292 
On the Teeth of the Ox, Sheep, and Pig, 
Fig. 4." 
to form the prism. The shape of eacli prism has been differently 
described by anatomists, but the commonly-received opinion 
appears to be that they are hexagonal. The figures added to 
assist this description show that form, tliough after I'epeatcd in- 
vestigations, I have by no means satisfied myself of its positive 
correctness. They seem to me rectangular in shape, and slightly 
flattened. 
Fi^. 4, A, siiows tlie prisms as a single layer; and it will be 
observed that, besides the minute canals between them, each prism 
is crossed by lines in some places double, representing connecting 
cross passages. In our best works on structural anatomy, these 
lines also are said to be produced 
by the walls of united cells. This 
point, however, requires further in- 
vestigation. I have often examined 
enamel v/hich has been very re- 
cently deposited on a. forminfj tooth, 
and have then failed to see any 
crossings on the separated bars, 
altliough tliese are to be observed 
in the structure when viewed in sec- 
tion after the ordinary manner. 
It may naturally be asked what is the use of the minute canals 
existing between the prisms ? A solution of this query is 
probably found in looking to the situation of the enamel in com- 
pound teeth. In them, as has been stated, this structure dips 
inwards, forming cups of greater or less depth in different teeth, 
which are always filled with Crusta (see figs. 2, 9, and 19). On 
the slightest reflection we perceive that tiie Onista here placed can 
only receive the fluid necessary for its support, tlirough the layer 
of enamel which is intei'posed between it and the dentinal tubes 
(fig. 9), and there seems to be no reason to doubt that the tulies, 
from the boundary of tlie Dentine, may extend to the canals be- 
tween the prisms, and thus supply the materials of nutrition to 
the Crusta within the cup.-f- 
* Fig. 4. Structure of enamel after Retzius, somewhat modified, a, vertical 
section showinj^ the enamel prisms with their interposed canals, n, transverse 
section showing the si'ppusrd hexagonal form of the prisms with tlie openings 
of the canals. Magnified .500 diameters. 
+ Mr. Tomes many years since drew attention to the fact that dentinal tubes 
passed in great numbers into the enamel in the kangaroo, and other animals of this 
class. From my own examinations I can also say that tlic dentinal tubes penetrate 
the enamel in the herbivora ; and it is more than probable that in coiixdliihfcd 
enamel the passages referred to are united with these dentinal tubes, and thus 
become continuous with them. In this view I am supported by Todd and Bow- 
man, who remark, in their work on Physiological Anatomy, tliat the enamel 
prisms " are arranged in the most suitable manner for percolation by the fluids 
