. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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) o


. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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


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