THE AUDITORY OSSICLES. 91 



of os orbicular* seu lenticulare, described as a separate bone, which indeed it 

 originally is in the foetus up to the sixth month. 



The length of the short process is about 3 to 3J mm., of the long process about 

 4J mm. The weight of the incus is very nearly the same as that of the malleus 

 (Blake). 



At the joints between the incus and malleus on the one hand and the incus and stapes on 

 the other the articular surfaces are tipped with cartilage and enclosed by a synovial membrane. 

 Riidinger describes both in this joint and in the articulation of the incus with the stapes an 

 iiiterarticular fibro- cartilage which subdivides the joint into two parts ; but according to 

 Brunner neither are synovial joints but are symphyses, the articular cartilages being united 

 by fibrous tissue. At all events the existence of an interarticular cartilage at the joint between 

 incus and stapes is doubtful, although most authorities admit its presence at the joint between 

 the malleus and incus. Some anatomists describe a synovial joint at the articulation of the 

 short process of the incus with the bone at the entrance to the mastoid cells. 



The stapes (figs. 00, 100), the third and innermost bone of the ear, is in shape 

 remarkably like a stirrup, and is composed of a head, a base, and two crura. The 

 whole bone measures 3 to 4 mm. in length and about 2J mm. in breadth. Its 

 weight is from 2 to 4 milligrams. The head (h) is directed outwards, and has 

 on its end a slight depression, covered with cartilage, which articulates with 

 the lenticular process of the incus. The lase () is a plate of bone fitting into 



Fig. 99. LEFT STAPES, VIEWED FROM BELOW. Magnified four times. (E. A. S.) 



A, outer extremity or head of the bone, with a shallow concavity for articulation 

 with the orbicular process of the incus ; c, constricted part or cervix. This is not 

 always so well-marked as in the present specimen, cr a, anterior crus ; cr p, 

 posterior crus ; b, base ; a, arch of the stapes. The bony .groove which bounds the 

 arch is shown in front and below ; above and behind it is concealed from view. 



the fenestra ovalis, but not quite closely, so that a slight amount of movement is 

 allowed. Its form is irregularly oval, the upper margin being curved, while the 

 lower is nearly straight (fig. 00, st). Its border is encircled by hyaline cartilage, 

 which also covers its vestibular surface. The margin of the fenestra ovalis has also 

 a covering of the same tissue (Toynbee), and the opposed cartilaginous surfaces are 

 closely connected by a network of elastic fibres passing between them, and forming 

 an especially dense ligamentous band near the tympanic and vestibular cavities 

 (Riidinger). The crura of the stapes diverge from a constricted part (neck, 

 fig. 00, c) of the bone, situated close to the head, and are attached to the outer 

 surface of the base near its extremities. The anterior crus (cr. a) is the shorter and 

 straighter of the two. The crura, with the base of the stapes, encircle a small 

 triangle or arched space (#), across which in the recent state a thin membrane is 

 stretched. A shallow groove runs round the opposed surfaces of the arch, and into 

 this the membrane is received. 



The formation and morphological relations of the auditory ossicles have already been 

 noticed in the Embryology (Vol. I., p. 167, et seq."). Suffice it here to recall the fact that 

 the incus and malleus are originally laid down as one piece of cartilage, which is continued 

 forwards as Meckel's cartilage along the first visceral arch, and that the stapes is formed by 

 ossification in cartilage which develops around an artery (stapedial or mandibular Fraser, 

 Salensky), which arises from the internal carotid, and passing into the tympanum through the 

 wall of the carotid canal, ascends over the promontory and anastomoses with branches of the 

 stylo-mastoid, 'middle meningeal, and ascending pharyngeal arteries. In rare instances this 

 artery remains, but it has usually disappeared before birth. In some animals (Cheiroptera, 

 Insectivora, Rodentia) it persists in the adult. In these cartilaginous foundations of the 

 auditory ossicles the formation of the individual bones occurs in the following manner : the 

 separation and articulation between the malleus and incus appears in the third month of 

 embryonic life ; the cartilaginous continuity between the malleus and Meckel's cartilage 

 disappears somewhat later, and its place is taken partly by ligamentous tissue (forming the 



