AUDITOKY ORGAN. 205 



at the blind end of the latter, that is, at the so-called cupula ter- 

 minalis (Fig. 167,*). The scala vestibuli is shut off from the 

 tympanic cavity (Ct t Ct) by the membrane of the fenestra ovalis, 

 to which the stapes is applied externally (Fig. 167, SAp, t) ; the 

 scala tympani is closed by the membrane of the fenestra rotunda 

 (Fig. 167, M}. 



On the floor of the bony cochlea, not far from the fenestra 

 rotunda, there is an opening (Fig. 167, d) into a narrow canal, the 

 aqueductus cochleae, or ductus perilymphaticus (P# t 

 D.p 1 ), which serves as a communication between the perilymph- 

 atic cavity and the peripheral lymphatic trunks of the head. 1 



The already-mentioned ductus endolymphaticus s. 

 aqueductus vestibuli is in relation with the endolymph 

 enclosed within the membranous labyrinth (Figs. 161 and 167, 

 D.e). It is an archaic structure, and is present from the lowest 

 Fishes (Myxinoids) up to Mammals, undergoing numerous varia- 

 tions and modifications in the Vertebrate series. In its primitive 

 form, the endolymphatic duct consists of a tube arising from the 

 inner wall of the sacculus, with the lumen of which it communi- 

 cates. Its upper end perforates the inner wall of the cartilaginous 

 or bony auditory capsule, passes into the cranial cavity, and 

 terminates by an expanded extremity (saccus endolymphaticus) in 

 the dura mater. Osmosis can thus occur between the lymph 

 contained in the endolymphatic and epicerebral lymph-spaces 

 respectively. 



In Elasmobranch s the ductus endolymphaticus opens on the posterior part 

 of the roof of the skull, and is thus in free communication with the sea- water. 

 In many Reptiles its free end comes to lie close under the roof of the skull 

 beneath the parieto-occipital suture, and in the Ascalabota the duct even 

 leaves the cranial capsule, passes back between the muscles of the neck, and in 

 the region of the pectoral arch becomes swollen to form a large folded sac, 

 from which finger-shaped processes extend right to the ventral surface of the 

 vertebral column and to the sub-mucous tissue of the pharynx. These pro- 

 cesses may also branch out in a labyrinthic manner into the orbit, and they 

 are always filled with a white semi-solid mass of otolithic substance, con- 

 sisting of very minute crystals of carbonate of lime : otolithic matter is also 

 present in the ductus endolymphaticus of all Vertebrates, at any rate in the 

 embryo. 



In Amphibians, and also in certain Teleosteans, the duct of either side, 

 by means of large sac-like enlargements, may come to lie close to its fellow, 

 either on the dorsal surface only, or on both dorsal and ventral sides of the 

 brain. The latter is the case in Anura, for instance. 



In Birds and Mammals the ductus endolymphaticus never passes out of 

 the cranial cavity, and in its general relations corresponds with the tube-like 

 ground-form described above. 



1 A ductus perilymphaticus can be plainly made out from Eeptiles onwards. It 

 arises in the cavum perilymphaticum on the outer side of the sacculus, then passes 

 along a deep furrow to the median wall of the cochlea, extends over the membrana 

 basilaris (scala tympani), passes through the foramen rotmidum, and comes into 

 connection with the epicerebral lymph-sinus. 



