EXTERNAL ASPECT OF THE RAT
Kill two adult rats with ether or chloroform. One of these is to be used for the study of the muscles, the other for the dissection of the remaining soft parts. Examine the external characteristics of the dead animal and determine the species to which it belongs by referring to the descriptions of species given above. Observe the general form of the trunk, and the characteristic shape of the head. Note the exposed, sharp, chisel-like, canine teeth which the rat uses in gnawing hard objects. The long hairs, or vibrissae, which project laterally from the animal's snout doubtless serve as feelers. Rats in the daytime appear to have poor vision while in the open, for their movements are then uncertain or slow. But when in contact with a wall, for instance, they run rapidly, indicating that they are then assisted by tactile stimuli through the vibrissae. The limbs are comparatively short, each being terminated by five digits (fingers on the fore limbs, toes on the hind ones). The thumb (pollex), which is so inconspicuous that it may be overlooked at first, bears a flattened, nail-like claw, but the claw at the end of each of the remaining four fingers is sharpened. The hind foot has five clawed digits. Numbering these digits from the median side, the first (hallux) and the fifth are shorter than the other three. Observe and determine the numbers of the footpads, or plantar tubercles, on the ventral surfaces of the fore and hind feet. In feeding on a morsel of food, rats are said to assume a sitting posture, and to hold the food between the fore paws while gnawing at it, like a
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