366 



BIOLOGY 



man has five fingers represented, the others have lost some of 

 these fingers, and one of them, the horse, A, has left but a single 



finger with the rudiments of 

 two others. That these other 

 fingers have been lost has been 

 proved by the study of the 

 paleontology of these animals; 

 for by tracing back their his- 

 tory through fossils it is found 

 that the ancestors of the horse 

 had at first five fingers, with a 

 type of hand similar to that of 

 man; later they had three and 

 finally only one finger. This 

 similarity in the structure be- 

 tween the frog and the higher 

 animals is shown in other parts 

 of the body besides the bone. 

 Figure 147 represents a section 

 through the body of a cat, giv- 

 ing a diagrammatic represen- 

 tation of the relation of the 

 organs in the upper part of 

 the body. This can be com- 

 pared directly with the anat- 

 omy of the same region of the 

 body of the frog, and while 

 there are many differences in 

 detail, the general structure is 

 evidently the same. The spi- 

 nal cord with the brain is found 

 on the dorsal surface of both 

 animals; the mouth, nostrils, 

 larynx, lungs, and oesophagus are, in essential features, iden- 

 tical. Thus it is evident from these comparisons that the 



di 



FlG. 147. A MEDIAN VERTICAL 

 SECTION OF A CAT, SHOWING 

 DIAGRAMMATICALLY THE RELA- 

 TIONS OF THE ORGANS 



cb, cerebellum ; 

 cr, cerebrum ; 

 hy, hyoid; 

 Ix, larynx; 

 oe, oesophagus; 



sp, spinal cord; 

 st, sternum; 

 th, thyroid gland : 

 tr, trachea. 



