212 COMPARATIVE ANATOMY. 



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

 tliymus). 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 (GUJi). 



(4) The tliymus gland (Gl.thy). 



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



(6) The liver (U). 



(7) The pancreas (Pa). 



To these may be added the gastric and intestinal glands 

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

 the wall of the gut. 



I. MOUTH. 1 



In Amphioxus the entrance to the mouth is provided with 

 cirrhi, and in C yclos tomes it is surrounded by a ring of cartilage : 

 all other Vertebrates are provided with jaws. 



Definite lips provided with muscles first appear in Mammals, 2 

 ;i in 1 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 

 Croups, 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 

 \\liirh the teeth are composed. These substances are, the enamel, 

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

 and ceni e i, I (Fig. 17:5, XX, Zll, ZC}. 



1 Tlio month of (In- I.:mi]nvy serves us a suctorial organ for attaching the animal 

 tn foreign objects. The larva- of Lcpidostciis ami Anura arc temporarily provided 

 with suctorial organs. 



AII exeepiion isseru, however, in l)i|iiiui, thi 1 lips of which are provided with 



Wcll-deVclnp '! 



