A Laboratory Manual of the Anatomy of the Rat/External Aspect of the Rat
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 squirrel. Note the scales on the tail. How are they arranged? Are scales characteristic of mammals? In what groups of vertebrates are scales usually found?
The anus, or opening through which the residual materials (feces) from the digestive tube pass, is immediately ventral to the base of the tail. The conspicuous swelling ventral to the anus of the male rat is the scrotum, which contains the two testes. Sometimes the testes lie up in the body cavity instead of in the scrotum, in which case the size of the latter is very much reduced. The external opening of the male's urogenital system is located mid-ventrally near the anterior end of the scrotum. Carefully insert one blade of a pair of scissors into this orifice and slit the skin, lengthwise of the animal, for three or four millimeters. Note that the foreskin (prepuce), which is hairy externally, surrounds the distal end (glans) of the penis. Examine now the corresponding region of the female. There are three openings in the female: the anus, ventral to the base of the tail; ventral to the anus is the vaginal orifice, or exit from the female reproductive organs; below this orifice the opening from the excretory system at the distal end of a prominent elevation. How do the relations between these openings in the female rat compare with the corresponding openings in other female mammals?
Carefully observe the hair color of your animal. If you are working with a wild rat scrutinize the distribution of the pigment within individual hairs. Is each hair of the same color throughout? Compare the hair color of the rat with that of other wild animals, if they are available either alive or mounted. Compare the color of the albino's eyes with the eye color of the wild rat.
If time permits, sketch the lateral view of the dead animal.