SKELETON. 



159 



Studies of the development of these ossicles in amphibia, sau- 

 ropsida, and mammalia. 



In the urodeles, where these elements first appear, the lateral 

 wall of the otic capsule is interrupted by an opening, the fen- 

 estra ovalis, in which a plate, the stapes, is supported by mem- 

 brane. In several urodeles and in all cascilians this stapes is 

 connected with the quadrate by means of a stapedial process 

 (see Fig. i6i, s, sp). This may be called the urodele type ; and 

 it is to be noted that here no tympanum (p. 73) occurs, the 

 first postoral visceral cleft undergoing reduction in -develop- 

 ment. 



In the anura and sauropsida the tympanum is well developed ; 

 and this is crossed by a rod, the columella, often differentiated 

 into three parts, which 

 reaches from tympanic 

 membrane to stapes, 

 which is situated as in the 

 urodeles. This columella 

 serves to conduct sound 

 waves across the tympanic 

 cavity to the internal ear. 

 In development it arises 

 behind the tympanum, 

 and when fully developed 

 it is bound to the pos- 

 terior wall by membrane. 

 It is therefore clearly 

 postspiracular in charac- 

 ter, and its connection with the ventral portion of the hyoid 

 (Fig. 168) indicates that possibly it is to be homologized with 

 the hyomandibular of the pisces. In these groups the quadrate 

 acts as a suspensor of the lower jaw, and has only a ligamental 

 connection — no articulation — with the stapes or columella. 



In the mammals two ^ ossicula intervene between the tympanic 

 membrane and the stapes. The more internal of these is the 

 incus, the outer the malleus. As will be remembered, the lower 



Fig. 168. Diagram of auditory ossicles and 

 related parts in the sauropsida, based on em- 

 bryos of Sieleporus tindulaius. c, columella; 

 ctj chorda tympani ; /, facialis ; h, hyoid ; hi, 

 hyoid branch of facialis; m, Meckel's dartilage; 

 fiiii, mandibular branch of facialis ; q, quadrate ; 

 s, stapes; /, tympanum. 



1 Frequently a third element is mentioned, the os orbiculare or os lenticulare, which 

 arises in the ligament tietween incus and stapes. 



