362 THE FACULTY OF HEARING. 



Acriditim ( cmrulescens ) lies at the back of the thorax on each side, 

 above the third pair of limbs. The integument forms a tirm ring, 

 in which is stretched a thin membrane (dram). On the ^Vsside of 

 this drum, and towards the middle, project two peculiarly shaped 

 processes of chitinous substance. The upper and smaller one is a 

 three cornered (pear-shaped) knob, whose apex is turned down- 

 ward. The lower and larger process takes the form of a hook, 

 made up by two shanks or limbs joined at an angle which projects 

 inwards from the drum. Of these two limbs, the shorter one 

 arises from one part of the drum and thickens as it arches inwards 3 

 the other, longer and broader, and channelled on its face, rises 

 from the opposite part of the drum, and, likewise arching inwards, 

 meets the first limb, so that at the middle of this incurved hook 

 the angle of junction of the limbs swells out into a small knob, 

 which is pierced with honeycomb-like pores. The small pear- 

 shaped knob before described is also pierced with numerous canals 

 or pores. 



The nerve (derived from the third thoracic ganglion) swells into 

 a ganglion as it approaches the central knob of the process above 

 described. From this (the distal end of the ganglion) proceeds a 

 bundle of finger-shaped nerve prolongations, each of which 

 contains in its axis a rod-like body surmounted by a sort of cap. 

 The rod appears as if hollow (on account of its refractive power?) 

 and is a mere thread as it starts from the ganglion, but enlarges as 

 it extends towards the bulbous extremity of the process. The 

 number of these processes varies from twenty to thirty, and as the 

 ganglion rests against the knob-like projection of the hooked 

 process above described, these nerves with their imbedded rod-hke 

 bodies run into the canals or pores opening on the tympanal 

 surface, and so reach it. 



The tympanum has a tensor muscle and special muscle nerve j 

 and adherent to a part of its inner surface, just where the nerve 

 ganglion lies, is found a large tracheal sac, whose function may be 

 conceived to be that of multiplying the resonance of sonorous 

 vibration. 



