145 



CHAPTER XVII. 



DIGESTIVE SYSTEM OF BIEDS. 



The digestive function is most potent and rapid in Birds, in 

 order to supply the waste occasioned by their extensive, frequent 

 and energetic motions, and in accordance with the rapidity of 

 their circulation and their high state of irritability.^ 



The parts to be considered mth reference to this function are 

 the rostrum or beak, the tongue, the oesophagus, the stomach 

 which is always divided into a glandular and muscular portion, 

 the intestines, and the cloaca: with these are connected the 

 salivary glands, the proventricular follicles, the liver and pancreas. 



§ 145. Beaks of Birds. — The beak consists of an ' upper 

 mandible,' supported by the maxillary and premaxillary bones, 

 and of a ^ lower mandible ' forined by the lower jaw. In place 

 of teeth these bones are provided Avith a sheath of horny fibrous 

 material, similar to that of which the claws are composed: 

 this sheath is moulded to the shape of the osseous mandibles, 

 being formed by a vascular substance covering these parts, and 

 its margins are frequently provided with horny processes or 

 laminae secreted by distinct pulps, analogous in this respect to 

 the whalebone laminae of the Whale. In a foetus of a Perroquet 

 nearly ready for hatching, the margins of the bill are beset with 

 white and round tubercles, arranged in a regular order, about 

 seventeen in the upper jaw, the foremost on the mid-line.^ These 

 tubercles are not, indeed, implanted in the alveolar border, but 

 form part of the sheath of the bill. Under each tubercle, 

 however, there is a gelatinous pulp, like that of a tooth, but 

 resting on the edge of the jaw-bones, and every pulp is supplied 

 by vessels and nerves traversing a canal in the substance of the 

 bone. These tubercles form the first margins of the mandibles, 

 and their remains are indicated by canals in the horny shcatli 

 subsequently formed, which contain a softer material, and which 

 commence from small foramina in the margin of the bone. 



' The Cormorant devours, in captivity, six or eight pounds offish daily; what may 

 be the amount iu its state of wild activity ! 2 xxxvi*. 



VOL. II. L 



