The law of the unlimited growth of the scal priform incisors is unconditional, and constant exercise and abrasion are required to maintain the normal and serviceable form and propor tions of these teeth. When, by accident, an op posing incisor is lost, or when by the distorted union of a broken jaw the lower incisors no longer meet the upper ones, as sometimes hap pens to a wounded hare, the incisors continue to grow until they project like the tusks of the elephant, and the extremities, in the poor animal's abortive attenipts to acquire food, also become pointed like tusks : following the curve prescribed to their growth by the form of their socket, their points often return against some part of the head,are pressed through the skin, then cause absorption of the jaw-bone, and again enter the mouth ; rendering masti cation impracticable, and causing death by starvation. In the Museum of the College of Surgeons there is a lower jaw of a beaver in which the scalpriform incisor has,by unchecked growth, described a complete circle; the point has pierced the masseter muscle and entered the back of the mouth, passing between the condyloid and coronoid processes of the lower jaw, descending to the back part of the molar teeth, in advance of the part of its own alveolus, which contains its hollow root. The upper jaw of a rabbit with an analogous abnormal growth of the scaleriform and acces sory incisors is represented in fig. 269.
Organs of digestion. —The order of Rodent quadrupeds cornprehends animals which are nourished by various kinds of food, both animal and vegetable substances forming the nutriment of some genera, whilst others live exclusively upon the fruit, bark, or leaves of trees, or upon tender succulent plants. The differences observable in the structure of the stomach and intestinal canal correspond to the variety of their food, and bear a relation to the structure of their dental apparatus.
campagnols (Arvicola), and the lemmings (Georychus, !linger), present a similar ar rangement.
Some genera, as, for example, the Canada rat (fig. 270.) are remarkable for the pos session of capacious cheek pouches, in which considerable quantities of food can be stored up, and which, like the crop of birds, may be considered as reservoirs, wherein nutriment can oe retained preparatory to its introduction into the stomach.
The type of stomach most common in this order is the following: the stomachal bag is formed by two distinct pouches, which are more or less separated from each other ; one portion, situated to the left of the cardia, is placed longitudinally, and is generally of a cylindrical or conical shape. This portion is frequently larger than the right portion ; it is lined internally with a thick epidermis, which terminates suddenly, and clearly indicates by its margin the boundaries of this compart ment of the stomach. The right compartment, which is situated more transversely and further back, is of a conical shape, the apex of the cone terminating at the pylorus. This second
portion has its walls thicker and niore mus cular than the former ; its mucous membrane .is not lined with epidermis, but presents the ordinary appearance. The distinction be tween these two portions is indicated ex ternally by a constriction. The cesophagus enters the first compartment very near to the point where it communicates with the second.
Such may be said to be the typical form of the stomach in this extensive order, but many families recede from it to a greater or less ex tent.
In the squirrels (Sciurus) for example, the stomach is not divided into separate cavities, but is of a pyriform or oval shape, giving off a conical or cylindrical portion, which ter minates in the pylorus. The first compart ment is lined internally with a thick epi dermis, which forms two oval lips, as it is prolonged around the opening into the second compartment, the lining membrane of which is simply mucous, without any apparent epi dermic covering. The ondatras (Fiber, Cuv .), In the Hudson's Bay lemming (Mus sonius, Gm.), the shape of the stomach is slightly different, it is situated transversely and much elongated, without any division into cavities ; the cardia opens at about the upper third of its anterior border ; the left cul-de-sac is cylindrical and of uniform size with the pyloric portion, which is bent for ward and to the left side.
The stomach likewise varies from the com mon type in the jerboa (Dipus, Gm .), and in the leaping hares of the Cape (Helamps). In the former it is globular, in the latter pyriform and longitudinal, with a large cardiac cul-de-sac directed forwards, a pyloric cul-de-sac, and a short cylindrical pyloric portion, which is bent forwards.
The rat moles (Spa lax, Guldensledt) are approximated to the lemmings and to the campagnols in the shape of their stomach, which is divided into two pouches, hiiving the cesophagus closely approximated to the pylorus.
In the muscardin avellenarius, L.) the stomach offers a peculiarity in its struc ture, which distinguishes it not only from the other species of this genus, but also from all other Mammalia, — the Ixsophagus imme diately beyond the diaphragm terrninates in a globular pouch, the walls of which are thick, glandular, and exhibiting internally numerous pores leading into crypts : this structure is separated by a constriction from the stomach properly so called. This latter organ forms a large cul-de-sac of a slightly oval shape, which gives off' anteriorly, and to the right side, a short bowel-like pyloric portion. In this animal, therefore, there are two stomachs, one of which corresponds with the glandular ' stomach of birds, as will be seen further on. The beaver exhibits traces of this structure.