298 



COMPARATIVE ANATOMY 



The first indications of a pars basilaris with its papilla basilaris 

 lagcnce are already met with in certain Urodeles, but they lie 

 inside the upper part of the lagena, close to the sacculus. 



In the Anura (Figs. 217 and 218) a higher condition is seen in 

 the presence of a peculiar small outgrowth of the thickened wall 

 of the lagena on which a definite region, supported by cartilage, 

 corresponds to the basilar membrane of higher types. The lagena 

 and pars basilaris now open close together but independently of 

 one another in the posterior part of the sacculus. Thus, in addition 

 to those present in Fishes, a new patch of nerve-endings is added, 

 viz., the papilla acustica basilaris lagcnce. 



The ductus endolymphaticus may give rise to large sac-like 

 enlargements containing calcareous matter which meet those of 

 the other side, either on the dorsal surface only, or on both dorsal 

 and ventral surfaces of the brain. The latter is the case in Anura, 



Ppl 



Ppl 



FIG. 218. DIAGRAM SHOWING THE RELATIONS OF THE LAGENA AND PARS 

 BASILARIS IN ANURA (A) AND SAUROPSIDA (B). The outer wall of the 

 sacculus faces upwards. (After H. Spencer Harrison.) 



D, ductus sacculo-cochlearis ; Ly, lagena ; MS, macula sacculi ; Pb, pars 

 basilaris with its cartilaginous framework ; Ppb, papilla basilaris ; Ppl, 

 papilla Iagena3 ; S, sacculus. 



for instance, in which the sac extends as an unpaired structure 

 along the whole vertebral canal dorsally to the spinal cord, giving 

 rise to paired outgrowths extending through the intervertebral 

 foramina and forming the characteristic calcareous bodies situated 

 close to the spinal ganglia. These consist of numerous inter- 

 communicating tubes lined by pavement epithelium and plentifully 

 supplied with capillaries. 



The perilymphatic cavity of Amphibians communicates medially 

 with the cranial cavity by a perilymphatic duct, and the same is 

 true of the Amniota. 



A further advance in structure as compared with Fishes is seen 

 in the gradual differentiation of a middle car. In the outer wall of 

 the auditory capsule is a membranous space, thefcncstra vestibuli 

 (ovalis), which is plugged by a cartilaginous stapedial plate, and 

 from the latter a rod-like cartilage or bone, the columella, iisually 

 extends outwards towards the quadrate and paraquadrate (p. 98). 



A tympanic cavity, with a tympanic membrane supported by a 



