. 



A pyloric valve usually exists in Birds ; it is placed, in most species, a little 

 below the gizzard, so that there is a short pyloric portion of the stomach inter- 

 vening between the gizzard and the duodenum. The pyloric valve is very 

 strong in some birds ; it is double in the gannet, and, in the ostrich, forms six 

 or seven ridges, which close the pylorus like a grating, permitting only small 

 stones to pass. 



The uses of the crop, proventriculus, and gizzard of Birds, are obvious. 

 The crop, absent or small in birds living on fruit, insects, small aquatic ani- 

 mals, or flesh, but reaching its utmost development in those which feed on 

 grain and other seeds, forms, like the ruminant paunch, a cavity for the reten- 

 tion and maceration, during several hours, of hard and dry food, so as to pre- 

 pare it for the solvent action of the proper gastric juice of the proventriculus, 

 and the grinding force of the gizzard. Seeds soften and swell under the influ- 

 ence of the scanty saliva, and of the more copious secretion of the crop. Pigeons 

 will sometimes devour so many dry peas, as to be almost suffocated when these 

 swell in the crop. The crop has been compared to the hopper of a mill, and 

 the gizzard to the millstones, the former and larger cavity receiving the food, 

 and delivering it, in successive suitable quantities, into the latter, which is so 

 much smaller. Whilst rearing its young, the mucous membrane of the double 

 crop of the pigeon becomes thicker and more vascular, its glands enlarge, and 

 secrete a milky-looking fluid, which mixes with the softened grain in the crop, 

 and is then, by an antiperistaltic action, regurgitated into the mouth, from 

 which in the manner above described (p. 573), it is taken by the young pigeon, 

 serving the same purpose as the lacteal secretion of the Mammalia. The pro- 

 ventriculus of Birds has been compared with the cardiac, and the gizzard with 

 the pyloric, portion of the mammalian stomach. The secretion of the proven- 

 triculus has the same digestive properties as those of the gastric juice of Mam- 

 malia. The gizzard, which, when it exists, forms an internal masticatory 

 organ, supplying the want of a masticatory -apparatus in the head, has evi- 

 dently a mechanical office. By the aid of pebbles, gravel, or sand, swallowed 

 especially by granivorous birds, it triturates the food. Such birds do not thrive 

 without a supply of pebbles or gravel ; and pigeons have been known to carry 

 these to their young. Grains of barley, inclosed in strong perforated tubes, 

 pass through the alimentary canal of the bird undigested, whilst meat, simi- 

 larly inclosed, is dissolved. TJnbruised corn, with its hard silicious coat un- 

 broken, is not soluble in gastric or intestinal juice. The gizzard of the ostrich 

 can flatten metal tubes, pulverize glass balls, and break or blunt the points of 

 needles and lancets, without injury to its hard internal coat. The grinding 

 of the stones in a bird's gizzard may be heard by the stethoscope. The move- 

 ments of the walls of this cavity are supposed to be slightly rotatory. In the 

 membranous gizzard of the cuckoo, as I, as well as others, have found, the 

 hairs of caterpillars are sometimes impacted in a regular spiral manner, as if 

 felted by a continuous movement of partial rotation ; balls of hairs spirally 

 disposed have also been seen. These facts have been often quoted, in support 

 of the view that, in all animals, intrinsic movements of the walls of the stomach 

 may occur. 



The intestine of Birds generally is, relatively to the body, shorter than that 

 of Mammalia, but longer than that of Reptiles. It varies in length and width, 

 as well as in the arrangement of the convolutions, and in the relative develop- 

 ment of the cceca. In the birds of prey generally, the intestine is not more 

 than twice as long as the body, including the bill, but in the osprey it is eight 

 times as long. It is longer in frugivorous and granivorous birds, and shorter 

 in the flesh-eating species. The duodenum always forms a long loop, embrac- 

 ing the pancreas. The remaining portion of the small intestine is variously 



