612 



THE NERVOUS SYSTEM. 



vestibuli. It terminates in a dilated and flattened cul-de-sac, 

 which lies within the cranial cavity between the layers of the 

 dura mater. 



Membranous Semicircular Canals. These are three in number, 

 in shape like the osseous canals, but are only about one-third their 

 diameter ; they open by five apertures into the utricle. The lining 

 of the canals forms papilliform elevations (Fig. 392). In each 

 ampulla is the ridge, crista acusiica, already described (p. 610). 



Membranous semicircular canal. 



v Epithelium of the 



membranous 

 canal. 



Blood-vessel. 



Wall of mem- 



branous 

 canal. 



- Ligament of 

 canal. 



Bone. 



Perilymphatic _ 

 spaces. 



Blood-vessel. ^ 



FIG. 392. Transverse section through an osseous and membranous semicircular 

 canal of an adult human being : a, connective-tissue strand representing a remnant 

 of the embryonic gelatinous connective tissue. Such strands serve to connect the 

 membranous canal with the osseous wall ; X 50 (after a preparation by Dr. Scheibe) 

 (Bohm and Davidoff ). 



Canal of the Cochlea. For the following description we are 

 indebted to Schafer's Essentials of Histology : " The periosteum, 

 a peculiar kind of connective tissue which covers the upper surface 

 of the lamina spiralis, is thickened, forming the limbus, also called 

 limbus lamince spiralis, and denticulate lamina by Todd and Bow- 

 man, and the edge is grooved, somewhat resembling the letter C, 

 the upper part of which is the labium tympanicum ; between these 

 labia is the sulcus spiralis (Fig. 393). The membrana basilaris, 

 already referred to (p. 608), extends from the labium tympanicum 



