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Rh 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 following 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 unthickened 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 development 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 structures under the head of horny tissues. Since Miescher’s discovery, however, that the vessels of bone also traverse only the medullary canaliculi, since Müller 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.