840 THE ORGANS OF SENSE. 



the malleus. On the back of the head, below this spur, is an oblique crest, the crista 

 mallei, to which is attached the lateral ligament of the malleus. The neck is the 

 slightly constricted portion immediately below the head. Flattened from before 

 backwards, its lateral surface is directed towards the membrana flaccida, whilst its 

 medial surface is crossed by the chorda tympani nerve. The manubrium or handle 

 is directed downwards, backwards, and medialwards from the neck, forming with the 

 long axis of the head an angle of 126 to 150. Its upper part is flattened from 

 before backwards, but towards the lower end it is twisted on itself, so that its 

 surfaces look laterally and medially ; moreover, the lower end is slightly curved, 

 the concavity being directed forwards and laterally. It is fixed, along its entire 

 length, to the membrana propria of the tympanic membrane by its periosteum and 

 by a layer of cartilage (Gruber). The cartilage intervenes between the manubrium 

 and the membrane, and must be regarded as a residue of that stage of development 

 when the entire malleus was cartilaginous. On the medial surface of the manubrium, 

 near its upper extremity, is a slight projection for the attachment of the tendon of 

 the tensor tympani muscle. The processus anterior, a slender spicule, springs from 

 the front of the neck and is directed forwards, towards the petro-tympanic fissure. 

 In the foetus it is the longest process of the malleus, but in the adult it usually 

 assumes the form of a small projection, since its anterior part is replaced by 

 ligamentous tissue. The processus lateralis may be looked upon as the upper 

 extremity of the manubrium projected laterally ; it is fixed to the upper part of the 

 membrana tympani by the cartilaginous layer already referred to, and to the 

 extremities of the notch of Rivinus by the anterior and posterior malleolar plicse. 



The incus (Fig. 714, A, C) may be likened to a prsemolar tooth with widely 

 divergent roots. It consists of a body, a crus longum, and a crus breve ; the crura 

 form with each other an angle of 90 to 100. The body and crus breve are situated 

 in the recessus epitympanicus. The body presents a more or less saddle-shaped 

 surface for articulation with the head of the malleus. This surface is directed 

 forwards, and its lower part is hollowed out for the accommodation of the cog-tooth 

 of the malleus; in front of this hollow it is prominent and spur-like. The crus 

 breve is thick, triangular in shape, and projects horizontally backwards ; its conical 

 extremity, covered with cartilage, is received into the fossa incudis in the postero- 

 inferior part of the epitympanic recess. The crus longum projects, almost per- 

 pendicularly, downwards from the body into the tympanic cavity, where it lies 

 parallel with, but T25 mm. behind and medial to, the manubrium mallei. Its lower 

 end is bent medialwards and narrowed to form a short neck, on the end of which 

 is a small knob of bone, the processus lenticularis, for articulation with the head 

 of the stapes. Until the sixth month of fcetal life this process exists as a separate 

 ossicle, termed the os orbiculare. 



The stapes (Fig. 714, E) consists of a head, a neck, two crura, and a base. 

 The head, directed lateralwards, is concave for articulation with the processus 

 lenticularis of the incus. The neck is slightly constricted, and from it the two 

 crura spring ; the tendon of the stapedius is inserted into the posterior aspect of 

 the neck. The crus anterius is shorter and less curved than the crus posterius. 

 Diverging from each other, the crura are directed medialwards and are attached one 

 near the anterior, the other near the posterior end of the base. The base almost 

 completely fills the fenestra vestibule, and, like it, is somewhat oval or reniform, 

 its anterior end being the more pointed. In the recent condition a membrane fills 

 the arch formed by the crura and the base, the crura being grooved for its reception. 

 In the child the crura of the stapes are less curved than in the adult, and the 

 opening bounded by them and the base is nearly triangular. 



Articulations of the Auditory Ossicles. The incudo-malleolar joint between 

 the head of the malleus and the body of the incus is diarthrodial, and may be 

 described as one of reciprocal reception. It is surrounded by an articular capsule, 

 and from the inner surface of the fibrous stratum a wedge-shaped meniscus projects 

 into the joint cavity and incompletely divides it. The incudo-stapedial articulation 

 between the processus lenticularis and the head of the stapes is of the nature of 

 an enarthrosis and is surrounded by an articular capsule. An interarticular 

 cartilage has been described as occurring in this joint, while some observers deny 



