636 Comparative Morphology of Chordates 



commonly possess tooth-germs arranged in two rows, one in front of 

 the other. Those of one row develop into functional teeth. The others 

 disappear or may persist as useless rudiments buried within the j;iws 

 (see p. 54). 



The various adaptive modifications of the teeth are mentioned in 

 the descriptions of the Orders of mammals. 



Salivary Glands 



A dry oral cavity would require a lining of cuticula or horn to protect 

 the living tissue of the oral surface. The most advantageous situation 

 for sense-organs of taste is within the mouth. The most effective means 

 of producing a sensation of taste is a solution of food substance acting 

 to stimulate sensory cells so situated as to be easily accessible to the 

 solution. The foregoing statements imply reasons why the oral cavity 

 must be kept wet. Passage of food through the mouth will be facilitated 

 if the surfaces are not only wet but also slippery. 



In all vertebrates the lining of the mouth is glandular. In aquatic 

 animals there is no difficulty about keeping the mouth wet, but 

 glands are important in supplying a lubricating mucus. The oral 

 mucous glands of anamniotes are mainly of the primitive unicellular 

 type and are widely distributed on the oral surfaces. In terrestrial 

 vertebrates the increased importance of oral glands is attested by 

 development of the more efficient multicellular glands. In terrestrial 

 amniotes small multicellular glands are more or less abundantly dis- 

 tributed over the oral surfaces. Some of them are serous, producing a 

 watery secretion, others are mucous. Added to these small and 

 diffusely distributed glands are larger and definitely localized glandular 

 complexes which, according to their location, are designated as labial, 

 buccal, lingual, orbital (on the floor of the orbit), or molar. Such 

 local glands appear in terrestrial amphibians (e.g., the intermaxillary 

 gland) and to a greater extent in reptiles and birds, but it is in mam- 

 mals that they are most conspicuously developed and acquire highest 

 functional importance by adding to their primitive functions the secre- 

 tion of substances which begin the digestion of food. 



Three pairs of large oral glands are especially characteristic of 

 mammals — the parotid, submaxillary, and sublingual (Fig. 183). 

 There are various other localized glands, but they are smaller and less 

 constant in their occurrence. The parotid is the largest, situated 

 superficially at the rear of the cheek and base of the external ear. Its 

 duct (Steno's duct) opens on the inside of the cheek. The disease 

 known as "mumps" is an infection of this gland. The submaxillaries 

 (more logically called submandibular) lie beneath the floor of the 

 mouth at the rear of the lower jaw. The two ducts (Wharton's ducts) 



