The Digestive System consists chiefly of the Alimentary
Canal and its glandular appendages, the former, beginning with the MOUTH,
is successively made up of the OESOPHAGUS, the STOMACH, the small intestine
or "ileum", and the large intestine or "rectum" (with the Caeca
when present), which last opens into the Cloaca.
The glandular appendages are either proventricular and other mucous glands,
imbedded in the walls of the Canal, or salivary glands, LIVER, and PANCREAS,
communicating with it through special ducts. The function of the System
is of two separate kinds: first
the preparation of the food, which
is effected in part mechanically and in part by chemically-acting secretions
of the accessory glands; and
secondly
the absorption of the "chyle",
or prepared nutritive fluid, by means of the LYMPHATIC SYSTEM.
The Digestive Process
The digestive process is as follows:- The food taken
into the mouth is swallowed and passes through the oesophagus into the
stomach, assisted in its descent by the secretions of the salivary and
mucous glands. When there is a Crop,
it is therein mixed with saliva and water, and assisted by the heat of
the body is softened aud acted upon in a preliminary way.
It then enters the stomach, where it meets with
the secretions of the proventricular or gastric glands. But beside being
acted upon chemically it is crushed and triturated in the gizzard, especially
in graminivorous and granivorous birds, which possess a strong muscular
stomach. Thus comminuted it is known as "chyme," and passes through the
pylorus into the small intestine, in the first loop of which, the "duodenum,"
it is mixed with the bile and pancreatic juice, these two fluids being
the secretions of the liver and the pancreas. Their principal action
is to convert its soluble parts into "peptones", which are to be conveyed
into the Lymphatic System, and so into the Blood.
Their absorption as chyle is effected by numerous "villi" or projections
which line the walls of the whole Canal from the pylorus to the cloaca.
At the beginning of the rectum the caeca, when
such are functional, receive the remaining chyme, and it is probable that
in them certain hitherto undissolved matter, as cellulose and possibly
chitin, is acted upon by methane, so as to extract as much nutrition as
possible from the food. After remaining a due time in the caeca,
their contents return to the retum, and are finally ejected through the
cloaca as faeces.
The Alimentary Canal
The walls of the Alimentary Canal are composed of
five layers,of which the innermost only is of "endodermal" origin,
the rest being "mesodermal" (see EMBRYOLOGY). These layers are:
-
The tunica serosa or adventitia,
which
is outermost and consists of partly elastic connective tissue.
-
A layer of smooth muscular fibres, transversely or
circularly arranged.
-
One of smooth muscular fibres, longitudinally arranged.
-
The tunica submucosa
of loose connective tissue,
which contains nerves, blood, and lymphatic vessels.
-
The tunica mucosa or innermost lining, composed
of epithelial cells, which give rise to mucous and various specific digestive
glands.
It is noteworthy that Birds and Reptiles differ from
Mammals in the succession of the two muscular layers (2 and 3), since in
the last the circular fibres are placed on the inside, next to the submucosa
(4),
while the longitudinal fibres together with the serosa (1) form
the outer wall. These layers vary considerably in the different parts of
the Alimentary Canal; thus the thickening of the walls of the gizzard is
due to the excessive development of the muscular, layers, while in the
oesophagus the mucosa is represented chiefly by ordinary epithelial
cells, comparatively few of which form simple mucous glands, though in
the region of the proventriculus its cells are transformed into large glands,
often closely packed and compressed, constituting the greater part of the
thickened walls. Again, in the gizzard no such specific, but only mucous
glands occur, the hardened secretion of which invests its cavity with an
additional cuticular lining.
Both the small and large intestines are characterized
by numerous villi protruding into the canal as excrescences of the two
innermost layers, and absorbing the prepared nutritive fluid. Beside the
ordinary mucous glands the mucosa
gives rise to two masses of specific
nature which as LIVER and PANCREAS grow out of the walls of the duodenum,
and thus indicate their point of origin only by their respective ducts.
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