OF DIGESTION.

363

middle one, or proventriculus, is wanting. Thus the chylifying por-tion of the intestine is formed in the several orders according to thedifferences of their food; for greater detail I refer to § 105.

If we now investigate the digestion of solid substances bv the assist-ance of the proventriculus we shall find that those, when of the animalkingdom, are swallowed wholly unchanged but in pieces, but, whenof the vegetable kingdom, they are already much comminuted andintimately mixed with the saliva. They consequently first arrive atthe large crop placed in front of the proventriculus, which in somecases, as in the Dylici, is thickly beset internally with glands, and thesuperior surface of the internal tunic is occupied with wrinkles, hornylines, and teeth (PI. XVII. f, 5 —']■). The secretion of these glands,is a dark brown sharp corrosive fluid, which strongly smells like Russia leather, it supplies the place of saliva, envelopes the food, makes it soft, andthus prepares it for digestion. The food, after having thus remaineda short time in the crop, advances by degrees into the infundibuliformorifice of the proventriculus, and thence into its narrow cylindrical orstar-shaped cavity, where it is easily comminuted, and transformed into auniform pap-like consistency. To produce this we observe in the crop, andparticularly in the proventriculus, a peculiar motion, which consists ofan alternating expansion and contraction. This contraction commencesat its anterior extremity, and gradually advances to the end of theproventriculus, whilst the earlier contracted portion again expands. Itthus greatly resembles the progressive advance of worms and footlesslarvae; it is called the peristaltic motion. It is most distinctly observedin the proventriculus, which also, of all the parts of the intestine, is sup-plied with the largest fasciculi of muscles (§ 104), and it here appearsas a contraction and distension of its internal cavity, produced by itsrhythmical contraction and expansion. By means of this contractionthe teeth and horny plates rub against each other, and thus grind thefood into a simple uniform pap, which is called chyme. In this statewe then find it in that portion of the intestine lying behind theproventriculus, which, as we have above seen, is supplied throughoutor partially with short blind appendages. These appendages, accordingto Rengger *, become shortened when the intestine is filled with food,and they then appear merely as lumps upon its surface. Its contents is

* Physiologischc Untersuchungcn uber den Thierischen Ifausbalt der Insekten.Tubing. 1817. 8vo.