THE NERVOUS SYSTEM 



187 



-cu. 



(5) proprioceptive, (6) chemical (taste and smell), (7) sound, (8) 

 light, and (9) gravitation (equilibrium). 



Tactile Organs. — The sensitivity to touch that characterizes 

 the integument of animals generally is brought about by the 

 presence in the integument of tactile 

 organs developed in connection with 

 nerve endings. The tactile organ itself 

 is usually composed of nonnervous 

 tissue. Among invertebrates the com- 

 monest type of tactile organ is the 

 tactile hair, examples of which are found 

 in arthropods (Fig. 119). Such a tactile 

 hair consists of a core formed of epider- 

 mal cells, covered by a thin cuticle, the 

 distal end of which tapers to a delicate 

 point. The base of the tactile hair is in 

 contact with a nerve ending so that any 

 movement of the hair serves as a stimu- 

 lus for the nerve ending. In mammals, 

 including man, the hair follicles are sup- 

 plied with sensory nerve endings that 

 are stimulated by movements of hairs. 

 Where hairs are lacking, tactile corpus- 

 cles of Meissner serve as end organs of 

 touch in the skin. These are elliptical 

 structures composed usually of several 

 nerve fibers enclosed by a capsule of 

 nonnervous cells and are located in the 

 subcutaneous tissue (Fig. 120). They 



r,v.end.__ 



Fig. 119. — Tactile end-or- 

 gan of a nerve fiber in the 

 tactile hair of a shrimp, Palac- 



monetes. cu., cuticle; hyp., 



are numerous in the finger tips, the hypodermis, which is invagi- 

 soles and palms, the nipples of the J*- •* ^'"l" 



mammary glands, the lips, and Other the hair; nv.end., nerve ending 

 u • r„ „ „„~„ , ^f +1-.,-. ol^i.-. in the lower part of the hair. 



hair-free areas of the skin. _ _ (From Dahlg v ren and Kepner , 



Pain Receptors. — Sensation of pain Principles of Animal Histoi 

 can be aroused everywhere in the 

 human skin by pricking with a sharp 

 needle. The receptors for cutaneous pain are thought to be 

 naked axis cylinders of sensory nerves distributed in the lower 

 layers of the epidermis. The nerves involved are sensory com- 

 ponents of cranial and spinal nerves. Some internal structures 



ogy, copyright, The Macmillan 

 Company. By permission.) 



