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Show 178 ON THE STlHJC'rUHE OF THE SJ<ULI,. n·o r1 y 1· n t o tl1 e 1,e -e1-1tei·ing angle between the vomer a11(l t.h o pre-1nax1' lla anc1 rna x1'lla· It is usually regarded aR the pa1atmo bone. · 1 · f 1 Before attempting to discuss the boino ~g1 s o t 1e~e s~voral t ·t . t of tl1e IJalato-qnadrate arch 111 the fish, 1t will be CODSJ uen S · c necessary to take into account the nature and arrangement of its post-oral facial bones. . In Man, the post-oral bones ar arranged. 111 two ar~b os~the 1uandibular and the hyoid an; or, more tn tly. sp~akm g, sm?c the hyoidean arch is r ally compos d of two, 1nd1cated by 1ts lesser and greater cornua, there are thr .e 1:0, t-oral arches .. In the Pike, the lower jaw forn1s a rnnncbbu1ar arch, obv10usly corresponding in a o· neral w<::y with t1at f .1\Ian ; behind this follows a second arch, auswenng to the . tyl01d processes, stylohvoid licraments, and cornua 1ninora f th human hyoid, but ., b • . . . . with much greater n1asses of b n - entenng 1nto 1ts composition; and this is succeeded by no fewer than fiv0 oth r ar ·hcs, the fonr anterior of which, supporting th gill fihun nt , arc termed the "branchial arches," whi1e the last pair, which carry no branchiro, and are much smaller than the ther , are called the " inferior pharyngeal bones." . . . The ymphysis of th lower JaW 1s form cl Ly the hgament_ons union of two bones, whi h arry the inferior t oth of tho P1ke, and correspond re pe tiv ly with the rami of the human mandible. But, b ides the denta1·y bones ( D, Fig. 65), each half of the lower jaw of the Pik ha. two other con titucnts, which.are not represent d in th hum, n low r jaw. One of these IS a small bone which forms tb 1 w r part of the angular process of the jaw. It is termed th os angulare, or angular ~ieee (A~, Fig. 65). The other is a large triangular bone, winch ~ts m betweeu the dentary and th angular, and is t rmed the articular ( os a'rticulare, A1·.), becau on its upp r surface it bears a concave articular fo a, into whi ·h the nclyle of the quadrate bone is rec ived (Fig. 71). The artilage which partially forms ~he walls of this fossa is continu d into a long tapering rod, whiCh lies upon th inner surface of tho arti ular and of the dentar~, and tenninate in a point hortly befor"' roaching the symphysis (Mck., Fig. 71). THE STRUCTUH.E OF THE PIKE'S SKULL. 179 This rod of cartilage affords a safe basis upon which to found a homological argumentation. For it most certainly corresponds with Meckel's cartilage in the human fcetus, and the dentary bone lies outside it, in just the same 'vay as the dentigerous ramus of the human Inaudible lies outside Meckel's cartilage. But the articular bone is an os ification in and around the proximal end of Meckel's cartilage in the Pike, just ns the malle~ts is · an ossification in and around the proxi1nal end of Meckel's car·tilage in the human fcetus ; and the os quadr-atum is related to tho os articttlare of the fish in the same way as the incus is related to the malleus. Hence it is to be concluded, in the absence of any evidence to the contrary, that the articular piece of the Pike's lower jaw answers to the malleus, and the quadrate bone to the incus. I am not aware that any evidence can be adduced against this view; but, on the other hand, the relations of the parts thus identified to the portio dura of the seventh nerve, in Man and in the Fish, seem to me to afford it much support. The portio dura in the former perforates the pars petrosa, and, after skirting the inner wall of the tympanum, external to the labyrinth, leaves the skull by the stylo-mastoirl foramen. Before it does so, however, it gives off a recurrent branch, the chorda tympani, which takes a very singular course-passino· between the pyran1id, which is the upper end of the hyoidea~ arch, and the tympanic bone, entering the tympanum, crossinO' the auditory ossicles to make its way out at the front wall of th: tympanum, between the tympanic and the squamosal then uniting with the .gustat~ry division of the trigeminal, and ~assing down along the Inner side of the ramus of the mandible with it. until eventually it leaves it to become connected with the sub~ maxillary ganglion. The. principal portion of the portio dura, on the other hand, 1nakes Its way out by the stylo-mastoid foramen, and is distributed to the facial muscles, some comparatively insignificant branches only, being furnished to the levators of the hyoidean apparatus and depressors of the lower jaw. But, as has been already s.tated, the facial muscles, so important and largely developed In Man, become insignificant in the lower Vertrbrates, N 2 |