SENSE ORGANS AND NERVOUS COORDINATION 575 



tations sometimes experience "phantom limbs," i.e., sensations that 

 appear to come from the missing part, for nerves in the stump that 

 were formerly connected with the missing part may be stimulated by 

 pressure or other factors. 



Vertebrates have many kinds of receptors, more than the usual 

 "five senses." There are chemoreceptors in the nose and mouth that 

 provide for smell and taste; various kinds of mechanoreceptors in 

 many parts of the body that detect touch, pressure, muscle stretch, 

 vibrations and balance; photoreceptors for light; thermoreceptors in 

 the skin and mouth for heat and cold; and free nerve endings in the 

 skin and internal organs, whose stimulation results in sensations of 

 pain. Most of these receptors are microscopic, consisting of only a iew 



Encapsulating 

 connective tissue 



Myelin sheath. 



Nerve 

 fibers 



xon 



B 



Conneccive 



tissue 



Shea-th 



Muscle 

 fibers 



TaLste bud 



^ — Pore 



Surface of 

 tongue 



■Ne-rve 

 fibers 



^ Surface fluid 



Supporting ce.ll 



Olfactory cell 



- Fibers to 

 olfactory bulb 



Figure 29.1. A group of mammalian receptors. A, Meissner's corpuscle found 

 beneath the epidermis, assumed to be sensitive to touch; B, Pacinian corpuscle found in 

 the dermis and many internal organs, sensitive to pressure; C, neuromuscle spindle, 

 sensitive to muscle tension (proprioception); D, taste buds between papillae on the 

 surface of the tongue; E, olfactory cells in the nasal mucosa. The olfactory cells are 

 known as neurosensory cells for they are both receptive and transmitting cells. (A, 

 Modified after Ranson; B and E, after Gardner; C, after Maximow and Bloom.) 



