OF DIGESTION. 155 



tongue. This fold, it may be mentioned here, is what produces the 

 appearance called tongue-tied, when too short. The tongue, then, 

 cannot be well put out of the mouth, and the infant cannot nurse 

 conveniently. A little time, however, relaxes the parts. 



One or other of the ducts which open below the tongue is subject 

 to become stopped up, and the saliva distends it into a soft- swell- 

 ing, which impedes the motions of the tongue in the actions of 

 speech and mastication. But the most annoying thing connected 

 with the salivary ducts is, when one of those running in the cheek 

 is divided, as by a cut ; it is then difficult to heal, and the saliva is 

 constantly running over the cheek. The quantity which is lost in 

 this way in such cases, is very great indeed. 



TEETH. 



The teeth are the hardest parts in the whole body. In the adult 

 they are thirty-two in number, eight upon each side of each jaw. 

 They are of four different kinds ; in front there are, on each side, 

 two incisors or cutting teeth, whose edge is like that of a chisel ; 

 next them is one eyetooth, which is pointed ; thirdly, there are two 

 small grinders; and lastly, there are three large grinders. Tha 

 teeth in the two jaws do not exactly meet ; the cutting teeth of the 

 upper jaw overlapping those of the under, while the grinders just 

 meet one another. We sometimes, however, see people whose 

 under teeth project beyond their upper, giving a peculiar appear- 

 ance of length to the lower part of the face. 



The crown is that part which appears above the gum, the body 

 is the thick part, and the fangs or roots penetrate down into the 

 socket. A cavity is seen in the body of the tooth, which is occu- 

 pied by a pulpy 'substance, containing some blood-vessels and nerves, 

 and a small canal is seen leading from the cavity into each fang, 

 and opening by a minute hole, through which the blood-vessels and 

 nerves enter. The bony part of the tooth which projects above the 

 jaw, and is destined to meet its fellows, and to come in contact with 

 the fluids in the mouth, is protected by the enamel, which is the 

 hardest substance in the body. It is thought not to be an organized 

 substance at all, as no blood or sensibility has been detected in 

 it, so that it seems to be merely a protection, to prevent the re- 

 mainder of the tooth from being worn. When broken off, it is 

 never replaced, and the tooth of necessity passes into decay. 



The structure of the bony part of the tooth is similar to that of 

 bone elsewhere, only it is much harder. Hence, by referring to 



