DIGESTIVE SYSTEM.
Illouth.—The various modes of locomotion, resulting from the different modifications of the osseous and muscular systems observable in the several families of Marsupialia, relate to the acquisition of as various kinds of alimentary substances, which necessarily require for their assimilation as many adaptations of the diges tive organs.
Food,—means of obtaining it,—instruments for preparing and mechanically dividing it,— cavities, canals and glands for chemically reducing and animalizing it,—form a closely connected chain of relationships and interde pendencies. The usual sequence of anatomical description has here been followed in com mencing with the consideration of the passive and active organs whose office it is to carry the stomach to the food and the food to the mouth, and we have now to describe the preparatory mechanical instruments in digestion, and the modifications and appendages of the alimentary canal.
The jaws of the Marsupialia are covered by well developed fleshy lips ; the upper lip is partially cleft in the Kangaroos, as in some of the Ro dents; the muzzle is clad with hair in the Great Kangaroo and a few other species of Macropes, but in other Marsupialia it is naked and generally red from the vascularity of the integument.
The masticatory muscles of the jaws consist in the Marsupial, as in other Mammalia, of the temporal, the masseter, the external and in ternal pterygoids, and the digastricus. They are chiefly remarkable for the large proportional size of the masseter and internal pterygoid; the great developement of the latter muscle is con stant in all the Marsupials, and is the condi tion of the peculiarly large and inflected angle of the lower jaw. The relative size of the masseter, as compared with the temporal mus cle is greater in the herbivorous than in the carnivorous species, but this difference is much less in the Marsupial than in the cor responding placental genera. The extent of origin of the temporal muscle is indicated by the various conditions of the temporal and pa rietal crests ; the inner surface of the zygomatic arch always affords origin to a portion of the fibres of this muscle, and in some species, as in the Koala, to all that portion of it which is inserted into the external fossa of the coronoid process and ascending ramus of the jaw, the fibres from the temporal and parietal bones being implanted on the inner side of the coro noid process. The masseter takes its origin by
a strong band of tendinous and carneous fibres from the inferior and anterior part of the zy goma ; the muscle expands as it is directed backwards, and is inserted into the ridge which bounds the external temporal fossa of the as cending ramus, and into the outer side of the inflected angle of the outer jaw. The external pterygoid takes its origin from the temporal plate of the sphenoid and the base of the pterygoid plate anterior to the sphenoidal bulls ; the fibres converge to be implanted into the inner projecting side of the condyle of the jaw. The internal pterygoid arises from the outer depression of the longitudi nally extended pterygoid plate already men tioned as characterizing the cranial structure of the Marsupials, and is implanted along the inner surface of the inflected angle of the jaw. The digastricus arises from the ex-occipital process ; its fibres expand, and are inserted into the lower margin of the maxillary ramus, anterior to the commencement of the inflected angle of the jaw. The preceding description is taken from a dissection of the Koala. The masticatory muscles of the Wombat differ only in their relative proportions ; the masseter in this gliriform Marsupial is single, presenting no trace of that subdivision and modified attach ments which adapt it to the protraction of the lower jaw in the true Rodents, and accordingly the structure of the joint of the lower jaw of the Wombat exhibits, as already described, a corresponding difference from the Rodent type.