812 



DISSECTION OF THE EAE. 



Arteries are 

 branches of 

 carotids. 



From 



internal 



maxillary, 



middle- 

 meniugeal, 



posterior 

 auricular, 



ascending 

 pharyngeal, 



internal 

 carotid. 



Nerves from 



several 



sources. 



Dissection 

 to prepare 

 the nerves ; 



outside 



tympanic 



cavity, 



and inside 

 cavity. 



Tympanic 

 nerve 



BLOOD-VESSELS. The arteries of the tympanum are furnishec 

 from the following branches of the external carotid, viz., internal; 

 maxillary, middle meningeal, posterior auricular, and ascending! 

 pharyngeal; and some offsets come from the internal carotid in the' 

 temporal bone. The veins join the pterygoid plexus, and the large 

 meningeal and pharyngeal branches. 



The internal maxillary artery supplies an anterior tympanic 

 branch (fig. 297, d), which enters the cavity through the Glaserian 

 fissure, and gives an offset to the membrane of the tympanum. 



The middle meningeal artery also sends fine twigs to the upper 

 part of the tympanum through small apertures in the roof of the 

 cavity. 



The stylo-mastoid branch of the posterior auricular artery, 

 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, 

 posterior tympanic (fig. 297, 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. 



One or two minute branches of the internal carotid artery reach 

 the anterior wall of the tympanum from the carotid canal. 



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. 300). 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. 633). 

 Supposing the nerve to be found, the student should place the 

 scalpel on the outer side of the Eustachian tube, and carry it back- 

 wards 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 Fallopius. 



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

 be followed in its canal ; and the branches in the grooves on the 

 surface of the promontory are to be pursued ; two of these, arching 

 forwards, pass to the sympathetic on the carotid artery and to the 

 Eustachian tube ; and two others are directed upwards beneath the 

 tensor tympani muscle. 



The course of the chorda tympani nerve can be seen on the 

 preparation used for the muscles. 



THE TYMPANIC BRANCH OF THE GLOSSO-PHARYNGEAL NERVE 



(fig. 300, 2 ; nerve of Jacobson) enters a special aperture in the 



