822 NERVOUS SYSTEM. 



" The opening of the tube is thus the result of the action of these three muscles: the 

 tensor palati, or dilator tubae, draws the hook of the cartilage outward, the cartilage 

 becomes less curved and the tube is widened ; the levator palati in contracting becomes 

 more horizontal, and draws the lower end of the cartilage inward and upward, thus 

 enlarging the pharyngeal orifice more than 3'". As soon as these muscles cease acting, 

 the elasticity of the cartilage restores the canal to its former condition." 



It is thus that the action of certain of the muscles of deglutition dilates the pharyngeal 

 opening of the Eustachian tube. If we close the mouth and nostrils and make several 

 repeated acts of deglutition, we draw the air from the tympanic cavity, and the atmos- 

 pheric pressure renders the membrane of the tympanum tense, increasing its concavity. 

 By one or two lateral movements of the jaws, we open the tube, the pressure of air is 

 equalized, and the ear returns to its normal condition. The nerves animating the dilator 

 tuba3 come from the pneumogastric and are derived from the spinal accessory. 



A smooth mucous membrane forms a continuous lining for the Eustachian tube, the 

 cavity of the tympanum, and the mastoid cells. In all parts, it is closely adherent to the 

 subjacent tissues, and, in the cavity of the tympanum, it is very thin. In the cartilaginous 

 portion of the Eustachian tube, there are numerous mucous glands, which are most 

 abundant near the pharyngeal orifice, and gradually diminish in number toward the 

 osseous portion, in which there are no glands. Throughout the tube, the surface of the 

 mucous membrane is covered with conoidal cells of ciliated -epithelium. The mucous 

 membrane of the tympanic cavity is very thin, consisting of little more than epithelium 

 and a layer of connective tissue. It lines the walls of the cavity, the inner surface of the 

 membrana tympani, is prolonged into the mastoid cells, and covers the ossicles and those 

 portions of the muscles and tendons which pass through the tympanum. On the floor of 

 the tympanic cavity and on its anterior, inner, and posterior walls, the epithelium is of 

 the conoidal, ciliated variety. On the promontory, roof, ossicles, and muscles, the cells 

 are of the pavement- variety and not ciliated, the transition from one form to the other 

 being gradual. The entire mucous membrane contains numerous lymphatics, a plexus 

 of nerve-fibres and nerve-cells, with some peculiar cells, the physiology of which is not 

 understood. 



We have thus given a general sketch of the physiological anatomy of the middle ear, 

 and shall not find it necessary to treat more fully of the cavity of the tympanum, the 

 mastoid cells, or the Eustachian tube, except as regards certain points in their physiology. 

 The minute anatomy of the membrana tympani and the articulations of the ossicles can 

 be more conveniently considered in connection with the physiology of these parts. 



General Arrangement of the Bony Labyrinth. The internal portion of the auditory 

 apparatus is contained in the petrous portion of the temporal bone. It consists of an 

 irregular cavity, called the vestibule, the three semicircular canals (13, 14, 15, Fig. 258), 

 and the cochlea (16, Fig. 258). The general arrangement of these parts in situ and their 

 relations to the adjacent structures are shown in Fig. 258. Fig. 261, showing the bony 

 labyrinth isolated, is taken from the beautiful photograph contained in Eudinger's atlas. 



The vestibule is the central chamber of the labyrinth, communicating with the tympanic 

 cavity by the fenestra bvalis, which is closed in the natural state by the base of the stapes. 

 This is the central, ovoid opening shown in Fig. 261. The inner wall of the vestibule 

 presents a small, round depression (the fovea hemispherica) perforated by numerous small 

 foramina, through which pass nervous filaments from the internal auditory meatns. 

 Behind this depression, is the opening of the aqueduct of the vestibule. In the posterior 

 wall of the vestibule, are five small, round openings leading to the semicircular canals, 

 with a larger opening below,- leading to the cochlea. 



The general arrangement of the semicircular canals is shown in Fig. 261 (6, 7, 8. 9, 



The arrangement of the cochlea (the anterior division of the labyrinth) is shown in 



