74 ZOOLOGY. 



In vertebrates the central nervous system consists primarily 

 of a tube with nervous walls the spinal cord which may be 

 specially enlarged and thickened at the anterior end to produce 

 the brain (Fig. 170). From the various parts of this cord the 

 nerves take their origin. 



105. The Peripheral Nervous System: Sense Organs. 

 We know by experimentation that in the lowest animals even, 

 or for that matter, in protoplasm, certain external conditions 

 produce definite responses or changes. We also know that 

 these external happenings and their responses, in our own 

 case, are accompanied by certain sensations, as touch or taste. 

 By inference, both from the nature of the response and from 

 the structure of organs, we reach the conclusion that the lower 

 vertebrates and higher invertebrates experience sensations in 

 some degree similar to ouY own. The classes of possible 

 stimuli have already been mentioned ( 20). Those producing 

 in us definite sensations are : simple contact stimuli, producing 

 the sensation of touch and pressure ; vibratory contacts, giving 

 rise to hearing and temperature sensations; chemical actions, 

 making possible sensations of taste and smell ; ethereal vibra- 

 tions, producing the sensation of light. In the lowest forms 

 of animals there are no specialized organs for the reception of 

 particular stimuli, and in such cases it is reasonable to infer 

 that the distinctness of the sensation cannot be very great. In 

 almost all animals, however, certain areas are specially suited 

 to be stimulated by special stimuli. 



106. Touch. There are two principal ways by which contact stimuli 

 are received among animals. Fibres of the central nervous system may 

 pass to the skin and end. among its outer layers as free nerve terminations, 

 or these fibres may become intimately united with one or more of the 

 cells of the epithelium. The most common of the tactile organs in verte- 

 brates are of the first class. Where the stimulus reaches the nerve through 

 a nervous epithelium, the epithelial cells often have special developments 

 such as hairs, bristles, and the like, whereby the possibility of contact with 

 external objects is increased. The appreciation of changes in temperature 

 is also associated with the general skin surface. 



107. Chemical Sense (including taste and smell). It is impossible 

 for us to distinguish between taste and smell in the lower animals. Indeed 



