STATIC TISSUES 



213 



sen.c. 



thus cutting the lumen down to one half its diameter at that point, which 

 is the widest point in the ampulla, and consequently in the whole length 

 of the tube. 



The sensory cells are placed on the edge of this ridge, a transverse 

 section of which, from Raja lavis, is represented in Figure 192, also 

 an enlarged figure of the same , 



structure in Amieurus by Figure 

 193. These cells are rather large, 

 and do not reach down to the 

 basement membrane, being sup- 

 ported by contact with support- 

 ing or sustentacular cells whose 

 proximal ends rest by broadened 

 bases on a well-developed mem- 

 brane. 



The sensory cells have, as cell- 

 organs of perception, peculiar fila- 

 ments projecting from their distal 

 surfaces. These filaments (Fig. 

 193) are grouped into a single 

 projection which is very delicate 

 and is partly embedded in a ge- 

 latinous coating which covers the 

 ridge. The nuclei are placed 

 about halfway in the height of 

 the cell and are oval, with a dis- 

 tinctive chromatin pattern. They 

 are larger and clearer than the 

 nuclei of the sustentacular cells. 

 These latter cells have the nucleus 

 a little lower than the middle and entirely below the row of static nuclei. 



Medullated nerve fibers enter the connective tissue of the ridge freely 

 and extend up in all directions to the epithelium. Passing through the 

 basement membrane, they usually lose their medullary sheath and divide 

 into smaller fibril-bundles or into single fibrils to innervate the sensory 

 cells, receiving from them an impulse when they are stimulated. 



This stimulation of the perceptory cells is probably performed by 

 currents of the fluid which fills all parts of the statocyst, including the 

 canals and their ampullae. When the body turns in any direction, the 

 fluids in such of the three canals as lie in or at a sharp angle to the plane 

 of motion pass backward or forward through the canal by their inertia, 

 and, flowing over the ridge so well placed in their path, stimulate the 

 sensitive cells on its edge by causing their delicate processes to bend and 



sup. 



FIG. 193. Sensory epithelium from median 

 septum of ampulla of catfish, Amieuris catus. 

 sen.c., sensory cells; sen.r., sensory rods; 

 supx., supporting cells, one of which shows 

 mitcsis; b.m., basement membrane through 

 which two nerve fibers pass. One fiber is 

 naked while the other carries its medullary 

 sheath into the epithelium. They both ramify 

 as nerve-endings on the bases of the sensory 

 cells. 



