648 



THE EAR. 



of the sacculus. The nerves of the ampullre enter the flattened or least 

 prominent side of the ampullo3, where they each form a forked swelling 

 (4, 5, fig. 460), which corresponds with the transverse septum already 

 described, in the interior of the dilatation. No nerves have been 

 found extending to any other parts of the semicircular canals. 



Structure. — Three layers can be distinguished in the membranous 

 walls of the semicircular canals, an outer fibrous stratum, an inner 

 epithelial lining, and between the two a homogeneous tunica propria. 

 These layers are not of equal thickness throughout, for at those parts 

 where the walls are in contact with and supported by the bone (fig. 

 461,6), or rather its periosteal lining, namely, along the side which is 

 turned towards the convexity of the semicircle, they are thinner than 

 at the rest of the circumference, where they lie free and arc bathed by 

 the perilymph. The difference in thickness affects the fibrous layer 

 and the tunica propria only, for the epithelium forms throughout a 

 lining of sim]3lc flattened cells. 



Fig. 462. 



Fig. 462. — Section of IMembranous Semicircular Canal, much Magnified (Riidinger)- 



1, outer fibrous layer ; 2, tunica propria ; 3, 6, pai^illiform projections with epithelial 

 covering ; 5, fixed side of the canal, \\itli very thin tunica propria without papillae ; 

 7, fibrous bands passing to periosteum. 



Thefihroiis layer (fig. 462, 1), which contains some irregular pigment- 

 cells, is apparently composed of ordinary fibrous tissue, similar to that 

 of the periosteum, with which it becomes continuous at the parts 

 where the two structures are in contact. It is especially developed at 

 the ends of the oval section, whence well-marked bands of fibrous 

 tissue pass to the periosteum (fig. 462, 7). From here, also, the more 

 delicate bands of fibrous tissue above described commonly arise, which 

 traverse the perilymph to become connected with the periosteum of the 

 opposite wall of the canal (fig. 461, 2). Both along these bands and 

 also more directly from the contiguous periosteum, numerous small 

 blood-vessels pass into the fibrous layer and there break up into a 

 coarse capillary network, the branches of which do not, in man, pass 

 into the tunica propria. 



