THE ORGANS OF TACTILE SENSATION. 379 



breeding" season the organs of the thumb undergo a retrogressive 

 change, which results in a network of spindle-shaped and branched 

 cells with fibres (Wiedersheim). 



c. The lateral sense-organs attain their highest development in 

 the tadpole ; in the adult the organs have undergone a retrogressive 

 change, the result of which is that the organs are diminished in 

 size ; the whole organ sinks by the formation of a tube, which is 

 then closed by a mucous secretion, consequently the organ is 

 functionless : with this loss in function and change in position of 

 the organs a corresponding diminution of the ramus lateralis nervi 

 vagi occurs (Merkel). 



Each lateral org'an(Fig. 236 1) consists of a slightly elevated papilla, 

 with the centre depressed, and in each may be distinguished a central 

 and a peripheral zone {o,p). The central zone consists of a group of 

 pear-shaped cells, with the narrower ends directed towards the free 

 surface ; each is connected below with a nerve-fibril, while above it 

 bears a short, stiff cilium. The cilia are enclosed in a delicate 

 hyaline tube (B), which is open superficially, closed by the papilla 

 below ; consequently the cilia are in direct contact with the sur- 

 rounding medium. The peripheral zone is a layer of pale, flattened, 

 cylindrical cells (/;) ; they possess, at their upper borders, a perforated 

 memhrana limitans {Ml), through which the cilia pass. These 

 cells serve as a support to the central cells, and are themselves 

 surrounded by ordinary epithelial cells. 



The distribution and arrangement of the lateral organs is alike in fish and larval 

 amphibia. They are most numerous on the head, where they surround the eye, and 

 are continued forwards to the snout and on to the lower jaw. All these organs on 

 the head are in connection with the trigeminal nerve. A line of these organs passes 

 from the hinder part of the circumference of the eye along the gill-cover to the neck, 

 where the lines of opposite sides are usually, though not constantly, joined by a 

 transverse line of the same organs ; thence the lines are continued along the trunk to 

 the tip of the tail. In fish there exists, as a rule, only one pair of such longitudinal 

 lines {Linea laterali») ; in anura-larvae, proteus, and in all salamanders there are 

 three pairs : of these one pair lies near the vertebral column, the second at the 

 junction of the flank with the abdomen, and the third corresponds to the lateral line 

 in fish (Malbranc). 



At an early developmental period each nietamere possesses one pair of such lateral 

 organs ; in later life, however, this simple arrangement is lost, and each segment 

 usually possesses a group of organs. 



The organs of the trunk and tail are supplied by branches of the Ramus lateralis 

 nervi vagi. Both in fish and amphibia a reparative process, by means of fission, 

 occurs ; as a consequence the organs are found in various stages of development. 



d. [The touch-corpuscles of Golgi and Lowe (Fig. 236 II) are 

 found on the joints of the digits. They are surrounded by con- 



