398 THE DIGESTIVE APPARATUS IN MAMMALIA. 



separates this expansion from the cutaneous integument, and the terminal inser- 

 tion of the levator labii superioris almqui nasi and the dilator naris lateralis ; in 

 the inferior lip, the tvft of the chin and its suspensory muscles — the levatores menti. 

 All these muscles having been studied in detail in the Myology (p. 274), there is 

 no necessity for their being again described. 



3. Labial glandules. — These form an almost continuous layer between the 

 mucous membrane and the orbicularis muscle. They are small secretory organs, 

 similar in their structure and uses to the salivary glands, and will be described 

 when these come under notice. 



4. Vessels and nerves. — The blood is carried to the lips by the palato-lahial, 

 and the superior and inferior coronary arteries. It is returned to the heart by the 

 satellite veins of the two last vessels. The lymphatics are very numerous, and 

 pass to the glands between the branches of the lower jaw. The nerves are of two 

 kinds : the motor, which are given off from the facial nerve, and are distributed 

 in the muscular tissue of the lips to cause its contraction ; the sensitive 

 nerves — from the maxillary branches of the fifth cranial pair — are distinguished 

 by their number and considerable volume, and are nearly all distributed to the 

 cutaneous integument, which they endow with an exquisite sensibility. 



Functions. — The lips serve for the prehension of solid and liquid food ; they 

 retain it in the mouth after its introduction thereto, and they prevent the escape 

 of the saliva. They ought also to be regarded — especially the upper lip — as veiy 

 delicate tactile organs. 



2. The Cheeks (Fig. 168). 



These are two membranous walls, which enclose the mouth laterally. In the 

 interior of the buccal cavity they are limited : behind, by the posterior pillars of 

 the tongue ; in front, by the lips, with which they are confounded around the 

 commissures ; above and below, by the furrow formed by the gingival mucous 

 membrane, where it is reflected from the molar arches on to the cheeks. 



The greatest diameter of the cheeks is antero-posterior, like that of the cavity 

 it encloses. The vertical diameter is very small, especially behind ; anteriorly, 

 however, the cheeks assume a certain amplitude by the separation of the jaws. 



Structure. — The cheeks are formed by the buccal nmcous membrane, external 

 to which we find muscular tissue and glands. Vessels and nerves are supplied for 

 the conveyance of nutritive fluids, sensibility, or the stimulus to contractility. 



1. Mucous membrane. — The external face of this membrane is closely attached 

 to the buccinator muscle, and to the inferior molar glands. Its free face presents, 

 at the level of the third upper molar tooth, the buccal opening of the parotid 

 duct, at the summit of a variably-sized tubercle. On the face of each dental 

 arch there is also remarked a linear series of little salient points, analogous to 

 the large parotideal tubercle ; these are the excretory orifices of the molar glands. 

 Its structure is the same as the mucous membrane of the li}3S. (It is of a pale 

 colour, and sometimes stained in patches with pigment.) 



2. Muscular tissue. — This is the buccinator or alveolo-labialis muscle already 

 described. It may be remembered that the external face of this muscle is covered 

 by the masseter, the superior molar glands, and the skin ; while the internal 

 responds to the mucous membrane and the inferior molar glands. 



3. Glands. — These are two masses of glandular lobules, known as the molar 

 glands. They will be described with the salivary glands. 



