286 



COMPARATIVE PHYSIOLOGY. 



[ milky-looking substance tliat is regurgitated into the mouth of 

 the young one, which is inserted within that of the parent bird. 



The proventriculus — an enlargement just above the gizzard 

 — is relatively to the latter very thin-walled, but provides the 

 true gastric juices. 



Certain plants digest proteid matter, like animals ; thus the 

 sun-dew (Drosera), by the closure of its leaves, captures insects, 

 which are digested and the products absorbed. The digestive 

 fluid consists of a pepsin-containing secretion, together with 

 formic acid. 



STRUOTURB, ARRANGEMENT, AND SIGNIFICANCE 

 OF THE TEETH. 



In a tooth we recognize a portion imbedded in the jaw (fang, 

 root), a free portion (crown), and a constricted region (neck). 



t 





Pia. 235. 



Pig. 287. 



Fia. 235.— Maguifled section of a canine tooth, showing its intimate structure. 1, 

 crown; 2, 2, neck; 3, fang, or root; 4, cavitas pulpse; 5, opening by which the ves- 

 sels and nerves communicate with the pulp; 6, 6, ivory, showing fibrous structure; 

 7, 7, enamel ; 8, 8, cement. 



Fig. 3.36. — A, transverse section of enamel, showing its hexagonal prisms; B, sepa- 

 rated prisms iChanve.au). 



Fig. 837.— Section through fang of molar tooth (Chauvean). a, a. dentine traversed 

 by its tubuli; 6, b, interglobular or nodular layer; c, c, cementtim. 



