=General Arrangement.=—The stomach of the ox is very large, and occupies
nearly three-fourths of the abdominal cavity. It fills all of the left
half of the cavity (with the exception of the small space occupied by
the spleen) and extends considerably over the median plane into the
right half.
It is compound, being composed of four divisions, viz., =rumen=,
=reticulum=, =omasum=, and =abomasum=.[107] The division is clearly
indicated externally by furrows or constrictions. The first three
divisions may be regarded as proventriculi or œsophageal sacculations,
the fourth being the stomach proper (in the narrower sense of the term).
The œsophagus opens into the stomach on a sort of dome formed by the
rumen and reticulum, and is continued through the latter by the
œsophageal groove. From the ventral end of the latter a groove traverses
the ventral wall of the omasum, thus giving a direct path to the
abomasum for finely divided or fluid food. The abomasum joins the small
intestine.
=Capacity.=—The capacity of the stomach varies greatly, depending on the
age and size of the animal. In cattle of medium size it holds 30 to 40
gallons, in large animals 40 to 60, in small 25 to 35. The relative
sizes of the four parts vary with age. In the new-born calf the rumen
and reticulum together are about half as large as the abomasum; in ten
or twelve weeks this ratio is reversed. During this period the omasum
appears to be contracted and functionless. At four months the rumen and
reticulum together are about four times as large as the omasum and
abomasum together. At about one and one-half years the omasum equals (or
nearly so) the abomasum in capacity. The four divisions have now reached
their definitive relative capacities, the rumen constituting about 80
per cent., the reticulum 5 per cent., the omasum 7 or 8 per cent., and
the abomasum 8 or 7 per cent. of the total amount.