Cattle 1853 - Lewis Family Farm

THE ALTERATION OF THE SLOOD. 215
ive process, and to prepare it for that function on which the life of
the animal is to depend; and also to prepare the intestines for the
discharge of their duty. When, after birth, it begins to be separated
from the parotid gland, it has no means of pouring its secretion into
the stomach, and it gradually dwindles away, and disappears.
THE BRONCHIAL TUBES.
The windpipe pursues its 'course down the neck, until it arrives at
the chest. It there somewhat alters its form, and becomes deeper
and narrower, in order to suit itself to' the triangular opening through
which it is to pass. It enters the chest, and preserves the same
cartilaginous structure until it arrives at the base of the heart, where
it separates into two tubes, corresponding with the two divisions of
the lungs. These are called the bronchial tubes. They plunge
deep into the substance of the lungs ; these presently subdivide ; and
the subdivision is continued in every direction, until branches of the
trachea penetrate every portion of the lungs. These are still airpassages,
and they are carrying on the air to its destination, for the
accomplishment of a vital purpose. The lungs of the ox afford the
most satisfactory elucidation of the manner in which these air-tubes
traverse that viscus. They can be followed until they almost elude
the unassisted sight, but the greater part. of them can be evidently
traced into the lobuU, or little divisions of the substance of the lung,
which are so evident here. The minute structure of these lobuli has
never been demonstrated ; but we may safely imagine them to con-
sist of very small cells, in which the bronchial tubes terminate, and to
which the air is conveyed ; and that these cells are divided from
each other by exceedingly delicate membranes.
THE ALTERATION OF THE BLOOD.
The blood has already been described as sent from the right ventricle
of the heart into the lung, and the blood-vessels dividing and
subdividing until they have attained a state of extreme minuteness,
and then ramifying over the delicate membrane of these cells. The
blood, however, is in a venous state ; it is no longer capable of supporting
life; and it is forced through the lungs, in order that it
may be rendered once more arterial, and capable of supporting life
and all its functions. For this purpose, these minute veins spread
over the delicate membrane of the cells, and for this purpose also, the
air has been conveyed to these cells by the bronchial tubes.
Now, the chemical, it may almost be said the vital, difference between
venous and arterial blood is, that the venous is loaded with
carbon, and deficient in oxygen. It here comes, if not in absolute
contact with atmospheric air; yet so close as to be separated only
by a gossamer membrane, which offers little obstacle to the power of