THE HUMAN INTESTINAL CANAL. 315 



necessary briefly to get a correct idea of the more 

 general conditions of the formation of the intestinal canal 

 in the developed Man. Not until this is known can the 

 development of the several parts be correctly understood. 

 (Cf. Plates IV. and V., vol. i. p. 321.) The intestinal canal in 

 the developed Man is, in all essential points, exactly similar 

 in form to those of all other higher Mammals, and, especially, 

 to that of the Catarhines, the Narrow-nosed Apes of the 

 Old World. The entrance to the intestinal canal is the 

 mouth-opening (Plate V. Fig. 16, 0). Food and drink pass 

 tirst through this into the mouth-cavity, in the lower part 

 of which is the tongue. The human mouth-cavity is hedged 

 with thirty-two teeth, attached in two rows to the two jaws, 

 the upper and lower. It has already been stated that the 

 series of teeth is formed in Man exactly as in the Catarhine 

 Apes, but differs from the corresponding part in all other 

 animals (p. 173). Above the mouth-cavity is the double 

 nose-cavity ; the two parts of this are separated by the par- 

 tition-wall of the palate. But, as we have seen, the nasal 

 cavity is not originally separated at all from the mouth- 

 cavity, a common nasal and mouth cavity being primarily 

 formed in the embryo, and this separates at a later period 

 into two separate stories by the hard palate-roof : the upper 

 is the nasal cavity, the lower is the mouth-cavity. The nasal 

 cavity is connected with certain air-filled bony cavities; 

 the jaw-cavities in the upper jaw, the frontal cavities in 

 the frontal bone, and the sphenoid cavities in the sphenoid 

 bone. Numerous glands of various kinds open into the 

 mouth-cavity, particularly many small mucous glands and 

 three pairs of large salivary glands. 



The human mouth-cavity is half closed at the back by 



