i 3 6 



EVOLUTION AND DISEASE. 



and are recognized as supernumerary ribs. In order 

 to find mammals with more than twelve pairs of ribs we 

 need not seek far from man, as the chimpanzee and 

 the gorilla possess thirteen pairs ; in the American 

 monkeys there are twelve to fifteen pairs, and in 

 lemurs twelve to sixteen pairs. 



The following specimen illustrates the reversion of 



..a 



FIG. 75. The mitral valve of a man's heart containing a 

 patch of muscle tissue, a. (After Dr. Ogle.) 



tissue. Dr. Ogle * has described a man's heart, which 

 contained a patch of muscle tissue in the anterior 

 segment of the mitral valve (fig, 75). The patch of 

 muscle was as large as a fourpenny-piece, and resembled 

 under the microscope the natural tissue of the heart. 

 1 " Trans. Path. Soc.," vol. ix. 



