THE ORAL CAVITY 159 



THE ORAL CAVITY. 



This region is lined with stratified epithehmn resting \\\Kn\ a 

 subepithelial C()nnecti^'e tissue which is thicker in some ])laces 

 than others. In the hard palate, when ])resent, the mucous mem- 

 brane is in close relation with bone, but o\-er the jaws striated 

 muscle lies below the mucous membrane. ]\Iucous, serous, and 

 mixed mucoserous glands are often present. Reptilian, avian, and 

 mammalian oral epithelium is usually stratified squamous in char- 

 acter. The mucous membrane which lines the oral cavity of the 

 frog is covered with stratified ciliated columnar epithelium in which 

 goblet cells are numerous. There are also frequent pocket-like 

 involutions lined with large goblet cells. In the tunica propria there 

 is diffuse lymphoid tissue and occasionally a lymphoid condensation. 



Teeth. Very conspicuous as oral modifications are the teeth of 

 mammals. They are hard structures derived in part from epithe- 

 lium and in part from connective tissue covering the jaws. They 

 develop from large papillse of connective tissue covered by epithe- 

 lium. Both tissues undergo a peculiar hardening process through 

 chemical transformations and deposits. The de\'elopment of the 

 teeth resembles that of the placoid scales of Elasmobranchs. The 

 gum consists of muscle, connective tissue, and stratified squamous 

 epithelium. 



Each tooth is di\"isible into a crown which projects above the 

 gum, and a root, or roots in cases of the larger teeth with divided 

 basal insertions, which tapers to fit into an alveolus or pocket in the 

 jaw bone. The tooth is composed mainly of dentin, \vhich surrounds a 

 pulp cavity and is thickest in the crown region where it is covered by 

 an epithelial deri^'ative, the enamel. Below the gum level, a thin 

 layer of cementum covers the dentin of the root. The dentin 

 and cementum resemble bone in structure, but the enamel is 

 formed mainly of deposits of inorganic (only 3 to 5 per cent organic) 

 salts in the epithelial cells and is a harder material than bone. 

 A canal passes through the root carrying blood vessels and nerves 

 to the pulp cavity. 



Tongue.— Although the tongue is generally represented in all 

 vertebrates, it varies widely in the extent to which it develops. 



Among the fish it is poorly represented, and is called a primary 

 tongue. Such a tongue is composed of a projection of dense con- 

 nective tissue from the floor of the mouth and is covered by a 

 mucous membrane similar to that of the oral cavitv. 



