The head in the Rodents is generally of small or moderate size in proportion to the body, and the skull is usually rather elongated, and flattened on the upper surface. The tympanic bullæ are generally of considerable size; the zygomatic arch is in nearly all cases well developed; but the orbits of the eyes are never closed behind, and only in certain families is there even a small process of the zygomatic arch behind the orbits, as an indication of possible closure. Of the vertebral column we need only say that the lumbar vertebræ are remarkable for possessing large transverse processes directed forwards, and that the tail varies greatly in length, being sometimes longer than the body, sometimes reduced to very small proportions, whilst between these two extremes almost every grade of development may be met with. The sternum, or breast-bone, is usually long and narrow. Collar-bones are nearly always present, but in a few forms they become rudimentary, or even disappear altogether. The pelvis is long and narrow. The limbs exhibit a very great variety in their development; in many, the two pairs are nearly equal in length, but in the majority the hind limbs are distinctly longer and more powerful than their fellows, and in some groups they attain a most disproportionate length, and serve almost exclusively as the organs of locomotion. On the other hand, in the great majority of the order, the fore limbs serve in a certain degree as hands, and are used for holding the food to the mouth; and in these the radius and ulna, which are always distinct bones, retain the power of rotation. The corresponding bones in the hind limbs (tibia and fibula) are, on the contrary, firmly anchylosed together in two great groups of the order. The feet have usually five toes, but sometimes this number is reduced to four, or even to three, in the hind feet. These toes are armed with claws, which, however, in one family, acquire more or less of the appearance of hoofs. In point of intelligence the Rodentia do not stand high. The brain is comparatively small, and the cerebral hemispheres show no traces of those convolutions of the surface which are characteristic of most Mammals (_see_ figures). The Capybara alone is known to have a few convolutions. The cerebellum is entirely uncovered by the hemispheres. The organs of the senses are generally well developed, and the eyes and external ears, especially, are often of large size. In the Mole Rats and some other burrowing forms, however, the external ears are entirely wanting, and the eyes are very much reduced in size, and in some instances even concealed beneath the skin. The intestinal canal is long, and in all but one family furnished with a distinct cæcum. [Illustration: BRAIN OF BEAVER (_from above_).] [Illustration: BRAIN OF BEAVER. (_Profile._)]