156 BIOLOGY: GENERAL AND MEDICAL 



complexity until it culminates in the great brain and 

 spinal cord of man, enclosed in the skull and spinal 

 column for protection. 



As the greatest development of the nervous system takes 

 place primarily where the nerves of the special senses are 

 received, we must look upon their appearance and develop- 

 ment as contributory to the development of the central 

 nervous system, and before progressing further with its 

 functions pause to consider how and why. 



Metazoan organisms are rarely so simple in structure 

 as not to be provided with some kind of outer protection 

 cuticle, shell, skin, etc. the purpose of which is to pre- 

 vent the interruption of the normal activity of the internal 

 cells with beginning specialization of function, by contact 

 with external forces. If cells are to perform one function 

 to the exclusion of others, they must be saved from the 

 necessity of performing those others, and to this end the 

 cuticle, composed of elements freed from the customary 

 irritability of protoplasm, was developed. This, however, 

 was not without its disadvantages, for not only is protec- 

 tion thereby afforded against influences that are injurious, 

 but quite as much against those that are beneficial. Spe- 

 cial adaptations for appreciating the useful influences 

 therefore made their appearance in the organs of special 

 sense, each of which consists essentially of specialized 

 protoplasm which is highly sensitive to some particular 

 form of energy manifestation, but relatively insensitive to 

 other forms of stimulation. Each sense organ possesses, 

 in addition, certain accessory parts adapted to concen- 

 trate the stimuli upon the essential sensitive protoplasm, 

 to intensify the form of the stimulus, or to so transform the 

 energy of the stimulus as to enable it to act more efficiently 

 upon the essential end-organ. 



Herrick says: 



"We may conceive the body as immersed in a world full of energy 

 manifestations of diverse sorts, but more or less completely insulated 

 from the play of these cosmic forces by an impervious cuticle. The 

 body surface is, however, permeable in some places to these en- 

 vironing forces, and in a definite fashion, one part responding to a 



