SEC. IV ANATOMY OF BIRDS 315 



very loosely attached, and even deciduous in some cases. The 

 muscular arrangement is chiefly in two great masses, called the 

 lateral muscles, converging Jo a central tendon ; between them inter- 

 mediate fibres may form a more or less distinct muscular belly. In 

 the most powerful gizzards, the muscular tissue is very dense and 

 dark coloured, the tendons brilliantly glistening, and the contained 

 " millstones " extremely callous. Such a gizzard is well displayed 

 by the common fowl or the goose. The opposite extreme is afforded 

 by the carnivorous and especially the piscivorous birds, whose soft 

 food requires little trituration, — it is all a matter of degree. How 

 readily this part of the canal responds to the regimen of the bird 

 is witnessed in the cock-of-the-plains {Centrocercus urophasianus) — 

 a bird whose gizzard is so slightly muscular as to appear like a 

 membranous bag, though its gallinaceous relatives have extremely 

 strong grinders. Its food is chiefly the buds and leaves of the wild 

 sage (Artemisia) and grasshoppers. Increased muscularity of the 

 gizzard has even been artificially produced. Birds whose grist is 

 heavy habitually swallow gravel, that these small stones may 

 mechanically aid in the grinding process. The action is so energetic, 

 that in auscultating a fowl when the mill is in full blast, the noise 

 of the grinding can be distinctly heard. The pebbles, in fact, have 

 a function which leaves " hens' teeth " not merely mythical. The 

 kind of motion impressed upon the opposing pads of cuticle is 

 alternating, — a rubbing back and forth to a slight extent. Peculiar 

 dispositions of the callous surfaces are found in some pigeons, with 

 corresponding peculiarity of the cross-section of the gizzard. In 

 some of the cuckoos a matting of impacted hairs of lepidopterous 

 insects has been mistaken for a coat of, the gizzard itself. In the 

 darter, which has a pyloric division or compartment of the gizzard, 

 this is nearly filled with a mass of matted hairs, a peculiar modi- 

 fication of the epithelial lining, serving to guard the pyloric orifice. 

 Folds of the lining membrane form a pyloric valve in many birds. 

 The pylorus, or the pyloric orifice, is that opening by which food 

 leaves the gizzard for the intestines ; the orifice of entrance from 

 the oesophagus is the cardiac. The two are always near together, 

 and sometimes adjoining. (In Fig. 101, \ ^ is on the central tendon 

 of the moderately muscular gizzard ; the cardiac orifice is between 

 j and k, and pylorus between I and L) 



The Intestine continues the alimentary canal to the cloaca. 

 Any difference in the length of the whole tract, relatively to that 

 of the bird, is chiefly produced by the foldings of the intestine, 

 especially in the upper portion of its course. The extremes of 

 proportionate length are perhaps not ascertained ; but known to be 

 from less than 2 : 1 to more than 8:1. In birds there is little or 

 no distinction between "small "and "large" intestine as to the 



