168 



COMPARATIVE ANATOMY. 



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

 ing definite masses, or "tactile corpuscles/' Each of these 

 surrounded by a nucleated connective-tissue investment, from 



is 



which septa extend into the interior, partially separating the 

 individual tactile cells from one another. 



FIG. 138. A TACTILE CORPUSCLE (END-BULB) FROM THE CONJUNCTIVA OF A 



MAMMAL. 



N, nerve (the neurilemma of which at t becomes continuous with the investment of 

 the tactile corpuscle ; K, K, nuclei in the investment ; N, the coiled termination 

 of the nerve (axis-fibre) passing to the tactile cells (T, T}. 



JK 



FIG. 139. A PACINIAN CORPUSCLE FROM THE BKAK OF THE DUCK. 

 (After J. Carriere. ) 



ZZ, cells of the nerve-sheath ; L, longitudinal, and Q, circular layers of the in- 

 vesting lamellae ; JK, central knob, with the two pillars of cells ; A, axis-fibre, 

 with protoplasmic investment, entering the corpuscle at A 1 ; MS, medullary 

 sheath ; Nl, neurilemma, which becomes continuous with the investment of the 

 corpuscle at f, t- 



In Mammals the tactile cells are either isolated, as, for instance, 

 on the hairless portions of the body, or they give rise to oval corpus- 

 cles, each consisting of a many-layered and nucleated investment, 

 into which a nerve passes, becomes twisted up, and ends in one or 

 more terminal ganglion cells (Fig. 138). 



