GLANDS OF THE MOUTH 325 



With their increasing physiological importance a greater 

 morphological complication both as regards number and arrange- 

 ment takes place. Their histological character also undergoes 

 changes, so that the most varied forms of glands may be 

 recognised. 



Amphibians. With the exception of the Perennibranchiata 

 Derotremata, and Gymnophiona, a tubular gland becomes developed 

 in all Amphibia from the anterior portion of the roof of the mouth 

 (cf. Fig. 193), the main mass of which in Urodeles lies in the cavity 

 of the nasal septum or premaxilla (intermaxillary or internasal 

 gland). In Anura its position is more anterior than in the Urodela, 

 and it is more largely developed ; but in both cases the ducts open 

 into the anterior part of the mouth. In Anura there is a second 

 gland (pharyngcal gland) present in the region of the internal 

 nostrils, the secretion of which passes partly into the latter and 

 partly into the pharynx. Numerous gland-tubes are also present 

 in the tongue of Amphibians, and in the Gymnophiona oral glands 

 are abundant. 



Reptiles. The oral glands in Reptilia show an advance on 

 those of Amphibia in being separated into groups. Thus not 

 only is there & palatine gland, homologous with the intermaxillary 

 gland, but lingual * arid suUingual, as well as upper and lower 

 labial glands are present. Chameleons and Snakes are distin- 

 guished by a remarkable richness in glands, which are most 

 specialised into definite groups in the latter. 



In poisonous Snakes the poison- gland becomes differentiated 

 from a portion of the upper labial gland. It is tubular in structure, 

 is enclosed in a strong fibrous sheath, and is acted upon by powerful 

 muscles, so that its secretion can be poured with great force into 

 the duct and thence into the poison-fang (Figs. 233 and 238). 

 The sheath of the poison-gland is formed by a pocket-like enlarge- 

 ment of the zygomatic ligament, and is compressed by the muscles 

 of the jaws. When the Snake strikes, the lower jaw is depressed 

 to its fullest extent and the quadrate, pterygoid, palatine, and 

 transpalatine are pushed forwards by the contraction of the 

 posterior pterygo-sphenoid and pterygo-parietal muscles, thus 

 causing the maxilla to move on its articulation with the prefrontal 

 and to erect the fang. The upper jaw is again brought into its 

 position of rest by the contraction of the anterior pterygo- 

 sphenoidal and transverso-maxillo-pterygo-mandibular muscles. 



The sublingual gland of a Mexican Lizard, Heloderma, is also 

 of poisonous nature. The secretion passes out through four ducts, 

 which perforate the bones of the lower jaw in front of the grooved 

 teeth (p. 317). 



1 In Lacerta, Anguis, and Pseiulopus there are numerous depressions of the 

 lingual epithelium lined by goblet cells, which, however, are not differentiated 

 into definite compound glands. 



