340 TERMINATION OF SENSOEY NERVES. 



Tactile end-organs of birds. On account of the light which they throw upon the struc- 

 ture of the end-bulbs of mammals, a short description of the tactile end-organs of birds may 

 be given here. 



It was noticed by Grandry that in the Boft skin covering the bill of certain birds, such as 

 the duck and goose, a peculiar form of end-organ exists consisting of two or more flattened 

 cells, inclosed in a common capsule of connective tissue, and receiving between them the 

 termination of the axis-cylinder (fig. 399). The structures in question have since been investi- 

 gated by several observers with the following results : 



The cells which form the corpuscle of Grandry are for the most part of no great thickness, 

 and the surfaces which are opposed to one another are flattened. Their protoplasm is stated 

 by Merkel to resemble that of nerve-cells, having a striated aspect, the striae being partly con- 

 centric with the periphery of the cell, partly passing radially through it. The nucleus has 

 also been compared to that of a ganglion-cell. There may be two only of these cells (which 

 are termed by Merkel " tactile cells," by others " protective or inclosing cells ") in a corpuscle 

 of Grandry, or there may be three or four or even more, piled the one on the other. When 

 numerous they may lose their regularity of arrangement. Occupying the interval between every 

 two cells is a flattened disk termed the " tactile disk," and according to the testimony of all 

 observers the axis-cylinder of the entering nerve-fibre ends in these tactile disks. According to 

 Merkel. the disk is itself, on the other hand, directly in continuity with one or both of the 

 cells between which it lies, but this continuity is not generally admitted. The tactile cells and 

 disks are all inclosed in a common capsule or sheath of connective tissue continuous with the 

 perineurium of the nerve and receiving also a lining from the nucleated sheath of Schwann. 

 From the capsule incomplete septa pass inwards between the flattened cells, as far as the edges 

 of the tactile disks, so that the septa look as if they were perforated to receive the disks. 

 Usually a single nerve-fibre passes to each corpuscle, and this may either lose its medullary 

 sheath on entering the corpuscle or may retain it for some part of its course, although it event- 

 ually, in any case, becomes lost. The axis-cylinder, passing between the capsule and the tactile 

 cells, divides into as many branches as there are tactile disks, in which, as already mentioned, 

 it finally terminates. 



It will appear from the above account that the chief point in which there is a difference of 

 opinion with regard to the structure of the simply constructed tactile corpuscles, is as to the 

 expansion of the axis-cylinder known as the tactile disk, whether this is prolonged or not into 

 the cells which cover it. The former view is taken, as we have seen, by Merkel, but the 

 opinions of most other observers are adverse to it. There is, however, this to be said in favour 

 of Merkel's view, namely, that when degeneration takes place as a result of the section of the 

 nerve, the degenerative process extends not only to the tactile disk but also to the cells which 

 cover it. 



These corpuscles appear to be developed as a result of the multiplication and down-growth 

 of some of the epithelium-cells which lie at the apex of a papilla (fig. 399, 0). The growth be- 



Fig. 399. TACTILE CORPUSCLES 



FROM THE DUCK'S TONGUE. 



(Izquierdo.) 



A, composed of three cells, 

 with two interposed disks, into 

 which the axis-cylinder of the 

 nerve, n, is observed to pass ; in 

 B there is but one tactile disk 

 enclosed between two tactile 

 cells ; C illustrates the develop- 

 ment of a tactile corpuscle like 

 the one shown in B ; e, deeper 

 cells of the epithelium covering 



the papillatecl surface of the tongue ; p, apex of a papilla, in which there is seen to be a downgrowth of 



epithelium-cells, the lowermost of which are developed into tactile cells. 



comes entirely cut off from the rest of the epithelium and surrounded by connective tissue, 

 whilst the cells within it are converted into the flattened " tactile cells," and a prolongation 

 of a nerve-fibre grows up into it. 



Corpuscles of Herbst (fig. 400). These, which form the principal mode of nerve-termi- 

 nation in the bird's skin, are in structure similar to large cylindrical end-bulbs. They have a 

 core consisting of nucleated cells, which are disposed transversely, and through the middle of 

 which the axis-cylinder passes. Their capsule is composed of an outer longitudinal fibrous 

 tissue, and an inner layer of strongly-marked fibres of a brownish colour, running trans- 

 versely or circularly. They most nearly resemble the innermost part of the Pacinian 

 corpuscles, i.e., the core and the innermost lamella. 



