112 



HISTOLOGY 



of the electric surface that is in contact with the ramifications of the nerve- 



ending. They are sharp, wedge-shaped, and very small (Fig. 106). 

 A brief examination of the electroplax of the "torpedo fish," Tetro- 



narce, should be made at this point to compare its highly specialized 



electric unit with the simpler and more rudimentary structure of the 



electroplax found in Raja. 



In Tetronarce the electric organ is composed of two masses of vertical 



columns, each column with six sides to fit closely with its neighbors. 

 Each column is a pile of very thin electro- 

 plaxes that lie with their two surfaces at right 

 angles to the column and occupy its entire 

 section, excepting that their corners are some- 

 what rounded. A portion of electric con- 

 nective tissue lies on each side of the electro- 

 plax and they are farther separated by a very 

 thin layer of white fibrous tissue. The columns 

 are bounded on their sides by comparatively 

 heavy walls of the white fibrous connective 

 tissue. 



Examination of a vertical section of a part 

 of the column shows the electroplaxes in cross 

 section. They are so thin when seen thus 

 that it is with great difficulty that they are 

 made to appear as more than a line with 

 several nuclei on it. The nuclei are clearly 



FIG. 106. Part of the nerve- made out to be of two kinds: a large round 



" IT-- one on the u pp. er side and reall y b^ng in the 



Nerve-ending seen be- electroplax, which is thinner than the diameter 



of the nucleus ; also a smaller and somewhat 



elongated nucleus of denser appearance, which 

 is clearly a connective- tissue element. This is usually lying on or near 

 the electric surface of the electroplax. A fairly well fixed specimen 

 of this tissue shows two visible layers of the structure. 



Surface studies of this tissue with the Golgi method show, as in Raja, 

 the electric end-organ of the nerve supply as well as the numerous small 

 rods that point down into the electric layer and are found on the elec- 

 trolemma only at such of its points as are in contact with the nerve end- 

 organ. This latter structure is as complicated as in the skate. Each of 

 the rods, instead of being a simple pointed wedge as in the skate, is of 

 stouter formation, and bears on its end a peculiar round knob. Some- 

 times two of these rods are united. 



The Golgi method also shows a reticulum in the cytoplasm that is 

 similar to that seen in the skate. In general, the electroplax of Tet- 



fiber. 



