1918.] Bai]s-i Peashad : Middle Ear of Indian Frogs. 99 



It is attached all along the circumference to the slightly up and in- 

 turned edges of the annulus tympanicus. The tympanic membrane 

 is thin along the margin but is specially thickened in the middle on 

 its inner side for the attachment of the distal end of the columella auris. 

 The membrane is formed of connective tissue fibres which radiate 

 from the central point of attachment of the columella to the periphery. 

 A few blood vessels, nerve cells and a large number of pigment cor- 

 puscles are also found scattered in the connective tissue. Along the 

 margin some unstriated muscle fibres are also to be seen. Internally 

 the tympanic membrane is lined by columnar epithelium {text fig. 2) 

 which is continuous lower down with the mucous membrane hning of 

 the tympanic cavity. 



Fig. 1. — Transverse section of the ear of R. ti(jrin(i. 



The Tympanic cavity and associated skeletal structures. — After the 

 removal of the tympanic membrane the tympanic cavity is seen to be 

 a funnel-shaped structure. The upper margin of the funnel is slightly 

 turned inwards for the attachment of the tympanic membrane 

 as seen in a transverse section {text fig. 1). The funnel-shaped 

 tympanic cavity has its longer axis directed downwards and backwards 

 from the anterior and upper side. This upper or outer portion of the 

 tympanic cavity is formed by the annulus tympanicus {A. T.) ; from the 

 lower end of the annulus tympanicus the tympanic cavity becomes 

 very much reduced and continues as a slightly depressed tubular struc- 

 ture, the cross section of it hence is not circular but slightly elongated. 

 The beginning of this second or inner portion of the tympanic cavity 

 may be termed the tympanic recess {T. r.), while the opening by which 

 it communicates with the internal ear is known as the fenestra oralis 

 (fig. 4, /. o.). From the ventral surface of this deeper portion of the 

 tympanic cavity a short wide eustachian tube puts the tympanic cavity 

 of each side into communication with the pharyngo-oral cavity. The 

 deeper portion of the tympanic cavity is bounded by the squamosal 

 and prootic bones anteriorly, by the prootic dorsally, by the carti- 

 laginous portion between the prootics and exoccipitals internally and 

 by muscles posteriorly. 



b2 



