THE TEETH. 97 



closed by a peculiar membrane, and the canaliculi are then also 

 seen to be hollow prolongations of cells. The intermediate 

 substance between the cavities is produced in one of the fol- 

 lowing wavs : either the walls of the cells become thickened, 

 and then coalesce to form an homogeneous substance, or, which 

 is much the more frequent mode, the intercellular substance 

 is developed in greater quantity, and a coalescence takes place 

 between it and the unthickened or only slightly thickened 

 cell-walls. I cannot positively assert that a blending of the un- 

 thickened cell-walls with the intercellular substance takes place 

 universally : I cannot do so, for instance, with respect to the 

 cartilages of the higher animals, and so far the mere coalescence 

 of the cell-walls is not a certain characteristic of this class 

 of tissues. Should it be found not to prevail universally we 

 must look for a distinctive character in the abundant develop- 

 ment of a firm intercellular substance — a peculiarity which 

 is presented by no other class. 



1. Cartilage and Bone. As these tissues have been already 

 treated of (pp. 15-33), the reader is referred to that part of the 

 work. 



2. The Teeth. The teeth were formerly classed with the 

 bones, but have of late been treated of as non-vascular struc- 

 tures under the head of horny tissues. Since Miescher's 

 discovery, however^ that the vessels of bone also traverse 

 only the medullary canaliculi, since Miiller observed that 

 the teeth, like the bones, afford gelatine by boiling, and 

 Retzius discovered osseous corpuscles in the ivory, it seems 

 more correct to class the teeth with the bones again, and 

 the more so, as we now know that the presence or absence 

 of vessels proves no essential difference in the growth. The 

 coalescence of the cell-walls which appears to take place in the 

 ivory of the teeth forms an additional reason for our classing 

 them with bone. The teeth, as is well known, consist of ivory, 

 osseous substance, and enamel. 



