CLUB-SHAPED CORPUSCLES 195 



c. Tactile-cells and corpuscles. 

 (Terminal ganglion cells.) 



In these structures there is no longer any direct connection 

 with the surface of the epidermis, and supporting cells are want- 

 ing. 



" Tactile spots," consisting of groups of touch cells, are met with 

 for the first time in tailless Amphibians, in which they are situated 

 mainly on small elevations, and are distributed over the skin of the 

 whole body (Fig. 154 A). In Reptiles they are found chiefly in 

 the region of the head, on the lips and sides of the face, and on 

 the snout, but in some cases (as in Blindworms and Geckos), they 

 extend over the whole body close to the scales. In Snakes and 

 Birds the tactile cells are confined to the mouth-cavity (tongue) 

 and to the beak (cere), and lie much more closely together, forming- 

 definite masses, or tactile corpuscles (Fig. 154D). Each of these is 

 surrounded by a nucleated connective-tissue investment, from which 

 septa extend into the interior, partially separating the individual 

 tactile cells from one another. In Mammals the tactile cells are 

 either isolated as, for instance, on the hairless portions of the body, 

 or they give rise to oval corpuscles, each consisting of a many- 

 layered and nucleated investment, into which a nerve passes, be- 

 comes twisted up, and comes into relation with one or more ter- 

 minal cells (Fig. 154 B, c). These are most numerous and highly 

 developed on the volar and plantar surfaces of the hand and foot 

 respectively, and on the conjunctiva and snout. 



d. Club-shaped corpuscles. 

 (Pacinian corpuscles.) 



From the Reptilia (Lizards, Snakes) onwards, club-shaped 

 corpuscles are present in addition to the above-described tactile- 

 organs. In these Reptiles they occur chiefly in the region of the 

 lips and teeth ; they have an elongated, oval form, and their structure 

 is simple. 



In the interior of each corpuscle is seen the continuation of 

 the axis-fibre of the nerve which becomes swollen distally, and 

 externally to this is a double column of cells which enclose the 

 club-shaped axis (Fig. 155). It is probable that a fine branch 

 is given off from the axis-fibre to each cell. The column of cells 

 is enclosed externally by an investment consisting of numerous 

 nucleated lamellae in which longitudinal and circular layers can 

 be distinguished. 



Organs of this kind are universally present, deeply situated, 



o 2 



