742 COMPARATIVE ANATOMY 



palatal glands near the choanae, although these latter glands are lacking 

 in the caecilians. There is a sub-lingual gland on either side in many 

 reptiles. Probably all secretions from salivary glands in snakes are poi- 

 sonous. There is only one known poisonous lizard (Heloderma). In 

 these the sub-lingual glands furnish the poison. Birds do not have 

 labial and internasal glands, but they do have numerous other ones which 

 open separately into the roof of the mouth. They also have anterior and 

 posterior sub-linguals and even sometimes "angle" glands at the angle 

 of the mouth, a condition sometimes supposed to be a remnant of the 

 labial glands in the Sauropsida. Mammals possess small labial, buccal, 

 lingual, and palatine glands imbedded in the mucous membrane of the 

 mouth, each of which opens through a separate duct. All of these glands 

 serve to keep the various surfaces moist. 



Many glands, however, have become specialized; for example, the 

 intermaxillary glands of frogs and toads (opening into the roof of the 

 mouth) secrete a viscid and sticky fluid which the tongue uses as it is 

 thrown out to catch and hold insects and other moving objects. So, too, 

 the buccal glands of poisonous serpents furnish the venom which is sent 

 forth through the poison fangs. These poison fangs, it will be remem- 

 bered, are teeth, and they are provided either with a groove along the 

 external surface or else they have a very small lumen through the center 

 of the tooth and act similarly to a hypodermic needle. Those glands 

 which assist in throwing out a thin, watery lubricant are called serous 

 glands, while those assisting in softening and dissolving dry food so that 

 it can be more easily swallowed are called salivary glands without regard 

 to their position. 



In mammals the salivary glands are the parotid, lying ventral to the 

 ear (swelling up in man when he has mumps), the submandibular (called 

 submaxillary in human anatomy), the sublingual, and the retrolingual. 

 This last one is closely associated with the submandibular. It is not 

 found in all mammals. 



The serous glands secrete a clear fluid without any salivary at- 

 tribute. The molar gland of ungulates or the voluminous orbital gland 

 of dogs are examples of this type. The orbital glands open into the 

 mouth-cavity close to the last upper molar. The submandibular is found 

 in the lower jaw beneath the mylohyoid muscle. Its duct (Wharton's 

 duct) opens near the lower incisor teeth. The retrolingual gland is near 

 the submandibular, its duct opening close to Wharton's. The sublingual 

 gland lies between the tongue and the alveolar margin of the lower jaw. 

 It empties through several ducts. The parotid opens through Stenson's 

 duct near the molars of the upper jaw. 



THE PHARYNX 



We have already described the pharynx as the cephalic end of the 

 digestive canal which is found between the cavity of the mouth and the 



