498 



OF NUTRITION. 



which cover the interior of the opercula of the dental sac, like an Epithelium. 

 These opercula attain a much greater development in the Molar teeth of Herbi- 

 vorous animals ; where they dip down into the midst of the pulp, and give 

 origin to the vertical plates of Enamel already mentioned ( 633). It has been 

 remarked by Mr. Lintott, that the line along which the opercula meet, on the 

 crown of the Human molar teeth, that is to say, the groove which separates 

 their tubercles, is by far the most frequent seat of incipient decay ; probably 

 from its tissue having been at the first least perfectly formed. The Cementum 

 is formed by the transformation of the inner layer of the dental sac itself; or 

 by matter effused from it as from the periosteum of Bone. In the Human 

 tooth, it is formed as a thin layer over the crown, as well as around the fang ; 

 but the former soon wears away. In the teeth of Herbivorous animals, its 

 vertical plates are formed from the Opercula ; which, as just mentioned, dip 

 down into the dental pulp. The amount of this deposit around the fangs may 

 be very much increased by an inflammatory state of the capsular membrane ; 

 and thus are produced those exostoses which are often very troublesome ap- 

 pendages to the Teeth. 



637. A very large proportion of the body, in the higher Animals, is com- 

 posed of a tissue, to which the name of Cellular is ordinarily given ; this term, 

 however, is so much more applicable to those structures which are composed 

 of a congeries of distinct Cells, and the use of it for both purposes is likely to 

 engender so much confusion, that it is to be wished that its application to this 



[Fig, 132. 



[Fig. 133. 



The two elements of Areolar tissue, in their natural rela- 

 tions to one another; l,the white fibrous element, with cell- 

 nuclei, 9, sparingly visible in it ; 2, the yellow fibrous ele- 

 ment, showing the branching or anastomosing character of 

 its fibrillae; 3. fibrilke of the yellow element, far finer than 

 the rest, but having a similar curly character ; 8, nucleolated 

 cell-nuclei, often seen apparently loose. From the areolar 

 tissue under the pectoral muscle, magnified 320 diameters.] 



Development of the Areolar tissue. 

 (white fibrous element); 4, nucleated 

 cells, of a rounded form; 5. G, 7, the 

 same, elongated in different degrees, and 

 branching. At 7, the elongated extremi- 

 ties have joined others, and are already 

 assuming a distinctly fibrous character. 

 (After Sehwami.)] 



