388 THE SPECIAL SENSES. 



of spongy material with a compact surface layer. Their general 

 shape and connections are illustrated in Figs. 169 and 170. To 

 understand the manner in which the chain of bones acts in con- 

 veying the vibrations from one membrane to the other some points 

 in their structure and connections may be recalled. The malleus 

 is about 8 to 9 mm. long, and has an average weight of 23 milli- 

 grams. Its long handle is imbedded in the tympanic membrane, 

 the tip reaching to the umbo. The large, rounded head projects 

 above the upper edge of the tympanic membrane and forms a true 



joint of a peculiar nature with the 

 incus. It has two processes in ad- 

 dition to the manubrium : a short 

 one, processus brevis, that presses 

 against the upper edge of the tym- 

 panic membrane, and a longer one, 

 the processus gracilis or processus 

 Folianus, which projects forward 

 and is continued by a ligament, the 

 anterior ligament, through which 

 the malleus is attached to the bony 

 wall of the tympanic cavity. Three 

 other ligaments are attached to the 



Fig. 170. The bones of the lt , ,. J . . 



middle ear in natural connections malleus, the external ligament , bind- 



(Helmholtz): M, The malleus: Mcp, 



the head; Me, the neck; Ml, the mg it to the external face of the 



cavity, the posterior ligament, and 

 ' long Pr cess: S> the superior ligament, the latter at- 



taching the upper part of the head 



to the roof of the tympanic cavity. By means of these ligaments 

 the bone is held steadily in position even after its connections 

 with the incus are loosened. The incus is somewhat more mas- 

 sive than the malleus, weighing about 25 milligrams. Its thicker 

 portion articulates with the head of the malleus, and it has two 

 processes nearly at right angles to each other. The shorter process 

 extends posteriorly and is attached by a ligament to the posterior 

 wall of the tympanic cavity; the long process passes downward 

 parallel with the handle of the malleus, but turns in at the tip 

 to form the rounded os orbiculare, which articulates with the 

 head of the stapes. This latter bone is extremely light, weighing 

 about 3 milligrams, its oval base being attached to the margins 

 of the fenestra ovalis by a short, stiff membrane. 



The Mode of Action of the Ear Bones. The movements of 

 the tympanic membrane are communicated to the tip of the handle 

 of the manubrium. As the handle moves in, the chain of bones 

 makes a rotary movement around an axis which may be defined as 

 the line passing through the attachment of the short process of the 

 incus and the anterior ligament of the malleus. The general posi- 



