A Laboratory Manual of the Anatomy of the Rat/Introduction
INTRODUCTION
The rat is one of the few wild mammals that thrives in civilized communities. It reproduces rapidly, consumes or otherwise destroys millions of dollars worth of property, and is the known, or probable, carrier of several parasites harmful to man. Some of these parasites are: Leptospira icterohaemorrhagiae, the spirochaete organism of infectious jaundice; germs which cause diseases in man following rat bites, for instance rat-bite fever; the tapeworm Hymenolepis diminuta, and possibly its relative H. nana; the round worm, Trichinella spiralis, which causes trichiniasis; and Bacillus pestis, the germ of plague. It is claimed that the plague, or "black death," killed 25,000,000 people in Europe during the fourteenth century, and that plague has caused the death of nearly 9,000,000 persons in India alone since 1896. The rapidity and ease with which rats can be bred has led to their extensive use in experiments by physiologists, geneticists, and others. It is evident, then, that an elementary knowledge of the habits, functions, and structures of the rat is of interest to the medical student, anatomist, agriculturist, and experimental zoologist.
The rat belongs to the order Rodentia, which includes over one-third of all existing species of mammals. Rodents possess long, chisel-like, curved incisor teeth (usually but two in the upper jaw, and never more than two in the lower), which grow continuously. The enamel is restricted to the front of the incisor, so that in gnawing the softer dentine is worn away at the tip faster than the enamel, thus keeping the tooth sharp. There are no canine teeth. Rodents are mostly herbivorous in diet, are usually small, and have clawed digits. The family Muridae, to which rats and mice belong, is said to include nearly half the genera of the Rodentia.
Four common species of this family are found in the United States. The common house mouse (Mus musculus) is less than two hundred millimeters in length when adult. The generic designation Epimys has been established for the rats, but the older term Mus is doubtless more familiar. Mus rattus rattus (the black, or house rat) is the rat of the Middle Ages in Europe. It probably reached that continent some six hundred years before the Norway rat arrived. The black rat preceded the Norway rat in North America, and occurs still in scattered localities in the northern United States. Its diagnostic characteristics are: the ears when laid forward extend at least to the middle of the eye; the tail is longer than the head and body; and is dusky all around; hind foot 33–37 millimeters; the color is blue-black above, and slaty below. A color variety, Mus rattus alexandrinus (roof, snake, or Alexandrian rat) is "grayish-brown above, white or yellowish white below" (Lantz). It is said to be common in the southern states near the coast. Mus norvegicus (the brown, gray, barn, wharf, sewer, or Norway rat) has, as a result of its adaptability, fecundity, strength, and ferocity, largely replaced the other species of rats in temperate climates. When well fed it is said that a female Norway rat can give birth to from sixty to a hundred young a year. It is more of a burrower than the black or roof rat, and is the common form found about bams, houses, warehouses, etc., in the United States. This species' characteristics are: "Ears moderate, when laid forward barely or not reaching eye; tail shorter than (rarely equal to) the length of head and body, darker above than below; color of body normally grey-brown above, white below; hind foot 38–46 millimeters" (Lantz). The common albino rat (Miis norvegicus albinus) is a variety of the Norway rat, and is now known only as a domesticated animal. Mus rattus also has been known to produce albinos. The large number of wild Norway rats that can usually be caught in human communities, together with the ease with which albinos can now be secured from dealers, makes the Norway rat a convenient form for laboratory study by large classes. Either the wild Norway rat or its albino variety can be used for dissection in connection with the directions and descriptions contained in this manual.