Page:EB1911 - Volume 25.djvu/200

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184
SKELTON

segments, the ceratohyal laterally and the basihyal ventrally where it fuses with its fellow of the opposite side. Sometimes an epihyal intervenes between the hyomandibular and the ceratohyal. Behind the hyoid arch are usually five branchial arches, though in Hept- anchus there are as many as seven. These are divided into a number of segments, and outside these there is often another series of arches called extra-branchials which are probably homologous with the branchial basket of the Cyclostomata.

The chimaeroid fishes are called Holocephali because in them the palato-quadrate bar is fused with the rest of the skull. In the bony ganoids and teleosteans (Teleostomi) the palato-quadrate bar ossifies to form the palatine, ecto-, meso- and meta-pterygoids and quadrate bones from before backward, while outside these is another row of dermal bones formed by the premaxilla, maxilla and jugal or malar.

In the lower jaw, Meckel's cartilage is ossified at its proximal end to form the articular bone, but distally it remains and is partly en- cased by the dentary, and more posteriorly by the angular, both of


Fig. 34.—Longitudinal and Vertical Section of the Skull of a Dog (Canis familiaris), with mandible and hyoid arch.

Anterior narial aperture.

Maxillo-turbinal bone.

Ethmo-turbinal.

Nasal.

Ossified portion of the raes- ethmoid.

Cribriform plate of the ethmo-turbinal.

Frontal.

Parietal.

Interparietal.

Supra-occipital. ExO Ex-occipital. BO Basi-occipital.

Periotic.

Basi-sphenoid.

Pterygoid.

Alisphenoid.

Orbito-sphenoid.

Presphenoid.

Palatine. Vo Vomer. Mx Maxilla. PMx Premaxilla.




Stylo-hyal.

Epi-hyal.

Cerato-hyal.

Basihyal.

Thyro-hyal.

Symphysis of mandible.

Coronoid process.

Condyle.

Angle.

Inferior dental canal.

which are membrane bones quadrate and the articular.

The mandible is displaced down- wards, to show its entire form; which the condyle is articulated.


The jaw joint therefore is between the In comparing this description with the section on human embryology it will be seen that certain bones, like the palate and pterygoids, which in the fish are ossifications in cartilage, become in the higher vertebrates membrane bones, and so it is clear that too great stress must not be laid on the histological history of a bone in determining its morphological significance.

The branchial arches of the Teleostomi closely resemble those of the Elasmobranchii except that they are ossified and that the extra- branchials have disappeared.

In the Dipnoi (mudfish) the suspensorium is autostylic, and either five or six branchial arches are present. In the Amphibia, too, the suspensorium is autostylic, the palato-quadrate bar remains largely cartilaginous, though its posterior part is often ossified to form the quadrate. The membranous premaxilla, maxilla, palatine, pterygoid, yuadratojugal and squamosal bones are developed in connexion with it, though it is interesting to notice that the pterygoid is sometimes partly cartilaginous and the quadrato- jugal is absent in the tailed forms (Urodela). In the lower jaw a splenial element has appeared, and in the frog a cartilaginous mento-meckellian bone develops close to the symphysis. In the larval stages there are rudiments of four branchial arches behind the hyoid, but in the adult these are re- duced in the Anura and their ventral ends are united into a broad basilingual plate.

In the Reptilia the site of the palato-quadrate bar is surrounded by the same series of bones that are found in the Amphibia, but in lizards and chelonians a para-quadrate bone is found which, according to E. Gaupp, is the precursor of the tympanic ring of mammals. In the crocodiles the maxilla and palate grow inwards to meet one another and so form a hard palate. The mandible has dentary, splenial, angular, surangular, articular and coronoid ossifications and in some cases a mento-meckellian as well. The quadrate bone with which it still articulates is becoming included in the wall of the tympanic cavity, and, according to H. Gadow, it is this bone and not the para-quadrate which will become the tympanic of mammals. The hyoid arch is sometimes suppressed in snakes, but in Sphenodon its continuity with the columella or stapes can be demonstrated.

The branchial skeleton is reduced with the cessation of branchial respiration and only the ventral parts of two arches can be seen; these unite to form a plate with the hyoid (basihyobranchial) and with this the glottis is closely connected. In birds the morphology of the visceral skeleton is on the reptilian plan, and, although the modifications are numerous, they are not of special interest in elucidating the problems of human morphology.

In the Mammalia the premaxilla, maxilla, palate and pterygoid bones can be seen in connexion with the region where the palato- quadrate cartilage lay in the lower Vertebrata (see fig. 34). The premaxilla bears the incisor teeth, and except in man the suture between it and the maxilla is evident on the face if a young enough animal be looked at. The maxilla bears the rest of the teeth and articulates laterally with the jugal or malar, which in its turn articulates posteriorly with the zygomatic pro- cess of the squamosal, so that a zygomatic arch, peculiar to mammals, is formed. Both the maxilla and palate form the hard

Calate as in crocodiles, though the pterygoid bone does not do so ut fuses with the sphenoid to form the internal pterygoid plate (see fig. 34, Pt). The mandible no longer articulates with the quadrate but forms a new articulation, by means of the con- dyle, with the glenoid cavity of the squamosal, and many modern morphologists, including the writer, are inclined to agree with H. Gadow that the quadrate has probably become the tympanic bone. In many mammals (e.g. Carnivora) this bone swells out to form the bulla tympani. The derivation of the auditory ossicles has been discussed in the section on embryology as well as in the article Ear. The presence of a chain of ossicles is peculiar to the Mammalia.

In many of the lower mammals (e.g. Ungulata and Carnivora) the hyoid arch is much more completely ossified than it is in man, tympano-, stylo-, epi-. cerato- and basihyal elements all being bony (see fig. 34). It is of interest to notice that in the hares and rabbits the body of the hyoid has occasionally been found in two pieces, indicating its derivation from the second and third visceral arches. The fourth and fifth arches, which form the thyroid cartilage in mammals, are considered in the article Respiratory System.

For further details see S. H. Reynolds, The Vertebrate. Skeleton (Cambridge, 1897); W. Flower, Osteology of the Mammalia (London, 1885); R. Wiedersheim, Comparative Anatomy of Vertebrates, adapted and translated by W. N. Parker (London, 1907) ; C. Gegenbaur, Vergleich. Anal, der Wirbeltiere, Bd. i. (Leipzig, 1901). (F. G. P.)


SKELTON, JOHN (c. 1460–1529), English poet, is variously asserted to have belonged to a Cumberland family and to have been a native of Diss in Norfolk. He is said to have been educated at Oxford. He certainly studied at Cambridge, and he he is probably the “one Scheklton” mentioned by William Cole (MS. Athen Cantabr.) as taking his M.A. degree in 1484. In 1490 Caxton writes of him, in the preface to the Boke of Eneydos compyled by Vyrgyle, in terms which prove that he had already won a reputation as a scholar. “But I pray mayster John Skelton,” he says, " late created poete laureate in the unyversite of Oxenforde, to oversee and correct this sayd booke . . . for him I know for suffycyent to expowne and englysshe every dyffyculte that is therin. For he hath late translated the epystlys of Tulle, and the boke of dyodorus siculus,[1] and diverse other works ... in polysshed and ornate termes craftely . . . I suppose he hath drunken of Elycons well." The laureateship referred to was a degree in rhetoric. Skelton received in 1493 the same honour at Cambridge, and also, it is said, at Louvain. He found a patron in the pious and learned countess of Richmond, Henry VII.’s mother, for whom he wrote Of Marines Lyfe the Peregrynacioun, a translation, now lost, of Guillaume de Deguilleville's Pelerinage de la vie humaine. An elegy " Of the death of the noble prince Kynge Edwarde the forth," included in some of the editions of the Mirror for Magistrates, and another (1489)

  1. The MS. of this translation is preserved at Corpus Christi College, Cambridge.