THE SENSE ORGANS 



567 



with the dendrites of a sensory nerve may become encapsuled by con- 

 nective tissue to form a Meissner's corpuscle. 



Free nerve terminations occur in the skin of all classes of chordates, 

 usually in the form of multiple arborizations or dendrites. These may 

 lie in the epidermis or in the corium, in either case, are located where 

 they may respond to changes in pressure. Such free nerve terminations 

 in the skin are found in all classes of vertebrates, and are believed to be 



CLUB-SHAPED 



TACTILE CORP. 



(KRAUSE) 



ENCAPSULED 

 NERVC- KNOT 

 (GOUGI-MAZZDNl) 



ENCAPSULED 

 GROUP OF 

 TACTILE CELLS 

 (MEISSNER) 



DIAGRAMS B.-D CUTANEOUS SENSE ORGANS IN CRANIATES WITH SECONDARY SENSE CELLS. 



Fig. 467. — Varieties of cutaneous sense organs in chordates. A, B, C show sensory 

 terminations in the skin of acrania. B' to D^ show varieties of terminations in craniotes. 

 C to C* and D' to D* respectively represent stages in the hypothetical evolution of 

 encapsuled nerve terminations. (Redrawn after Plate.) 



the sensory mechanism of painful sensations arising in the skin. The 

 sense of touch is believed to depend chiefly upon the tactile cells or corpus- 

 cles in the corium, of which various forms occur. 



Meissner's corpuscles, present only in primates, are located in the 

 corium papillae of the palms and soles, and in the external genital organs. 

 Each corpuscle consists of a group of tactile cells surrounded by a rela- 

 tively thin envelope of connective tissue and connected with one or more 

 nerve fibers. The non-medullated nerve fibers twists spirally among the 

 tactile cells, each of which is in contact with a reticular nerve termination. 



In birds and reptiles nerve endings are connected with tactile cells of 

 Merkel but without a connective tissue capsule. These tactile cells 



