RAINEY, ON DENTAL TISSUES. 213 
organs concerned in its formation. The latter comprises the 
dentine-pulp, the enamel-pulp, and the osteo-dentine or 
bone-pulp. These are all composed of areolar tissue, vessels, 
and nerves, and are provided each with an epithelium. The 
former consists of a hard part, made up of dentine and 
enamel, and a soft part. This latter is limited to the im- 
mature organ, and, having the same relation to the calcified 
portion of a young cusp that the membranous edge of a 
flat bone has to the ossified part, I shall call it the mem- 
branous matrix of the cusp. This being the part where the 
process of calcification commences, and on which the pro- 
gressive stages of that process admit of being easily ex- 
amined, it will require to be described with some degree of 
minuteness. And to make this perfectly intelligible, a general 
view must first be taken of all the parts concerned in the 
formation of a cusp, and of the cusp itself. This will be 
best done by referring to the following diagram (fig. 1), 
which is intended to represent all the parts as they are 
found in the ossifying cusp of a mammal before the tooth has 
passed through the gum. ° 

c, capsule ; e-p, enamel-pulp; d-p, dentine-pulp; m, matrix, undulated, 
and dividing into e-m, enamel-matrix, as the external, and d-m, dentine- 
matrix, as the internal layer; e, enamel; d, dentine. 
A cusp, in which the process of calcification has made but 
little progress, is best adapted for this examination. In 
such a cusp, when seen by a low magnifying power, a mere 
shell of tooth-substance, with a membranous border ex- 
tending from its lower margin—the membranous matrix—is 
distinguishable. (Pl. XI, fig. 4). The relative proportions 
of the hard and soft parts will vary as a cusp approaches the 
