THE INTERNAL EAE 699 



branous sacs are contained within the irregular cavity of the vestibule. 

 The structure of the utriculus, therefore, requires no further description. 



THE SEMICIRCULAR CANALS 



The semicircular canals or ducts are three in number, the posterior, 

 superior, and lateral. The last is also horizontal in its position; the 

 first two are vertical, but are so placed as to form a right angle with 

 one another. The posterior lies in the long axis of the petrous bone 

 and its plane is therefore more nearly sagittal, while that of the superior 

 canal is more nearly coronal. Each canal forms something more than 

 half a circle, its two ends opening separately into the cavity of the vesti- 

 bule, with the exception of the posterior and superior canals whose in- 

 ternal ends open by a common orifice, the canalis communis. The un- 

 joined orifices of the posterior and superior canals, as also the outer 

 extremity of the lateral canal, present a marked dilatation at their 

 termination in the vestibule. These dilatations are known as the am- 

 pulla. They .lodge the neuro-epithelial patches, the cristaa acusticae. 

 The osseous and membranous canals are of similar shape; the 

 latter is, of course, contained within the former. 



The membranous semicircular canals open into the utricle. They 

 do not entirely fill their bony canal, but, like the utricle and saccule, 

 lie in contact with the periosteum at one surface only, this surface being 

 that of the outer wall or periphery of the semicircle, while iPthe re- 

 maining portion of the circumference of the cylindrical bony duct, the 

 membranous canal is loosely united to the periosteum of the osseous 

 wall by a retiform connective tissue whose loose meshes are filled with 

 perilymph and lined with mesenchymal epithelium. 



The wall of the membranous canal is similar in structure to that 

 of the saccule and utricle and consists of an ectodermal epithelium, a 

 membrana propria, and a fibrous tunic. Each of the three ampullae 

 presents a marked differentiation of the epithelial lining, which is there 

 raised in the form of a prominent crescentic fold, inappropriately termed 

 by the older anatomists the crista acustica, from its supposed connection 

 with the auditory function. Like the maculae of the saccule and utricle, 

 the cristae are supplied by the vestibular nerve and are concerned with 

 the function of equilibration. 



The utricle and saccule represent the original anlage of the ear 

 the otocyst from which the canals and cochlea arise as evaginations ; 

 they correspond most closely also to the f ear' of certain invertebrates, 



