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 ways: 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. : 
