i68 



NA TURE 



\ytdy 2, 1874 



of the hyoid arch as a chain of auditory ossicles, for the 



purpose of communicating the vibration of the tympanic 

 membrane to the stapes. 



Certain noteworthy peculiarities may be mentioned, with 

 regard to the investing bones, the chief being the fusion 

 of the parietal and frontal into a single bone (Fr.Pa), the 

 dagger-like form of the para-sphenoid, and the addition of 

 a horizontal bar to the upper end of the squamosal which 



J-ig. JX 



Fig. 13.— Skull of fully adult A.volotl, under view (x 2 diam.), the investing 

 bones being removed from the right side. I.N, inter-nasal plate ; p, 

 pedicle, a, ascending process, and o, otic process of the suspensorium. 



seems to answer to one of the bony plates developed in 

 ganoids in the temporal region, while the vertical portion is 

 clearly the homologue of the prc-opercular. An extremely 

 small membrane-bone is also developed in connection 

 with the external nasal opening : this is the septo- 

 maxillary (S.lNIx), which is interesting from its reappear- 

 ance in lizards, snakes, and birds. 



In the cartilaginous brain-case the form of the trabe- 

 cula; is entirely lost by the complete union of those arches 

 below, so as to form a solid floor of cartilage within the 

 para-sphenoid, and by the formation of a roof of lilce cha- 

 racter beneath the fronto-parielals : the latter is inter- 

 rupted by a large anterior and a pair of small posterior 

 fontanelles. Just behind the inter-nasal plate a £tout dice- 

 box-shaped ossification is developed (G) overlaid above 



J-'-'j II 



Via. I4.-Skull of Common Frog (x s). Ty, tjmpanic 



annulu:, tympaiiicu ': .\l..\!cl;, mento-metUlian. 



by the fronta's and below by the para-sphenoid ; this is 

 the girdle-bone (" os en ceinture " of Cuvier), and answers 

 to the hinder part of the ethmoid, the fore part of the 

 pre- and orbito-sphcnoids, and the pre-frontals. In its 

 posterior half this bone contains a single cavity, in which 

 are lodged the olfactory lobes of the brain, but in its anterior 

 moiety a veitical partition (mescthmoid) divides it into 

 two chamber.':, through which the nerves of smell pass to 

 the nasal sacs. 



Only a single bone occurs in the auditory capsule — the 

 prootic, which extends backwards, so as almost to meet 

 the exoccipital ; the opisthotic, epiotic, and stapes remain 

 entirely cartilaginous. 



The palatine (Fig. 15, Pi) is a slender bone not provided 

 with teeth ; the pterygoid is 3-ranged, having an anterior 

 process coming into relation with the palatine, a posterior 

 articulating with the auditory capsule, and a descending bar 

 which runs along the inner side of the suspensorial carti- 

 lage ; the two latter help to inclose the eustachian open- 

 ing (Eu). The suspensorium does not present that clear 



Fig. 15. — Skull of Frog, under i 

 the right side. P.N, poiterioi 



2). the investing bones removed from 

 Eu, aperture of eustachian tube. 



division into pedicle, ascending process, and otic process 

 which is observable in the axolotl ; the second of these 

 is, in fact, represented only by fibrous tissue, while 'the 

 pedicle and the otic process are completely fused with the 

 auditory capsule. 



There is no articular bone in the mandible, but an in- 

 teresting ossification (M.Mck) of IMeckel's cartilage takes 

 place at the point of union of the two rami. This is the 

 symphysial ossification or "mcnto-meckelian'' bone ; it 

 has been found in the sturgeon and also in early stages 

 of the human subject. 



The hyoid arch is divided into two portions, an 

 upper, which subserves the function. of hearing, and a 

 lower, which supports the tongue. The first of these 

 (Fig. 16) is a hammer-shaped apparatus, partly cartilagi- 

 nous, and partly bony, the handle of which articulates with 

 the stapes (St), while the head is fitted into the drum- 

 membrane (Fig. 14, Ty).* The parts of this ossiculum 

 auditus have been named by Prof. Huxley, in their rela- 

 tion to the stapes, inter-, medio-, extra-, and supra- 

 stapedial ; taken together they answer to the hyo-mandibu- 

 lar and symplcctic of a fish. The medio-s'.apedial (M.St) 

 is ossified ; the other portions of the apparatus are 



Fig. iG —Ear-bones of Frog ( x 4). i.st, iiiter-stapedlal ; m.st, medio- 

 stapedial ; e.st, extra-stapedial ; s.st, supra-st.ipedial. 



cartilaginous. The tongue-cartiLtge is a shield-shaped 

 plate consisting of basi-hyal ia its anterior and basi- 

 branchial in its posterior part, and connected with the 

 skull by two slender, spring-like rods, the stylo-liyals 

 (St.Hy), which are fused with the auditory capsule ; these 

 answer to the anterior or lesser horns of the hyoid bone 

 of man, the greater horns being represented by the ossi- 

 fied first hypo-branchials or thyro-hyals (H.Br, i) which 

 embrace the larynx. 



* The annulus tympanicus (A.T). or ring of cartilage which supports the 

 drum-membrane, would seem to answer rather to the external ear of a mam- 

 mal than to the tympanic bone. 



