Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/15

 

 

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 