214 The Vertebrate Organ Systems 



Some special receptors have already been mentioned, including those 

 in the walls of the auricles of the heart and the carotid sinus. In addi- 

 tion to these, there are receptors in the walls of the stomach which give 

 rise to hunger sensations, some in the throat which give the sensation 

 of thirst, and many others. In addition, there are special end organs 

 in the skeletal muscles which are important in the maintenance of body 

 position. This is commonly called the kinesthetic sense. 



Chemical Receptors. — The receptors for smell and taste stimuli 

 are known as the chemical receptors because they respond to the stimu- 

 lation of chemicals in solution. Both senses are extremely sensitive; 

 in some animals, the sense of smell is especially well developed. In 

 the dog a greater area is devoted to the perception of odor than in man. 



The taste buds (Fig. 66,C) are located over the surface of the 

 tongue, the roof of the mouth, the pharynx, and the palate. Each taste 

 bud is a flask-shaped organ located in the epithelial layer, and connected 

 to the surface by means of a short passage. It opens to the outside 

 by means of the small taste pore. Within the taste bud are the taste 

 receptors proper which have fine hairlike processes projecting into the 

 lumen of the taste bud. Unmyelinated fibers lead from these receptors 

 to the brain where the stimulus is interpreted by the cerebral cortex. 



There are four primary taste sensations: sweet, sour, bitter, and 

 salt. There is a specificity of reaction among the taste receptors with 

 each one reacting only to one of these taste stimuli. Those which are 

 most sensitive to sweet and salt are particularly abundant at the tip of 

 the tongue; those which respond to sour are along the margins, while 

 those stimulated by bitter materials are toward the back. In children the 

 taste buds are most abundant at the tip of the tongue; perhaps that 

 is why they enjoy licking candy. 



The receptors for odors are among the simplest of all the sense re- 

 ceptors (Fig. 66,D). They are simply the ciliated end organs of afferent 

 nerves. These receptors are located in a relatively limited area of 

 the nasal mucosa, a convenient spot for the constant sampling of in- 

 coming air currents. In man they occupy an area of about 5 sq. cm. 

 These receptors are constantly bathed with fluid, thus the gases enter- 

 ing are dissolved before affecting the sensory endings. 



Inasmuch as the nasal and mouth cavities are so closely associated, 

 the senses of taste and smell are often confused. The enjoyment which 

 food flavors arouse is usually a combination of stimulation of both of 

 these senses plus that of the consistency of the food. Some flavors are 



