VESSELS AND NERVES OF TYMPANUM. 677 



pharynx through the Eustachian tube, and is prolonged into the mastoid 

 cells through the apertures leading into them. 



It assists to form part of the membrana tympani, and of the secondary 

 membrane in the fenestra rotunda ; it is reflected also over the chain of 

 bones and the muscles, ligaments, and chorda tympani nerve. In the 

 tympanum the membrane is thin, not very vascular, and secretes a watery 

 fluid ; but in the lower part of the Eustachian tube it is thick and more 

 vascular, is provided with numerous glands, and its epithelium is lami- 

 nated. 



Its surface is covered with a columnar ciliated epithelium, but on the 

 membrana tympani and the ossicula it is laminar and unciliated. 



BLOOLJVESSELS. The arteries of the tympanum are furnished from 

 the following branches of the external carotid, viz., internal maxillary, 

 posterior auricular, ascending pharyngeal ; and some offsets come from the 

 internal carotid in the temporal bone. The veins join the middle menin- 

 geal and paryngeal branches. 



The internal maxillary artery supplies the tympanic branch (fig. 240, 

 <f) (inferior, p. 93), which enters the cavity through the Glaserian fissure, 

 and distributes a branch to the membrane of the tympanum. 



The stylo-mastoid branch of the posterior auricular artery (p. 86), 

 entering the lower end of the aqueduct of Fallopius, gives twigs to the 

 back of the cavity, and the mastoid cells. One of this set, superior tym- 

 panic (fig. 240, e), anastomoses with the tympanic branch of the internal 

 maxillary artery, and forms a circle around the membrana tympani, from 

 which offsets are directed inwards. 



Other branches from the ascending pharyngeal, or from the inferior 

 palatine artery, enter the fore part of the space by the Eustachian tube. 



NERVES. The lining membrane of the tympanum is supplied from the 

 plexus (tympanic) between Jacobson's and the sympathetic nerve : but 

 the muscles derive their nerves from other sources. Crossing the cavity 

 is the chorda tympani branch of the facial nerve. 



Dissection (fig. 243). The preparation of the tympanic plexus will 

 require a separate fresh temporal bone, which has been softened in diluted 

 hydrochloric acid, and in which the nerves have been hardened afterwards 

 in spirit. 



The origin of Jacobson's nerve from the glosso-pharyngeal is first to be 

 sought close to the skull ; and the fine auricular branch of the pneumo- 

 gastric may be looked for at the same time (p. 113). Supposing the nerve 

 to be found, the student should place the scalpel on the outer side of the 

 Eustachian tube, and carry it backwards through the vaginal and styloid 

 processes of the temporal bone, so as to take away the outer part of the 

 tympanum, but without opening the lower end of the aqueduct of Fal- 

 lopius. 



After the tympanum has been laid open, Jacobson's nerve is to be fol- 

 lowed in its canal ; and the branches in the grooves on the surface of the 

 promontory are to be pursued : one of these arching forwards joins in the 

 carotid plexus ; and two others directed upwards, unite with the large 

 superficial petrosal, and the facial nerve. 



The connections of the chorda tympani nerve can be seen on the prepa- 

 ration used for the muscles. 



The tympanic branch of the glosso-pharyngeal nerve (fig. 243, 2 ), nerve 

 of Jacobson (p. 112), enters a special aperture in the temporal bone, to 



