216 MANUAL OF ANIMAL BIOLOGY 



lens of the eye is an accessory structure which focusses the light 

 rays on the sensitive cells of the retina. 



A. Contact Stimulus or Sense of Touch. The sense of touch is 

 located in the skin. Scattered through the skin there are special- 

 ized sensory end organs, known as tactile corpuscles, which lie 

 just under the epidermis. These tactile corpuscles consist of a 

 small group of flattened cells, in close connection with which are 

 tiny nerve endings which respond to contact stimuli ; other sensory 

 areas in the skin are sensitive to heat, cold, pain, etc. 



B. The Chemical Sense. The skin of the Frog as a whole is 

 sensitive to chemical stimuli. If, for example, a drop of weak acid 

 is placed on the skin of any region it will stimulate sensory cells 

 there present, and the animal will endeavor to wipe off the acid. 

 However, the chemical sense is best developed in the mouth and 

 in the olfactory organs. The sense of taste is primarily a chemical 

 one, and there are many groups of specialized sense cells scattered 

 over the surface of the tongue and also, more or less generally, 

 over the entire lining of the mouth. The taste organs consist of 

 epithelial cells together with elongated sensory cells. Each of the 

 latter is connected by fine processes with a nerve. The various 

 chemical substances in the food stimulate, or excite, these sensory 

 cells, and the impulses thus set up are received by the central 

 nervous system over the connecting nerve fibers. (W. f. 146, B.) 



The sense of smell located in the nasal cavity is also a chemical 

 sense. The air which is drawn into the nasal cavity through the 

 external nostrils comes in contact with sensory cells which line this 

 region. These cells are extremely sensitive to very minute quanti- 

 ties of chemical substances present in the air. A microscopic study 

 of the tissue lining this cavity shows that this inner membrane is 

 made up of a basal connective tissue portion and an outer layer 

 of olfactory epithelium. The latter contains three types of 

 cells, the olfactory, basal, and interstitial. The olfactory cells 

 are the true sensory cells. They are greatly elongated, and at the 

 outer free end of each, which forms a portion of the lining of the 

 nasal cavity, there are a number of fine protoplasmic processes. 

 These structures, in some way, are influenced by the chemical 

 substances present in the air, and the resulting impulses are passed 

 to the connecting nerve. (W. f. 146, A.) 



C. The Sense of Hearing and of Position. The ear is a sense or- 

 gan which is adapted for receiving two types of stimuli ; those of 



