The Head and Neck. 



171 



(a) The tympanic membrane (membrana tympani) is stretched 

 almost vertically across the lower end of the external acoustic 

 meatus. 



(b) The tensor tympani is a slender muscle, the origin of which 

 forwards from the alisphenoid is concealed. It is inserted 

 on the manubrium mallei. 



(c) The stapedius is a minute muscle arising from the periotic 

 bone above the cochlear fenestra and inserted on the stapes. 



(d) The chorda tympani is a delicate nerve which crosses the 

 tympanic cavity, lying between the long crus of the incus and 

 the manubrium mallei. 



The nerve is a continuation of the intermediate nerve, a sensory root 

 of the facial, which arises independently of the chief or motor root 

 and joins the facial in the facial canal of the periotic bone. Its 

 peripheral, connections are with the lingual nerve and the" submax- 

 illary ganglion. 



(e) The internal carotid artery 

 traverses the carotid canal 

 of the tympanic bone. By 

 breaking away the posterior 

 portion of the bulla, the 

 entrance of the vessel into 

 the external carotid foramen 

 may be seen. 



The dissection of the parts of 

 the ear as here outlined includes 

 only the external ( and middle 

 portions together with the asso- 

 ciated acoustic nerve and its 

 entrance to the periotic bone. 

 The parts of the internal ear 

 (Fig. 51) are not readily made 

 out without the use of special 

 methods, though their position 

 may be estimated by carefully 

 breaking away the surface of the 

 ventral portion of the periotic. 

 They include (1) the bony laby- 

 rinth, consisting of a series of con- 

 nected spaces lodged in the in- 

 terior of the petrous bone, and 

 comprising the cochlea, vestibu- 

 lum, and the bony semicircular 

 canals; and (2) the mem- 

 branous labyrinth, consisting of a 

 second series of spaces contained 

 within the first, and comprising 

 the duct of, the cochlea, the 

 sacculus, the utriculus, and the semicircular ducts, together with their 

 connections and the endolymphatic duct and sac. The membran- 

 ous labyrinth contains the sensory portion of the ear and its cavity 

 is occupied by a fluid material, the endolymph. The wall is 

 separated from that of the bonv labyrinth by an extensive perilym- 

 phatic space also occupied by a" fluid material termed the perilymph. 



Fig. 51. Diagram of the parts of the 

 ear in vertical projection. To ; show the 

 general relations of the structures covered 

 by the dissection. ' 



p, petrous portion of the petrotympanic 

 bone; t., tympanic portion (bulla tym- 

 pani). 



c, cochlea; c.s. bony semicircular 

 canals; c.t., tympanic cavity; d.c, 

 cochlear duct; d.e., endolymphatic duct; 

 d.m., dura mater; d.s., semicircular 

 ducts; i.e., cochlear fenestra; f.v., vesti- 

 bular fenestra; i., incus;, m, malleus; 

 m.a.e., external acoustic meatus, ter- 

 minating at the tympanic membrane; 

 m.a.i., internal acoustic meatus; s., 

 sacculus; s.e., endolymphatic sac; St., 

 stapes; t. a., auditory tube; u., utriculus*; 

 v, vcstibulum; VIII, acoustic nerve. 



