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98 HUMAN BIOLOGY 



such instances the highly developed extremity with its 

 extra skin and muscle is represented by a slight local enlarge- 

 ment in the cord to meet the increase of function. Otherwise 

 this structure is extremely uniform throughout its length. 



Far different is it with the brain. This central organ Hes 

 within the head and the head, as is well known, carries the 

 chief sense organs of the body. In a typical vertebrate, such 

 as the frog, there are three pairs of these organs, the nasal 

 cavities, the eyes, and the ears. Of these the foremost are 

 the nasal cavities, the nerves from which enter that part 

 of the brain that is designated the hemispheres. The nasal 

 cavities being chiefly concerned with smell, this region of the 

 frog's nervous system may therefore be designated as the 

 olfactory brain. The nerves from the frog's eyes enter 

 the second important part of this central organ, the optic 

 lobes, and hence this region may be called the visual brain. 

 Finally the nerves from the ears terminate in the anterior 

 part of the medulla oblongata in close proximity to the 

 cerebellum. This region might therefore be supposed to be 

 the auditory brain, but it is well known that the ears of 

 vertebrates are organs of a complex nature and that they 

 have quite as much to do with enabling the animal to 

 maintain an upright position and with other matters of 

 equilibrium and of posture as they do with hearing. In 

 fact in such creatures as the frog where the sense of hearing 

 is in a relatively undeveloped state, the ears are in all 

 probability more concerned with positional relations than 

 with hearing. Hence this portion of the central nervous 

 organs may be designated as the positional brain without 

 however denying to it a number of other functions, one of 

 which, for instance, is hearing. In this way three important 

 functional regions may be distinguished in the brain of the 

 frog, the olfactory, the visual, and the positional, reflecting 

 the three important sense organs, the nasal cavities, the 

 eyes, and the ears. 



On first inspection scarcely any resemblance can be seen 

 between the brain of a frog and that of a man. Instead of a 

 stem moulded into several lobes as the brain of the frog is, 

 the human brain seems to be a more or less oval mass 

 covered externally by a most intricate system of convolutions 



