~ 
136 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 
I'1G. 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. 
* “Trans. Path. Soc.,” vol. ix. 
