THE Al'DITOKV OSSK'I.KS. SO 



lateral plate, and from the adjacent membranous part of the floor of the tube, 

 parallel to which its fibres course towards their insertion into the soft palate. 

 When the muscle contracts the membranous floor of the tube is tightened, and is 

 also raised near the ostium pharyngeum by the thickening of the muscular fibres 

 which run just below it, and this helps to open the lower orifice during swallowing. 



Lastly, some muscular bundles, forming the salpingo-pharyngeus of Santorini, 

 arise from the lower and fore part of the mesial plate of cartilage, and passing down 

 immediately beneath the mucous membrane, join the palato-pharyngeus (Vol. II., 

 p. 308). This muscle tends to drag the mesial plate backwards and downwards, 

 and thus assists in opening the tube during deglutition. 



The anterior wall of the tympanum, below the orifice of the Eustachian 

 tube, is formed by a bony plate forming part of the petrous portion of the temporal 

 bone, which is hollowed out by air-cells, and here separates the tympanic cavity 

 from the vertical part of the carotid canal. Rarely there are one or more deficiencies 

 in this wall. 



SMALL BONES OF THE EAR. Three small bones (ossicula auditns) are 

 contained in the upper part of the tympanum : of these, the outermost (malkus) 

 is attached to the membrana tympani ; the innermost (stapes) is fixed in the 

 fenestra ovalis ; and the third (incus), placed between the other two, is connected to 

 them by articular surfaces. They form together an angular and jointed connecting 

 rod between the membrana tympani and the fenestra ovalis. 



The malleus or hammer-bone (fig. 1)7), (18 mm. to 10 mm. long, weight 

 24 milligr.), consists of an upper thicker portion, with a tapering lower portion, and 



m 



Fig. 97 A. THE LEFT MALLEUS OF THE ADULT VIEWED FROM THE OUTER SIDE. Magnified four times. 



(After Helmholtz. ) 



c, capitulum ; a i, grooved articular surface for the incus; c, its prominent lower margin ; d, cervix, 

 in, end of the manubrium ; b, processus brevis ; pr.gr, processus gracilis, here represented only by a 

 short stump, the rest of the process having been converted into ligament ; a, ridge to which the external 

 ligament is attached. 



Fig. 97 B. LEFT MALLEUS OF A CHILD VIEWED FROM BEFORE. Magnified four times. (E. A. S.) 



The lettering is the same as in the previous figure. The processus gracilis is here complete. The 

 angle which the manubrium forms with the rest of the bone is seen in this view. 



two processes. The upper end is formed by the rounded head (capitulum) (c), 

 on the posterior surface of which is an elliptical depressed surface (a.i) with 

 prominent margins, which passes obliquely downwards and inwards, and serves for 

 articulation with the incus. The articular surface shows two principal facets, 

 nearly at right angles to one another. They are separated by a constriction and by 

 an obliquely crossing crest or edge. These facets look respectively, the upper and 

 larger one backwards, the lower one inwards ; and each principal facet is subdivided 



