214 
GENEBAL OBNITHOLOGY. 
lacteals already described (p. 199), and the unassimilable refuse of the food becomes excremen- 
titious. 
Caeca (Lat. ccecus, blind; in the nom. pi. cceca; sing, ccecum). — The blind guts/' so 
called because they end in culs-de-sac, are of two kinds. One is the umbilical ccecum, or 
vitelli^ie ccecum, a rudimentary, or rather vestigial, structure, the remains of the open duct by 
which the cavity of the umbilical vesicle (an embryonic organ) communicated with that of the 
intestinal tract. It is ordinarily not to be noted at all ; but it is said by Owen to have been 
found half an inch long in the gallinule, an inch in the bay ibis, and dilated into a sac an inch 
in diameter in the Apteryx. The structures ordinarily called cceca, or cceca coli, for they are 
usually paired, are pouches or diverticula which set off from the intestine proper at the junc- 
tion of the ileum with colon ; but there is nothing in the intestine itself to mark this point, so 
that when caeca are absent, as frequently happens, no distinction of ileum from colon or rectum 
is appreciable. No part of the intestinal tract is so variable as the caecal; so that presence or 
absence of these appendages furnishes zoological characters now-a-days taken very commonly 
into account in framing genera and famihes. There are no caeca, as in the turkey- 
buzzard and some pigeons ; there is a single small caecum in herons. From a condition of 
extremely small size, like little buds upon the intestine, caeca are found to elongate to extraor- 
dinary dimensions ; and the large specimens are frequently saccate or clubbed, with slender 
roots. In geese and swans the caeca are a foot long, more or less ; in some grouse they are 
said to be a yard long. In the ostrich, the mucous membrane is thrown into a spiral fold. 
However developed, the physiology of these intestinal appendages is, the detention of food until 
all its nutritive qualities are absorbed, and increase of the absorbent surface. 
The Cloa'ca (fig. 101, 1 k) or sewer," 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 uro-genital and digestive or- 
gans are behind-hand 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 com- 
partment 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 
(9) or paired sperm-ducts (<J), from ovary or testes. The urine of birds not being liquid 
requires no more of a bladder than the sinus furnishes. The same cavity contains the penis of 
those birds, as the ostrich and drake, M^hich are provided with an organ of copulation. A 
peculiar anal gland, the bursa fabricii (see frontisp.), also opens into the cloaca. Refuse of 
digestion, the renal excretion, the spermatic secretion, and the product of conception, are dis- 
charged by a single anal orifice, the two former en masse. 
Being intimately related to dietetic regimen, and so to the habits of birds, the alimen- 
tary 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. 67). Its char- 
acters, 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 caeca and even the gall- 
bladder may be present or absent in neighboring 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 
