206 ElEilENTS OP OENITHOLOGY. 



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 flexor profundus digitorum, which 

 generally sends its tendons to the three front toes. The other 

 muscle is the flexor longus hallucis. It usuaU3'^ 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 tlie ventral surface of the lungs, and is most complete in 

 the Ostrich and Apteryx. Even in those birds, however, it ia 

 not continuous, and the apex of the heart passes backwards 

 through it. 



The ALiMENTAEy 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 systeni 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 vrith 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 



* gee ante, p. 200. t See ante, p. 183. 



