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Show 688 MR. G. E. DOBSON ON THE [June 7, hyal and ceratohyal bones being connected with the base of the tongue by the membranous walls of the pharynx alone. The hyoid bone (figs. 1, 2, 3) is remarkably formed : the basi-hyal is ankylosed behind to a pair of long spatulate thyrohyals ; in front it articulates on either side by a synovial joint, permitting very free rotatory motion, with a cartilaginous ceratohyal, which also articulates by its upper extremity with a large, very peculiarly shaped cartilaginous epihyal, which is circular in outline, having its inner side flat or slightly convex, its outer deeply concave from below backwards and upwards, and near its articular extremity a prominent external projection, across which the tendons of the mylohyoid and hyo-glossus muscles, above described, glide ; the ceratohyal muscle extends from its posterior margin to the thyrohyal bone ; and the stylo-hyoid muscle is inserted into its upper side. Its outer surface, as above described, is deeply concave in the adult male, and is lined by part of the neck of a sac, which extends outwards and backwards from the pharynx, under cover of the integument and the sterno-mastoid muscle, across the clavicle to the anteroinferior part of the thorax. Another sac, anterior to this, on each side of the neck, also extends outwards from the pharynx, and will be described further on (vide infra, fig. 4). The walls of the evidently highly extensible pharynx are attached anteriorly to the base of the skull and to the hack of the fauces and tongue (which is free between the hyo-glossi and stylo-glossi muscles; for there is no trace of genio-hyoid or genio-hyo- glossi muscles, and the body of the organ appears to be chiefly made up of a few muscular fibres with much interposed fat), extending forwards for some distance between its inferior surface and the mylo-hyoid expansion-posteriorly, to the anterior margins of the basi-, cerato-, and epihyal bones, and laterally, by an oblique ligamentous band, to the sterno-mastoid muscles. Immediately behind the mouth, in the adult male, are two large oval apertures (0*4 inch in antero-posterior diameter) in the sides of the pharynx, leading into a pair of large sacs, which extend outwards under cover of the integument beneath and behind the ears. The apertures open opposite each other; so that, if the sacs he cut open from without, the pharynx appears to be transfixed. These, which may be called the anterior pharyngeal sacs, are separated on each side of the neck from the posterior pharyngeal sacs (above described) by the sterno-mastoid muscle, and by a ligamentous septum passing inwards from it to the side of the pharynx, and outwards to the integument. Both pairs of sacs are absent in the female ; their presence therefore indicates a secondary sexual character, of which the nearest analogue in Mammals appears to be the laryngeal sac of the Orang1. 1 It is an interesting fact that the nearest known analogues of these pharyngeal sacs are to be found not among Mammals but in Birds, asin certain species of Otis and Tctrao. This might lead us to think that this peculiar secondary sexual character was in some way related to the volant powers of the animals. It must, however, be recollected that these birds are noted for their terrestrial habits. |
