SEC. IV ANATOMY OF BIRDS 317 



of these intestinal appendages is the detention of food until all its 

 nutritive qualities are absorbed, and increase of the absorbent 

 surface. 



The Cloa'ea (Fig. 101, \ Ic) or "sevrer," very well named, is the 

 termination of the bowel, — an oval or globular enlargement of the 

 rectum, of sufficient capacity at least to contain the completely 

 shelled egg. For, not as in placental mammals, the urogenital and 

 digestive organs are behindhand in their evolution, and do not 

 entirely lose connection with each other. Nor is there in birds 

 any distinct bladder ; but a cavity, originally that of the allantois 

 of the embryo, persists in common with that of the intestines, and 

 is the cloaca. Such incomplete distinction between the two as there 

 may be, by a folding of mucous membrane or partial compartment 

 of the whole, results in cloaca proper, and urogenital sinus, in which 

 latter are the papillose orifices of the ureters, one on each side, 

 from the kidneys ; and of the single oviduct (?) or paired sperm- 

 ducts ( (J ), from ovary or testes. The urine of birds not being 

 liquid requires no more of a bladder than this sinus furnishes. The 

 same cavity contains the penis of those birds, as the ostrich and 

 drake, which are provided with an organ of copulation. A peculiar 

 anal gland, the hursa Fabricii, also opens into the cloaca. Refuse of 

 digestion, the renal excretion, the spermatic secretion, and the 

 product of conception, are discharged by a single anal orifice, the 

 two former en masse. 



Being intimately related to dietetic regimen, and so to the 

 habits of birds, the alimentary canal varies greatly, — even more 

 than my slight sketch shows, — and consequently affords good 

 zoological characters in the details of its construction. But of all 

 the anatomical systems, this is the one most variable as a matter of 

 physiological adaptation (see p. 103). Its characters, even when they 

 seem weighty, are therefore peculiarly liable to be fallacious as 

 indices of natural affinities, and must be applied with discreet 

 caution to morphological classification. Such are commonly only of 

 generic significance. Thus in pigeons the cseca and even the gall- 

 bladder may be present or absent in neighbouring genera. 



Alimentary Annexes. — Some of these, as the salivary glands, 

 have been noticed already. The two most important bodies 

 connected with the digestive tract, and properly considered adjuncts, 

 are the pancreas and the liver. The former is that kind of 

 lobulated salivary gland which in mammals is called the "sweet- 

 bread." It lies in the duodenal loop, along which its loosely 

 aggregated lobes extend. Its ducts, formed by the successive union 

 of smaller efferent tubes, are two or three in number ; they pierce 

 the intestine a little below its commencement at the pylorus, and 

 pour into the canal the pancreatic juice, which has the property of 



