4 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



C. THE BUCCAL OR MOUTH CAVITY 



Note that the two jaws fit together very tightly; this is essential for respira- 

 tion, as will be seen later. Open the mouth to its widest extent, cutting the 

 angle of the jaws if necessary (A). Consult Holmes, chapter iv, pp. 68-73. 



1. Roof of the mouth cavity. The edge of the upper jaw is covered by a 

 fold of skin, the upper Up fold. Just within this fold and concealed by it is a 

 semicircular row of teeth, the maxillary teeth, borne upon the edges of the 

 maxillary and premaxillary bones of the skull. It is necessary to grasp the lip 

 fold with a forceps and pull it outward in order to reveal the teeth. Obtain a 

 dried skull, examine the teeth with a hand lens, and note the following points. 

 The teeth are not set into sockets, that is, cavities in the jaw, as are our own teeth, 

 but they are fused by their bases and sides to the margins of the jaw, only their 

 tips projecting freely above the edge of the bone. Each tooth is a hollow cone, 

 consisting of two parts separated by an indistinct groove: an upper part, the 

 crown, composed of dentine, and coated externally with a shiny material, the 

 enamel; and a lower part, the root, composed of cement. The teeth are replaced 

 when lost and two or three sets of new teeth may usually be seen at the bases of 

 the old ones. 



Returning now to your own specimen, locate within the row of maxillary 

 teeth a deep groove, the sulcus marginalis. At the tip of the jaw this groove 

 deepens into a pit, the median subrostral fossa; on each side of this is an eleva- 

 tion, the subrostral puhinar or cushion; and lateral and adjacent to this is another 

 depression, the lateral subrostral fossa. 



Within the sulcus marginalis is the roof of the mouth cavity, properly speak- 

 ing. The anterior extremity of this is occupied by two oval openings, the internal 

 nares or choanae; they are the internal openings of the cavities of the nose, whose 

 external openings were already noted. The two pairs of nares, therefore, with 

 the cavities of the nasal chambers, constitute the respiratory passage through 

 which air is drawn into the buccal cavity. Between the choanae are two patches 

 of wmerine teeth, located upon the wmer bone. The greater part of the roof of 

 the mouth is occupied by two large rounded prominences, where the eyes are 

 located, and, as already observed, the eyes can be withdrawn into the mouth 

 cavity. At each side of the posterior end of the roof is an opening, the entrance 

 to the Eustachian tube (auditory tube in more recent terminology). Where does 

 it lead ? Consult Holmes, p. 69. 



2. Floor of the mouth cavity. The edge of the lower jaw forms a ridge 

 which fits into the sulcus marginalis. At the tip of the lower jaw is an elevation, 

 the prelingual tubercle, and on each side of this a depression, the prelingual fossa. 

 Note how exactly these fit into the elevations and depressions of the upper jaw. 

 Are teeth present on the lower jaw ? The greater part of the floor of the mouth 

 is occupied by the tongue. Note the size, shape, and attachment of this organ, 

 and find out how it is used in catching prey (Holmes, pp. 26, 70-71). Turn the 



