COMPARATIVE ANATOMY. 
the thick muscular globes of the granivorous 
genera. But there are in both many inter- 
mediate links between these extremes, and 
at the same time considerable analogies in 
the structures, which are apparently tlie 
most opposite. This is particularly observ- 
able in the course of the muscular fibres, 
and in the callous structure and appearance 
of the internal coat ; in which points, many 
of what are called membranous stomachs, 
have a great resemblance to those of tlie 
gallinte. 
Both parts, but particularly the muscular, 
are very strong in the gizzard (ventriculus 
biilbosus) of granivorous birds. We find 
here, instead of a nniscular coat, four im- 
mensely thick and powerful muscles, ri?. 
a large hemispherical pair at the sides 
(laterales), and two smaller ones (intermedii) 
at the two ends of the cavity. All the four 
are distinguished, both by the unparalleled 
firmness of their texture, and by their 
peculiar colour, from all the other muscles 
of the body. 
The internal callous coat must be consi- 
dered as a true epidermis ; since, like that 
part, it becomes gradually thicker from 
pressure and rubbing. It forms folds and 
depressions towards the cavity of the sto- 
mach ; and these irregularities are adapted 
to each other on the opposed surfaces. The 
cavity of this curious stomach is compara- 
tively small ; its lower orifice is placed 
very near the upper. Every part of the 
organ is, indeed, calculated for producing 
veiy powerful trituration ; and tliis is still 
further promoted by the well-known in- 
stinctive practice of granivorous birds, of 
swallowing small hard stones with their 
food. 
The end and use of swallowing these 
stpnes have been very differently explained. 
Caasalpinus considered it rather as a medi- 
cine than as a common assistance to diges- 
tion; Boerhaave, as an absorbent for the 
acid of the stomach ; Redi, as a substitute 
for teeth ; according to Whytt, it is a me- 
chanical irritation adapted to the callous 
and insensible nature of the coats of the 
stomach ; Spallanzani rejected all supposi- 
tion of design or object, and thought that 
the stones were swallowed from mere stu- 
pidity. There scents not much -sagacity 
to be discovered in this opinion, when we 
consider that these stones are so essential to 
the due digestion of the ' corn, that birds 
grow lean without them, although they may 
be most copiously supplied with food. This 
paradoJfical opinion has, however, been 
already refuted by J. Hunter, and G. For- 
dyce. Blumenbach thinks that the stones 
kill the grain and deprive it of its vitality, 
which otherwise resists the action of the 
digestive powers. Thus it has been found 
that if the oats and barley, given to horses, be 
previously heated, tlie animal only requires 
half the quantity, and thrives equally well. 
Reptiles and birds have their nostrils ter- 
minated by two longitudinal slits on the 
palate ; they ■ have no velum palati, nor 
epiglottis. 
The oesophagus of the serpent kind is of 
immense magnitude ; for these reptiles swal- 
low animals larger than themselves, which 
are retained for a considerable time in the 
tube, and descend into the stomach by 
degrees, where they are slowly subjected to 
the action of the gastric juice. The whole 
process sometimes occupies many days, or 
even weeks. There is har dly any distinc- 
tion between oesophagus and stomach. 
From the peculiar formatioir of the nose 
of fishes, and from their respiring by means 
of gills, their fauces have no connection 
with any nasal cavity, or glottis. 
The oesophagus is of great width in fishes , 
and is distinguished with difficulty in many 
cases from the stomach. These animals 
swallow their food whole without subjecting 
it to any mastication ; anfi if the stomach 
will not hold the whole, a part remains in 
the oesophagus, until that w'hich has de- 
scended lower is digested. The alimeirtary 
canal is genei'ally very short ; sometimes 
extending straight from the mouth to the 
anus with very little dilatation, as in the 
lamprey (petromyzon marinus). 
The Crustacea, and some insects, are fur- 
nished with organs of mastication of similar 
structure. Their mouth is fornred of two 
or more pairs of jaws placed laterally. These 
move from without inwards, and vice versa ; 
whereas those of red-blooded anirrrals move 
from above downwards, and back again. 
The parts which are termed the lips of in- 
sects are two bodies, of which one is placed 
above or in front of the jaws, and the other 
below or behind tlrein. The palpi or feeler s 
are articulated to tire jaw s. All insects, which 
have jaws, possess the power of masticating 
hard animal and vegetable substances ; for 
these parts are of a firm horny texture, and 
in many cases are very large when compar- 
ed with the size of the animal. 
The locusts (grylli), the dragon-fly (libel- 
lula), the beetles, and particularly the 
lucanus cervus, or stag-beetle, and the sta- 
pbylinns maxillostis, are examples in which 
