212 COMPARATIVE ANATOMY. 



any rate they are formed on the same type as glands (lungs, thyroid, 

 thymus). Mesoblastic elements are added to them secondarily. 



Beginning from the mouth the following appendicular organs 

 of the alimentary canal may be distinguished : 



(1) Salivary glands (Fig. 172, Gls). 



(2) Mucous glands. 



(3) The thyroid gland (Glth). 



(4) The thymus gland (GLthy). 



(5) The lungs (pulmones) (air-bladder) (Lg). 



(6) The liver (Z6). 



(7) The pancreas (Pa). 



To these may be added the gastric and intestinal glands 

 (peptic glands, glands of Lieberkuhn, &c.), which are embedded in 

 the wall of the gut. 



I. MOUTH. 1 



InAmphioxus the entrance to the mouth is provided with 

 cirrhi, and inCyclostomesitis surrounded by a ring of cartilage : 

 all other Vertebrates are provided with jaws. 



Definite lips provided with muscles first appear in Mammals, 2 

 and are most strongly developed in Monkeys, especially Anthropoids : 

 the space between them and the jaws is spoken of as the 

 vestibulum oris. This may become extended on either side 

 to form cheek -pouches, which serve as food reservoirs (many 

 Monkeys and Rodents). 



The organs of the oral cavity may be divided into three 

 groups, viz. the teeth, the glands, and the tongue. 



Teeth. 



Both epiblast and mesoblast take part in the formation of the 

 teeth. The epithelium of the mouth grows inwards so as to give 

 rise to the so-called enamel-organ, which meets with dome- 

 shaped elevations of the underlying connective-tissue, the tooth- 

 germs or tooth-papillae. Both these tissues come into the 

 closest relations with one another, and, by very complicated pro- 

 cesses of differentiation, give rise to the different substances of 

 which the teeth are composed. These substances are, the enamel, 

 dentine (ivory), which is permeated by a fine system of canals, 

 and cement (Fig. 173, ^Sf, ZB, ZC). 



1 The mouth of the Lamprey serves as a suctorial organ for attaching the animal 

 to foreign objects. The larvre of Lepidosteus and Anura are temporarily provided 

 with suctorial organs. 



3 An exception is seen, however, in Dipnoi, the lips of which are provided with 

 well-developad muscles. 



