278 
On the Teeth of the Ox, Sheep, and Pig, 
often laminated, but still in neither can it be said that true 
teeth are present. 
Tlie gizzard of the bird is beautifully adapted to the office it 
has to perform. It is a hollow cavity, the walls of which are 
composed of two powerful but short muscles, with smaller ones 
superadded. These muscles are remarkably thick in proportion 
to their length, and with then- tendons are so arranged as to 
give a grinding or partial rotatory motion to the whole organ. 
The inner surface of the gizzard is lined with a very dense 
cuticular membrane, which is thrown into ridges, the better to 
act mechanically upon the food. 
In addition to this arrangement the bird is led instinctively to 
swallow numerous small pebbles to further assist in the reduc- 
tion of its aliment. If deprived of these earthy matters, as is 
well known, the health of the creature will soon suffer, because it 
cannot extract the same amount of nutriment from its food. It 
would appear that there is even a greater exercise of the instinc- 
tive faculty in the selection of these earthy materials than 
we might at first suppose. Dr. Crisp, who has dissected nume- 
rous birds at the Zoological Gardens, informs me that he has 
very frequently found one pebble much larger than the others, 
forming a kind of fulcrum for the smaller to move upon, and 
thus greatly increasing the mechanical or grinding power of the 
gizzard. 
The food which is swallowed by the bird first enters the crop, 
Avhere it is retained a short time to be softened by the secretions 
of this organ. From this receptacle it goes to the gizzard, 
passing through the proventriculus, a short canal which connects 
the two cavities together, and where the true digestive fluid, 
{/antric juice, is produced. Within the gizzard the food is ground 
down and mixed at the same time with the gastric juice which 
enters the organ from the proventriculus above. After being 
sufficiently digested and comminuted, it passes onwards into 
the intestinal canal. 
Such, in a few words, is an explanation of the digestive 
process as we find it in the bird. The gizzard has sometimes 
been compared to a mill, and the crop to the hopper from which 
it receives the supply. 
It is evident from the preceding remarks that the teeth belong 
to the membrane lining the digestive canal rather than to the 
skeleton or Ijony parts of the frame. To further elucidate this 
part of my subject, however, I will add a few observations, of a 
very general nature, on the teeth of fishes. In tliis tribe we find 
for the most part that the teeth are located either upon the mem- 
brane of the mouth or at the commencement of the gullet. They 
are produced by the membrane, and are not, as in mammals, im- 
