68 



THE NERVOUS SYSTEM 



after further divisions, they end among the epithelial cells (Fig. 41). This type 

 of nerve ending is found in the skin, mucous membranes, and cornea. Similar 

 endings are also found in the serous membranes and intermuscular connective 

 tissue. 



We do not know what form the endings of the afferent unmyelinated fibers 

 may take, but it is not unlikely that they also ramify in the epidermis like the 

 terminal branches of the myelinated fibers just described. It seems certain 

 that at least a part of the free nerve endings in the epidermis are pain receptors. 

 In the central part of the cornea, the tympanic membrane, and the dentine 

 and pulp of the teeth, such free nerve endings alone are present, and pain is the 

 only sensation that can be appreciated. 



Some of the nerve-fibers which enter the epidermis end in disk-like expansions 

 in contact with specialized epithelial cells (Fig. 42). These have been known 



Fig. 42. Merkel's corpuscles or tactile disks from the skin of the pig's snout. The nerve- 

 fiber, n, branches and each division ends in an expanded disk, m, which is attached to a modified 

 cell of the epidermis, a; c, an unmodified epithelial cell. (Ranvier, Herrick.) 



as Merkel's touch-cells on the supposition that the endings in question are tactile 

 receptors. 



Encapsulated Nerve Endings. Among the encapsulated nerve endings are 

 the corpuscles of Meissner. These have quite generally been regarded as tactile 

 end organs and are located in the corium or subepidermal connective tissue of 

 the hands and feet, forearm, lips, and certain other regions. They are of large 

 size, oval, possess a thin connective-tissue capsule, and within each terminate 

 one or more medullated fibers (Fig. 43). Within the capsule the fibers lose their 

 myelin sheaths, make a variable number of spiral turns, and finally break up 

 into many varicose branches, which form a complex network. To another 

 type of encapsulated end organ belong those known as the end bulbs of Krause. 

 One of these is illustrated in Fig. 44. They are found in the conjunctiva, edge* 

 of the cornea, lips, and some other localities. 



