INFERIOR EXTREMITY OF THE HEAD. 69 



CHAPTER IV. 



INFERIOR EXTREMITY OF THE HEAD. 



A. The Mouth (Fig. 25). 



Situation ; Divisions. The mouth is a complex region which 

 occupies the inferior extremity of the head and represents the entrance 

 into the digestive apparatus. It is elongated from before to behind, 

 and comprised between the two jaws. We recognize in it the following 

 parts, which we will first study : 1, the lips; 2, the teeth and the gums ; 

 3, the bars; 4, the lingual canal; 5, the tongue; 6, the palate. We will 

 afterwards pass to the examination of the mouth in general. 



i. The Lips (Figs. 19, 25, and 26). 



Situation; Limits; Anatomical Base. The lips are two 

 movable musculo-cutaneous curtains, placed at the entrance of the 

 mouth, whose opening they limit. 



They are distinguished as superior or anterior and inferior, or pos- 

 terior. 



Physiologically, they are organs necessary to the prehension of 

 food, and they serve as auxiliaries in mastication. The inferior lip, 

 in relation to the exterior, to a small degree supports the bit, whose 

 impulses are, in a certain measure, first received by it. 



The superior lip, the more mobile, is limited above by the 

 extremity of the nose ; on the sides by the cheek and the nostrils ; by its 

 free border, finally, it is in contact with its congener. The inferior 

 lip is limited behind by the chin and laterally by the cheeks. 



The lips present for study two faces, external and internal ; two commissures, 

 right and left ; and two borders, free and adherent. Each is composed of three 

 layers, an external or cutaneous, an internal or mucous, and a middle or muscu- 

 lar, besides blood-vessels and nerves. The muscular layer is constituted by the 

 orbicularis oris muscle and the muscles of the face which attach to it. The arte- 

 ries are the superior and inferior coronary, passing through the centre of their 

 substance. 



The external face of each lip is covered by fine and very adherent skin, 

 provided with two kinds of hairs : the one consisting of long, stiff, and scattered 

 hairs, called tentacles, and deeply embedded in the subcutaneous tissue and even 

 in the muscles ; the other, very fine, short, and numerous, belonging to the ordinary 

 hairs of the coat. The former are provided with a nerve terminal at the base of 

 their papilla, which makes them delicate organs of tactile sensation for the 

 animal. To a large degree these supersede the function of the hand in quadru- 



