308 



THE RISE OF ANIMAL LIFE 



any animal begins with the end organs 

 which receive stimulations from the out- 

 side world. These sense organs are well de- 

 veloped in the frog and are very similar 

 to those in man. To begin with, the nose 

 bears a pair of nostrils with valves on them 

 so that air may be taken in intermittently. 

 The nostrils open into small but well-devel- 

 oped nasal passages which are lined with 

 sensory cells that join the olfactory nerve 

 (Fig. 13-31). Tests indicate that the frog 

 does rather well in identifying its odorifer- 

 ous world and makes use of this sense in 

 orienting itself in its environment. 



To the frog, as well as other animals, the 

 eye is one of the most important organs of 

 sense. It differs from the human eye in minor 

 details only. For example, the lids will not 

 cover the eye completely; to close it all the 

 way, the frog must pull the eye down into 

 its socket. The typical six muscles for mov- 

 ing the eye in all directions are present, 

 just as is the case in all higher vertebrates, 

 and these will be studied later. The lens is 

 fixed in place so that the focus cannot 

 be changed as in man by altering its own 

 shape, or as in the fish by moving back 

 and forth. Therefore, the frog sees clearly 

 only at one distance, and it is near-sighted 

 in air and far-sighted under water. The 

 rods and cones of the retina, which are the 

 parts of the eye sensitive to light, are scat- 

 tered, rather than concentrated in one spot 

 as in man. Consequently, the frog probably 

 does not see as distinctly as higher forms 

 do. Due to the position of its eyes the frog 

 does not possess stereoscopic vision and 

 therefore cannot see depth. Although the 

 frog eye is considerably inferior to that of 

 mammals, it appears adequate to the needs 

 of the animal. 



The conspicuous eardrum of the frog is 

 exposed to the outside world, whereas 

 in higher forms it is buried deep inside the 

 head. Lying beneath the drum is a cavity 

 in which a single bone, the columella ( one 

 end of which is homologous with the stapes 

 of higher animals), extends from the thin 



eardrum to a tiny bit of sensory tissue 

 which is stimulated by the vibrations as 

 they are passed to it, through first the drum 

 and then the bone. Because of the rather 

 primitive nature of the auditory organ, the 

 frog probably hears most notes at the same 

 pitch, that is, while it might hear a thud or 

 a chirp, they would both sound the same. 

 The organs of equilibrium (semicircular 

 canals) are similar to those of both lower 

 (shark) and higher (man) forms. 



The frog possesses a lateral line system 

 only during the tadpole stage. All fish have 

 such a row of sacs extending along each 

 side of the body which are sensitive to vi- 

 brations and movement of the water. It is 

 interesting to note that in the evolution onto 

 land these structures were lost, and cer- 

 tainly they were not sufficiently sensitive to 

 detect similar movements in air. No higher 

 animals possess any organs that resemble 

 the lateral line system of fish and the tad- 

 pole. 



With the exception of a few major modi- 

 fications there is remarkable similarity be- 

 tween the brain of the frog and man. The 

 brain is proportionately much larger in 

 man and the spinal cord sends out three 

 times as many nerves. The foreshortened 

 body of the frog accounts for the fact that 

 there are so few spinal nerves. When tlie 

 brain and cord are dissected out and viewed 

 as a unit, the brain seems to be no more 

 than a slightly expanded anterior end of the 

 cord (Fig. 13-32). Starting at the base of 

 the brain and progressing forward, the five 

 parts of the brain can be seen. 



The first enlarged portion is the medulla 

 oblongata ( myelencephalon — 5 ) which 

 gives rise to most of the cranial nerves ( Fig. 

 13-31). These have to do with most of the 

 automatic functions of the body, just as 

 they do in man. A slight projection which 

 runs transversely across the medulla is the 

 cerebellum ( metencephalon — 4) which is 

 much smaller in the frog than in most other 

 vertebrates. This may be owing to its func- 

 tion in muscular coordination, which is 



