VESSELS OF THE LABYRINTH. 



109 



ganglion of the cochlear branch lies within the modiolus, at the base of the spiral 

 lamina (ganglion spirale). All the cells of these several ganglia are bipolar. The 

 nerves of the ampullse enter the flattened or least prominent side of the ampullse, 

 where they each form a forked swelling, which corresponds with the crista acustica, 

 in the interior of the dilatation. 



Vessels of the labyrinth. The internal auditory artery, a branch of the 

 basilar, accompanies the auditory nerve in the internal auditory meatus, and divides 

 into branches for the vestibule and cochlea. Those of the vestibule supply the 

 membranous labyrinth and the endosteum, and small vessels ensheathed by tibrous 

 tissue pass across the cavity containing the perilymph. The blood is chiefly 

 collected into the internal auditory veins which accompany the artery and open 

 into the inferior petrosal sinus, but some is conveyed to the inferior petrosal sinus 

 by fine veins in the aqueductus vestibuli and aqueductus cochleae. Small arterial 

 branches from the vessels of the dura mater and also from the stylo-mastoid artery, 

 and from vessels of the middle ear, supply the bony walls of the labyrinth, but do 

 not appear to anastomose with the arteries of the membranous labyrinth. 



Structure of the utricle, saccule, and semicircular canals. Three 

 layers can be distinguished in the membranous walls of the semicircular canals, an 



Fig. 121. SECTION OF ONE OF 



THE HUMAN SEMICIRCULAR 



CANALS. (Riidinger.) Mag- 

 nified. 

 1, osseous wall ; 2, fibrous 



bands with included blood-vessels, 



united at 3 with the periosteum ; 



4, membranous canal with its 



three layers ; 5, short fibrous 



bands (with intervening spaces) 



uniting the membranous canal 



firmly to the periosteum ; 6, 



union of its outermost layer with 



the periosteum. 



outer fibrous stratum, an 



inner epithelial lining, and 



between the two a tunica 



propria. These layers are 



not of equal thickness 



throughout, for along the 



side which is in contact 



with and supported by the 



bone (fig. 122, 6), they are 



thinner than at the rest of 



the circumference, where 



they lie free and are 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 simple flattened cells. 



The Jilrous layer (fig. 122, 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 con- 

 tact. It is especially developed at the ends of the oval section, whence well-marked 

 bands of fibrous tissue pass to the periosteum (fig. 122, 7). More delicate bands of 

 fibrous tissue traverse the perilymph to become connected with the periosteum of the 

 opposite wall of the canal (fig. 121, 2). Both along these bands and also more 

 directly from the contiguous periosteum, numerous small blood-vessels pass into the 



