102 



STRUCTURE OP EAR. 



on each side of the tympanum, and enables it freely 

 to transmit the atmospheric vibrations. 



These trachete, though formed on a similar plan, 

 present many variations, corresponding to those of 

 the tympana, and showing that the tympana and 

 the tracheae stand in intimate connection with one 

 another. For instance, in those species where the 

 tympana are equal, the tracheae are so likewise ; in 

 Gryllotalpa, where the front tympanum only is de- 

 veloped, though both tracheal branches are present, the 

 front one is much larger than the other ; and where 

 there is no tympanum, the trachea remains compara- 

 tively small, and even in some cases, according to 

 Graber, undivided. 



The tibia is thus divided into three parts, as shown 

 in the diagram (Fig. 64), the central 

 portion being occupied by the two 

 tracheae (Fig. 64, tr, tr). 



Of the other two spaces, one (the 

 lower one in the figure) is occupied 

 by the muscles, nerves, etc., while 

 the other is mostly filled with blood, 

 which thus surrounds and bathes the 

 auditory vesicles and rods {ar). 



The acoustic nerve — which, next 

 to the optic, is the thickest in the 

 body — divides soon after entering 

 the tibia into two branches ; the one 

 forming almost immediately a ganglion, the supra- 

 tympanal ganglion, to which I shall refer again pre- 

 sently; the other passing down to the tympanum, 

 where it expands into an elongated flat ganglion, known 

 after its discoverer as the organ of Siebold (Fig. 65), 

 and closely applied to the anterior trarhese. 



Fig. 6-t.— Section through 

 the tibia (leg) of a 

 Weconema, x about 

 150. Ir, tr. The two 

 trachea; ar, the audi- 

 tory rod. ' 



