Chap, iv.] GIZZARD OF BIRDS. 163 



developed. Two leading types of stomach have been 

 distinguished in bony fishes ; the so-called siphonal, in 

 which the two halves of the stomach are bent upon 

 themselves, as in the salmon, and the csecal form, in 

 which the upper or cardiac portion gives off a long 

 blind sac. The walls of the stomach are richly pro- 

 vided with muscle in the mullets. 



The simplicity of the resophagus is found to extend 

 to the stomach in most Amphibians and Reptiles, and 

 the chief modification which we observe is an increas- 

 ing tendency for the stomach to be set at right angles 

 to the long axis of the body. In the crocodile the 

 walls are very muscular, and call to mind those of 

 some birds. In the latter group, the members of 

 which are, it will be remembered, toothless, the so- 

 called stomach is divided into two more or less 

 distinct portions. The anterior of these (the 

 proventriculus) has, as a rule, walls which are 

 richly provided with glands that exert a chemical 

 action on the food, while the posterior portion, or 

 gizzard, has walls of considerable thickness, owing 

 to the great development of its muscles ; it seems to 

 have only a mechanical action on the food. Two 

 chief types of gizzard may be distinguished, the 

 simpler, which is found in such birds as are carni- 

 vorous or insectivorous or live on soft fruits, has the 

 walls much less thick than in the other type ; the 

 muscles radiate out from two tendinous centres, 

 which are placed one on either side of the stomach, 

 and the muscular fibres pass from one to the other. 

 The crocodile also has such centra tendiiiea. 



The complicated form of gizzard, or gizzard par 

 excellence, is distinguished from, the more simple by 

 the great development of its lateral muscles, and by 

 the conversion of its lining wall into a strong horny 

 layer, which is particularly thick, and may even form a 

 hard pad on either side (Fig. 73). The tendinous patches, 



