206 ELEMENTS OF ORNITHOLOGY. 
The muscles which go to the toes become tendinous when 
they pass down beside the tarso-metatarsal bone, and some of 
their tendons may pass through small bony canals before men- 
tioned * as existing in the calcaneal process. There are two 
special muscles which arise from the hinder surface of the tibia 
and fibula and serve to bend the toes; each muscle ending in 
one or more tendons, which are implanted one into each digit. 
One of these muscles is the flewor profundus digitorum, which 
generally sends its tendons to the three front toes. The other 
muscle is the flewor longus hallucis. It usually ends by sending 
a single tendon to the hallux. The tendons of these muscles, 
as they are situated on the under, or plantar, surface of the foot, 
are spoken of as the plantar tendons (fig. 160). : 
The diaphragm is a sheet of membrane and muscle which 
covers the ventral surface of the lungs, and is most complete in 
the Ostrich and Apteryx. Even in those birds, however, it is 
not continuous, and the apex of the heart passes backwards 
through it. 
THE ALIMENTARY SYSTEM. 
Birds eat more, in comparison with their bulk, than do 
animals of any other class of vertebrates. They feed on animal 
substances in the majority of species, and most of all on insects. 
The system of organs devoted to this function is the alimentary 
canal, with the various parts annexed to it. The alimentary 
canal itself consists of the mouth, gullet, crop, stomach, gizzard, 
intestine, cloaca, and vent. The parts annexed to it, and which 
assist it in performing its great function of digestion, are the 
salivary glands, the pancreas, and the liver. Other parts form- 
ing no part of the alimentary system may nevertheless commu- 
nicate with the cavity of the alimentary canal. Such are: the 
nostrils, the ears, the lungs, the kidneys, and the generative 
glands. 
The mouth is bounded and enclosed by the bill, which we 
have already described, and with the shape of which it corre- 
sponds. Into the roof of the mouth the nostrils open by a 
slit which is generally single, and the ears open into it more 
posteriorly, the usually single opening being that common to 
the two eustachian tubes t. At the lower part of the mouth is 
* See ante, p. 200. t See ante, p. 183. 
