464 CHORDA TYMPANI. 



near the basis of the pyramid, and passing downwards and 

 forwards, goes out at an aperture in the fissure of the cavity 

 for the head of the lower jaw. In this course it crosses the 

 long leg of the incus, and the manubrium of the malleus, 

 and passes between them, being in contact with the manu- 

 brium. 



This nerve comes off from the portio dura immediately 

 before it emerges from the foramen stylo-mastoideum, and 

 after passing a short distance through a small canal, enters the 

 cavity of the tympanum as above described. After passing 

 out of the cavity of the tympanum, it joins the lingual branch 

 of ihe fifth pair of nerves. 



It was believed by the late Mr. John Hunter, that the chorda 

 tympani is not simply a branch of the portio dura, but that 

 it is the twig of the reflected branch of the spheno-palatine 

 nerve, which, entering the os petrosum by the Vidian Fora- 

 men, joins the portio dura ; and after passing with it a consider- 

 able distance, leaves it at the place above described.* 



There is the greatest reason to believe that the principal object of the structure 

 above described, is to transmit to the labyrinth those impulses of the air 

 which occasion sound. The membrana tympani, with the small bones and 

 their muscular appendages, seem to be the agents for this purpose. The 

 Eustachian tube and the mastoid cells are subservient parts. The effect of 

 the chorda tympani in the cavity has not been ascertained. 



With this view of the subject, it is surprising that persons, in whom the mem- 

 brana tympani has been destroyed, should enjoy the sense of hearing in a 

 very considerable degree of perfection. Such, however, is the fact. 



It is necessary thai there should be a communication between the cavity of the 

 tympanum and the exiernal air, in order that the function may be duly per- 

 formed. 



This is evinced by the deafness which results from the obstruction of the Eus- 

 tachian tube, and the cure of this deafness by relieving that obstruction ; as 

 well as by the salutary effects of opening the membrana tympani ■,-\ and even 

 of instituting a communication through the mastoid cells, in cases where the 

 obstruction of the Eustachian could not be removed. 



It has been supposed that the Eustachian tube has the effect of transmitting sound 

 to the ear, and particularly the voice of the individual of whose structure it is a 

 part ; and it is certain that we hear our voices very distinctly, (although pecu- 

 liarly modified,) when the external orifices of the ears are closed : but the well- 



* I have followed Mr. H. in this dissection, and have no doubt of his opinion being cor- 

 rect. — H. 



t See Dr. Sim's paper on this subject, in the first volume of Memoirs of the Medical 

 Society of London ; and Astley Cooper's in the London Philosophical Transactions. 



