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 deter- 

 mine 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 ani- 

 mal's snout doubtless serve as feelers. Rats in the day- 

 time appear to have poor vision while in the open, for their 

 movements are then uncertain or slow. But when in con- 

 tact 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 incon- 

 spicuous that it may be overlooked at first, bears a flat- 

 tened, 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 sur- 

 faces 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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