760 THE SENSE ORGANS AND SKIN OF THE HORSE 



inates under the dura mater on the posterior part of the internal surface of the 

 petrous temporal bone in a dilated blind end, the saccus endolymphaticus. 



3. The membranous semicircular canals (Ductus semicirculares) correspond 

 in general to the osseous canals already described, but it may be noted that while 

 the ampullse of the membranous canals nearly fill those of the osseous canals, the 

 other parts of the membranous canals only occupy about one-fourth of the bony 

 cavities. 



4. The cochlear duct (Ductus cochlearis) is a spiral tube situated within the 

 cochlea. It begins by a blind end (Caecum vestibulare) in the cochlear recess of 

 the vestibule, and ends by a second blind end (Caecum cupulare), which is attached 

 to the cupola of the cochlea. The vestibular part is connected with the saccule 

 by the ductus reuniens. The duct is triangular in cross-section, and it is usual to 

 regard it as having three walls. The vestibular wall or roof, which separates the 

 cochlear duct from the scala vestibuli, is formed by the very delicate membrana 

 vestibularis (of Reissner), which extends obliquely from the lamina spiralis ossea 

 to the outer wall of the cochlea. The tympanic wall or floor intervenes between 

 the cochlear duct and the scala tympani; it is formed by the periosteum of the mar- 

 ginal part of the lamina spiralis and the membrana basilaris, which stretches be- 



Fig. 570. — Left Membranoi'r L.\bvrinth (En- Fig. 571. — Schematic Sectional View of Labyrinth 



LARCKij). (Enlarged). 



1, Cochlea; 2, fenestra vestibuli; 3, fenestra 1, 2, 3, Superior, external, and inferior semi- 



cochlese; 4, ductus endolymphaticus; 5, superior, circular canals; 4> utricle; 6, saccule; 6, cochlea; 



6, external, 7, inferior semicircular canal. (After 7, auilitory nerve. (After Ellenberger, in Leisering's 



EUenberger, in Leisering's Atlas.) Atlas.) 



tween the free edge of the lamina and the outer wall of the cochlea. The outer 

 wall consists of the fibrous lining of the cochlea, which is greatly thickened to form 

 the ligamentum spirale cochleae. 



Structure.' — The membranous labyrinth consists in general of an outer thin 

 fibrous layer, a middle transparent tunic, and an internal epithelium, composed of 

 flattened cells. But in certain situations special and remarkable structures occur, 

 among which are the following: (1) The maculae acusticae appear as small whitish 

 thickenings of the inner walls of the saccule and utricle. The epithelium here 

 consists of two kinds of cells, viz., supporting cells and hair cells. The latter are 

 flask-siiaped and are surrounded })y the fusiform supporting cells. The free end 

 of each hair cell bears a stiff, hair-like process composed of a bundle of cilia. Fibers 

 of the saccular and utricular branches of the vestibular nerve form arborizations 

 about the basal parts of the hair cells. Adherent to the surface of the macula 

 are fine crystals of lime salts, embedded in a mucoid sul)stance, and termed otoconia. 



(2) The cristae acusticae are linear thickenings of the wall of each ampulla of the 

 membranou.s semicircular canals. Their structure is similar to that of the maculae. 



(3) The spiral organ of Corti (Organon spirale) is an epithelial elevation which is 

 situated upon the inner part of the membrana basilaris, and extends the entire 

 length of the ductus cochlearis. It is very complicated in structure, but consists 



1 For the finer structure reference is to be made to the histological literature. 



