158 EXTRACTS FROM FOREIGN PERIODICALS. 
tions of the Academy of Stockholm.—M. Purkinje, says the author .ot 
the letter, has pushed his researches on enamel further than myself; but 
I think I have gone further than M. P. with regard to the osseous portion. 
The two preparations sent with this letter will enable you to verify the 
accuracy of my results. M. Purkinje and myself have acknowledged that 
the osseous substance is chiefly composed of undulating fibres, and of hol¬ 
low cylindrical canals, which radiate from the centre of the pulp towards 
the surface; I have ascertained that they almost uniformly ramify without 
communicating with each other. Under the microscope they resemble vessels 
filled with a white substance. The same structure is observed in the teeth of all 
vertebrated animals. Since these observations have been published, I have dis,r 
covered, that others similar to them have been made by Leuwenhoeck, but no 
one appears to have noticed them. The preparations which I send are of human 
teeth, the one a vertical, the other a horizontal slice, and in the middle of the 
crown. M. Purkinje and I have found the cortical substance of Tenon, 
surrounding the roots of human teeth. This substance closely resembles bone 
in structure; it has the same porosities, with undulating canals, but it wants the 
small blood vessels, the cylindrical tubes, and the radiating canals. 
6. The Nature of Dartoid Tissue. —^In a paper read before the Academie 
des Sciences, on the 19th of September, 1836, by M. Thomson, the author 
attempts to prove that the dartoid tissue is not a distinct tissue. In certain 
animals, he observes, as the Stag, the Sheep, the Goat, &c., the dartos so evi¬ 
dently presents the aspect of muscular tissue as to leave no doubt as to its being 
such. In man, indeed, the resemblance is not so striking, but an attentive exami¬ 
nation proves that it exists in all the essential points. In fact, the fibre of the 
dartos exhibits a slightly rosy tint; it is homogeneous and transparent; cut 
across, it presents a square section; in short, its characters are those of muscular 
fibre. 
• 7. On the Influence of Atmospheric Pressure.— M. Dombres Firmas, in a 
memoir read on the same day as the preceding, asserts that man, in a healthy state, 
can support, without difiiculty, the great variations of atmospheric pressure. We 
are not favored with the facts and reasoning adduced in support of M. Firmas’ 
theory. 
8, Observations on the specific characters of the large Cetacea, or 
Whales.' —It is often very difficult, says M. Vanbeneden, to distinguish between 
the diflerent species of Whales, without examining fresh specimens, or at least, 
without a comparison of the crania. Still we find a character equally important 
in the situation of the ears, although it has not hitherto been noticed, and the 
application of which would be much more frequent.; A voyager would have 
much less difficulty in referring to the bones of the ear than to the entire cra¬ 
nium, and would obtain, by this means, the specific characters with equal pre- 
