OSSEOUS SYSTEM. (COMP. AN.\T.) 



835 



attached to the extremity of 

 the upper poition of one of 

 the thiee last branchial 

 arches. 



Condition oft/icos liyoidcs 

 in Reptiles. The condition 

 of the os hyoides in a per- 

 fect Reptile is very simple 

 when compared with that 

 of the Fish, or even, as is 

 most strikingly apparent in 

 the amphibious Reptilia, 

 with that which it exhibits 

 previous to the accomplish- 

 ment of the metamorphosis 

 which changes the mode of 

 respiration from that ofa fish 

 inlo that of the Frog. In the 

 adult Reptile, indeed, the 

 composition of this bone 

 gives no indication of its pre- 

 vious complexity of struc- 

 ture, consisting only of the 

 remains of the anterior cor- 

 nua (26, a) and a broad hatchet-shaped disc 

 forming the body of the bone. In Lizards its 

 structure remains more complicated, resembling 

 that of Birds. The body is generally simple, 

 with two and sometimes three sets of cormia- 

 like appendages connected with it. From the 

 fore part of the body projects along and slender 

 process, more or less cartilaginous, which pe- 

 netrates the substance of the tongue. The ante- 

 rior pair of cornua are variously folded, and the 

 posterior placed differently in different genera; 

 while the third pair, which is but seldom met 

 with, seem rather to be prolongations of 

 the body of the bone than separate elements 

 appended to it. In theChelonian Reptiles the 

 hyoid apparatus varies remarkably in form in 

 different species. It generally consists of a 

 central part, which is frequently itself divisible 

 into several pieces, and of two or sometimes 

 three pairs of cornua. Moreover, beneath the 

 anterior part of the body, there is suspended 

 a bone or cartilage, which is sometimes double 

 and represents the special bone of the tongue, 

 which in Birds is articulated to the fore part 

 of the body of the os hyoides. 



The os hyoides of the Crocodiles is the sim- 

 plest met with in the class of Reptiles, its 

 central portion being a mere broad cartilagi- 

 nous plate, convex below, concave above ; its 

 anterior part having a semicircular form, while 

 its posterior margin is hollowed out into a con- 

 cave edge; there are no remnants of cornua 

 visible, and the os hyoides here seems to per- 

 form the duties of epiglottis, hyoid, and thy- 

 roid cartilage. 



Metamorphosis of the os hyoides. Pro- 

 fessor Bell having already described the most 

 remarkable changes which the branchial appa- 

 ratus of the Frog undergoes during its meta- 

 morphosis, it would have been needless to 

 recur to the subject again in this place, were it 

 not for the purpose of collating the facts there 

 recorded with the series of changes we are now 

 discussing, and indicating the nature of the 

 respective bones delineated in a preceding 



Fix. 4-17. 



volume (vide Article AMPHIBIA, \o\.\. figs. 21, 

 22, 23, 24, 25, 26). We shall, however, embrace 

 the opportunity afforded of adding a few circum- 

 stances to those there recorded, extracted from 

 the observations of M. Martin St. Ange, con- 

 nected with this most remarkable and interest- 

 ing process. 



Some days before the birth of the Tadpole 

 the os hyoides consists of a single median 

 piece, of a pair of broad cartilaginous plates 

 situated on each side of the former, anteriorly, 

 and of two other similar plates occupying a 

 like position behind, to each of which last are 

 appended four separate styliform pieces repre- 

 senting the branchial arches, making thirteen 

 pieces in all. 



Examined a little after birth, the whole car- 

 tilaginous frame-work is found to have increased 

 considerably in breadth, more espscially the 

 eight last-mentioned cartilaginous styles upon 

 which the branchial vessels run, sufficiently 

 indicating their nature moreover, they become 

 united together by their distal extremities so as 

 to form a series of arches, as represented in Jig. 

 21, vol.i. p. 98, ate. At this point of its deve- 

 lopement the hyoid system of the Frog is at its 

 maximum of complexity, and we will therefore 

 pause to examine the elements that enter into 

 its composition. The median piece (Jig. 21, ft, 

 vol. i. p. 98) represents, according to Geoffrey, 

 the gloHSohi/af, basihyal, and vroliyul elements of 

 the Fish. The elements marked will be the 

 stylhyal bone, suspending the whole from the 

 tympanic bone of the skull (e), while the broad 

 pieces c, c, regarded by the same author as 

 being dismemberments of the larynx, imme- 

 diately sustain the branchial arches. 



At that period, when in consequence of the 

 changes that take place in the circulation of the 

 Tadpole the branchial vessels are to be oblite- 

 rated, the condition of the os hyoides too 

 becomes rapidly changed. The cartilaginous 

 arches become diminished, especially in length, 

 and at last become completely absorbed except- 

 ing two remnants, which are found appended to 



3 H 2 



