202 



VERTEBRATE SKELETON 



ossicle of the chain, the incus. This in turn is hinged on the meilleus, 

 a bone with a body and two processes, a manubrium which is attached 

 to the tympanic membrane, and an anterior (Folian) process extend- 

 ing towards the petro-tympanic fissure. This chain of bones trans- 

 mits sound waves across the tympanic cavity from the tympanic 

 membrane to the inner ear through the vestibular fenestra. 



In development the malleus arises by ossification of the hinder end of Meck- 

 el's cartilage, from which it is later cut off and comes to lie within the tympanum, 

 enveloped in the mucosa of that cavity. From its body a manubrium grows 

 downwards and becomes enveloped in the tympanic membrane, while a short 

 part (Folian process) of the Meckelian extends forwards in the gap between 

 petrosal and tympanic bones. The articulation of the malleus with the incus 

 is the same in details as that of quadrate and articulare. 



Fig. 210. — Early ossicula auditus and related parts in A, Crocodilus (Parker, '83) 

 and B, C, lateral and medial sides of parts in 16 mm. human embryo (Bromann, '99). 

 Nerves black, cartilage stippled, membrane bones white; as, stapedial ring; bm, body 

 of malleus; ct, chorda tympani; e, epipterygoid process; /, facial nerve; g, goniale; h, 

 hyoid; /z5, hyostapes; i, incus; Ic, longcrusof incus; m, manubrium mallei; mk, Meckelian; 

 oc, otic capsule; os, otostapes; p, pterygoid process; pd, dorsal process; pq, pterygo- 

 quadrate; sc, short crus of incus; /, tympanic bone. 



The incus, in the early stages, articulates with a ridge on the otic capsule, 

 just as does the quadrate of the Sauropsidan, and the resemblance between the 

 two is strengthened by a close connexion of rather dense tissue (fig. 211) between 

 incus and squamosal. The stapes occupies the same position as in reptiles and 

 unquestionably is the same as at least a part of the reptilian stapes. In reptiles 

 there is no case known of articulation of quadrate and stapes, but such occurs 

 in Urodeles (p. 119) and Gymnophiona. 



The articulare and the proximal part of the reptilian Meckelian are associated 

 closely with two membrane bones, goniale and angulare, the former covering 

 its lower side and extending up on the medial surface, while the angulare is 

 ventral to the goniale and covers the lateral side of the cartilage. In the develop- 

 ing malleus there are two bones (fig. 212) in exactly the same relative positions. 

 Of these the angulare was homologised above with the tympanic of mammals. 



