ORGAN OF HEARING. 



543 



Not much is known of the veins of the laby- 

 rinth. The internal auditory artery is accom- 

 panied by a corresponding vein which carries 

 away the blood from the labyrinth, and empties 

 it into the superior petrosal sinus. Another 

 vein, says Weber,* goes perhaps from the laby- 

 rinth through a small opening in the cleft of 

 the aqueduct, and empties itself into the trans- 

 verse sinus. 



Of the veins of the cochlea, some, according 

 to Breschet, accompany their arteries ; others 

 enter the sinus, lodged in the substance of the 

 outer margin of the spiral lamina. Near the 

 base of the cochlea, this sinus communicates 

 with the veins of the vestibule. 



Nothing is known of the absorbents of the 

 labyrinth. 



Fig. 247. 



A section of the cochlea parallel to its axis, showing 

 the distribution of the vessels in its interior. It is the 

 veins that are delineated, but their distribution is 

 almost the same as that of the arteries. Magnified. 

 (From Breschet.) 



a, a. Veins accompanying the trunk of the coch- 

 lear nerve, and penetrating the nervous branches 

 across the spiral lamina ; b. first anastomoses at the 

 periphery of the osseous zone ; c, c. second anasto- 

 moses at the periphery of the middle zone ; d. last 

 ramuscules, which are almost parallel, occupying 

 the membraneous zone ; e, e, e. venous sinus in 

 the peripheral margin of the membraneous zone. 



II. ACCESSORY PARTS OF THE APPARATUS 



OF HEARING. 



Of these parts, the auricle collects the so- 

 norous undulations, and the auditory passage 

 conducts them to the middle ear or tympanum, 

 where they are modified and transmitted to the 

 sensitive part of the apparatus, the ear-bulb. 



A tympanum is found in reptiles, birds, and 

 Mammifera, a perfect external ear only in the 

 Mammifera. The lip-like folds of skin before 

 the membrana tympani, in some birds and rep- 

 tiles, may, however, be considered rudiments 

 of an external ear. Among the Mammifera, 

 the Cetacea have no auricle, and only a very 

 contracted auditory passage. 



It was said that the tympanum exists in a 

 greater number of animals than the cochlea. 

 This refers to a discovery made by Weber ,f 

 that a prolongation of the swimming-blad- 



* In Hildebrandt's Anatomic. 

 t De Aure et auditu Hominis et animalium, 

 Lipsis, 1820. 



der in the herring, in the cyprinoid fishes, 

 in silurus glanis, and in several species of 

 cobitis, has a connexion with the membraneous 

 labyrinth in the same manner that the prolon- 

 gation of the mucous membrane of the throat, 

 forming essentially the tympanum and Eusta- 

 chian tube, is extended to the surface of the 

 labyrinth; and moreover, that in all those 

 fishes, with the exception of the herring, there 

 exist bones analogous to the tympanic ossicles 

 in the higher animals. 



1. The middle ear, or tympanum, and its ap- 

 pendages. 



The cavity of the typanum, cavitas tympani; 

 Fr. came du tympan ou du tambour; Germ, die 

 Trommelhohle oder Paukenlwhle. The cavity 

 of the tympanum is a space lying at the peri- 

 pheral surface of the ear-bulb, and measuring 

 from above downwards as well as from before 

 backwards about four-tenths of an inch, and 

 from without inwards about three-twentieths of 

 an inch. It is bounded internally by the outer 

 wall of the osseous labyrinth ; externally by a 

 vibratile membrane, the membrana tympani, 

 and that portion of the temporal bone into 

 , which it is framed. Anteriorly a canal, the 

 a' Eustachian tube, leads from it into the throat ; 

 | and posteriorly and superiorly it communicates 

 with the mastoid cells. 



The cavity of the tympanum is traversed by 

 a chain of small bones, extending from the 

 membrana tympani to the vestibular fenestra, 

 and is lined by a very delicate membrane of a 

 fibro-mucous character, which is prolonged into 

 all its sinuosities and dependent cavities. This 

 membrane is moreover reflected on the parts 

 which traverse the cavity, and envelopes them. 

 The lining membrane of the tympanum is con- 

 tinuous through the medium of that of the 

 Eustachian tube, with the mucous membrane 

 of the throat. 



A condition essential to the due performance 

 of the function of the tympanum is that the 

 external air have free access to its cavity. 



Examined on the dry bone, the inner wall 

 of the tympanum presents a considerable emi- 

 nence ; behind and below which is an opening 

 somewhat of a triangular form, and in a fossa 

 above it another opening, about twice the size 

 of the preceding, and of an ovoid shape. From 

 the description which has been already given 

 of the osseous labyrinth, it will be immediately 

 perceived that the eminence in question, called 

 the promontory, is that which the commence- 

 ment of the cochlea forms ; that the opening 

 below and behind it is the fenestra rotunda or 

 cochlear fenestra, and the opening above it the 

 Jenestra ovalis or vestibular fenestra. 



The surface of the promontory is marked by 

 a groove ; sometimes instead of a groove there 

 is a canal. This groove is continuous below 

 with a canal, several lines in length, which 

 opens in that depression in the partition be- 

 twixt the lower orifice of the carotid canal and 

 the foramen lacerum posterius. Above, in front 

 of the vestibular fenestra, the groove again runs 

 into a canal which proceeds forwards and up- 

 wards between the canal for the internal muscle 

 of the malleus and the commencement of the 



