164 DIGESTION. [CHAP, xxm 



pressure. The canine and feline races employ the tongue to lap 

 fluids ; the giraffe twines this organ around the leaves and branches 

 of trees, and detaches them with force. The ant-eaters have a 

 remarkably long tongue, covered with a slimy secretion ; this they 

 protrude, and upon it entrap their victims. The camelion among 

 reptiles, and the woodpecker among birds, have each a tongue enor- 

 mously developed for the purposes of prehension: to these many 

 other striking examples might be added. 



The cavity of the mouth, in which mastication is conducted, is 

 bounded, first, by the palate or roof of the mouth, a fixed and hard 

 surface formed by parts of the upper maxillary and palate bones, 

 supporting a dense fibrous structure, lined with closely adherent 

 mucous membrane, and fitted to act as a resisting surface against 

 which the tongue may press the food ; and, secondly, by the cheeks, 

 lips, and tongue, which, in reference to the present function, may be 

 classed together as tactile and muscular organs, designed to handle 

 the food while subjected within the mouth to the action of the teeth, 

 and then to forward it into the pharynx. Projecting into the mouth, 

 above and below, is an arched series of teeth, or grinding organs, 

 firmly fixed by roots into the alveoli of the upper and lower maxillary 

 bones. Those of the upper jaw are immoveable, or only moveable 

 with the entire head; but those of the lower jaw are capable of up- 



not easily separable from the areolar and other tissues which lie below it. 

 These two, the epithelium and the basement membrane, may be regarded as 

 the constituents of a simple mucous membrane, although the latter cannot be 

 everywhere traced, for example, in the interior of the hepatic lobules. The 

 office of the basement membrane seems to be in all cases to sustain the epi- 

 thelium, and to shut in or cover over those tissues which may be regarded as 

 internal to the elements of the simple mucous membrane. Professor Goodsir 

 considers that the basement membrane is covered with points, which he terms 

 centres of nutrition, from which the development of the particles of epithe- 

 lium proceeds ; but we cannot accede to this view ; first, because the best 

 examples of basement membrane display no such points ; and, secondly, be- 

 cause such a supposition does nothing to explain the successive growth of 

 the particles. 



The tissues which lie under the simple mucous membrane, as now sketched 

 out, are areolar tissue, blood- and lymphatic vessels, and nerves, and, in some 

 situations a peculiar papillary tissue. These, in their several forms and pro- 

 portions, greatly modify the characters of the membrane, as it is presented to 

 the naked eye, and contribute largely to our common notions of the structure 

 of the skin, mucous membranes, and glands. The areolar tissue forms the 

 cutis vera of the skin (vol. i. p. 406), and the corresponding part of the great 

 mucous tracts ; while in glands it is generally in much smaller quantity. 

 The blood-vessels and other textures are modified in various ways, as will 

 hereafter be noticed in detail. 





