I5i6 



HUMAN ANATOMY. 



Trabecuht 



Membranous 

 canal 



to them in number, name and form. They are closely united along their convex 

 margins with the bony tube CFig. 1270), whilst their opposite wall lies free in the 



perilymphatic space, 

 ^^70' being attached only by 



irregular vascular con- 

 nective tissue bundles, 

 ligamenta labyrin- 

 thi canaliculorum, 

 which stretch across 

 this space. Like the 

 bony canals, each of 

 the membranous tubes 

 possesses an ampulla, 

 which in the latter is 

 relatively much larger 

 than in the former, 

 being about three times 

 the size of the rest of 

 the tube. The part of 

 the ampulla corre- 

 sponding to the con- 

 vexity of the semicir- 

 cular canal is grooved 

 on the outer surface at 

 the entrance of the 

 ampuUary nerves. On 

 the corresponding in- 

 ternal surface is a pro- 

 jection, the septum 

 transversum, which partially divides this space into two parts and is surmounted 

 by the crista acustica, which contains the endings of the vestibular nerves. The 

 crescent-shaped thickening beyond each end of the crista is called the planum 

 semilunatum. 



Perilymphatic, 

 space 



Trabecule 



Bonv wall 



Transverse section of superior semicircular canal, showing relations of 

 membranous to bony tube. X 35. 



Structure of the Utricle, Saccule and Semicircular Canals. — The vestibule and the bony 

 semicircular canals are lined by a very thin periosteum composed of a felt-work of resistant 

 fibrous tissue, containing pigmented connective tissue cells. Endothelium everywhere lines the 

 perilymphatic space between the membranous and osseous canals, covering the free inner sur- 

 face of the periosteum, the fibrous trabeculae, and the outer or perilymphatic surface of this 

 part of the membranous labyrinth. 



The walls of the utricle, saccule and membranous semicircular canals are made up of (a) 

 an outer fibrous connective tissue lamella and ( b) an inner epithelial lining, the latter consisting 

 throughout the greater part of its extent of a single layer of thin flattened polyhedral cells. Be- 

 neath the epithelium, especially in the region of the maculae, is (r) a thin, almost homogeneous 

 hyaline fnembrane, with few cells. This middle layer presents in places on its inner surface 

 small papillary elevations covered by epithelium. On the concave side of each of the 

 semicircular canals is a strip, the raphe, of thickened epithelium in which the cells become low 

 cylindrical in type. In the plana semilunata they are cylindrical in type. Over the regions 

 receiving the nerve-fibres, the macuke acusticce and the cristas acusticte, the epithelium 

 undergoes a marked alteration, changing from the indifferent covering cells into the highly 

 specialized neuroepithelium. 



The maculae acusticae are about 3 mm. long by 2 mm. broad, the macula of the saccule 

 bemg a little narrower (1.5-1.6 mm. ) than that of the utricle (2 mm.). At the margin of these 

 areas the cells are at first cuboidal, next low columnar, and then abruptly increase in length, until 

 they measure from .030-.035 mm., in contrast with their usual height of from .003-. 004 mm. The 

 acoustic area includes two kinds of elements, the sustentacular or fibre-cells and the hair-cell.'^. 

 The sustentacular cells are long, rather narrow, irregularly cylindrical elements and extend the 

 entire thickness of the epithelial layer, resting upon a well-developed basement-membrane by 

 their expanded or divided basal processes- At a variable distance from the base, they present a 

 swelling enclosing an oval nucleus and terminate at the surface in a cuticular zone. The cylin- 

 drical hair-cells are broader but shorter than the sustentacular cells, and reach from the free 

 surface only as far as the middle of the epithelial layer, where each cell terminates usually in a 



