SENSE ORGANS. 



69 



Fig. 70. Gran- 

 dry's corpuscle, 

 after Bohm and 

 Davidoff. k, axis 

 cylinder of nerve. 



Fig. 71. Pa- 

 cinian corpuscle. 

 //, axis cylinder of 

 nerve. 



birds, two or more biscuit-shaped 

 cells are included in a connec- 

 tive tissue, while the connecting 

 nen'e becomes flattened out into 

 disks between each two cells. A 

 more complicated type is found 

 in the corpuscles of Vater or 

 Pacini, — elliptical structures 

 composed of layers of cells like 

 the layers of an onion, into the 

 centre of which projects the 

 axis cylinder of a sensory nerve. 

 Under the heading of tactile cor- 

 puscles (Wagner's or Meissner's corpuscles) are 



included club-shaped aggregations of cells, around which are 



coiled the terminal fibrillae of a nerve. These last are scattered 



all over the body in the amphibia, but are more restricted in 



their distribution in the higher 



groups. 



Among tactile organs must also 



be enumerated the long facial hairs 



(vibrissse) of mammals, the base of 



each being surrounded by a network 



of nerves. Besides these special 



tactile organs there are numerous 



free nerve terminations in the epi- 

 dermis of all vertebrates from cyclo- 



stomes to mammals to which sensory 



functions must be ascribed. 



The ears in all vertebrates are 



paired structures on either side of 



the head between the seventh and 



ninth nerves. In the most highly 



developed ears three portions are to 



be distinguished, — inner, middle, 



and outer, — the first of which only 



is sensory and essential, and is the 



only part occurring in the fishes ; 



Fig. 72. Meissner's corpuscle 

 from human finger, after Law- 

 dowski from Wiedersheim. a, 

 fibrous tissue envelope; b, cor- 

 puscle with its cells; «, entering 

 medullated nerves ; n', ramifica- 

 tions of nerves ; n" , club-shaped 

 nerve terminations. 



