278 



AVES. 



the outer side of the internal angular process. 

 This ligament assumes a fibro-cartilaginous 

 structure at its anterior part : it is attenuated 

 internally, and is situated between the two 

 bones in the outer part of the capsular ligament. 

 At the posterior part of the joint a strong 

 fibrous band extends from the end of the mas- 

 toid process to the internal angular process of 

 the lower jaw, so as to restrain the forward 

 movement of the jaw. 



The skull presents fewer varieties of form in 

 birds than in any other class of vertebrate ani- 

 mals. With the exception of a few species, in 

 which the beak assumes what may almost be 

 termed a monstrous development, it has the 

 form of a pretty regular five-sided pyramid, of 

 which the occiput forms the base, and the an- 

 terior extremity of the beak the apex. 



The posterior facet or base of the pyramid 

 is formed by the upper and larger portion of 

 the occiput, together with part of the temporal 

 bones. It is the smallest facet of the head, 

 and is larger in the transverse than the vertical 

 diameter. It presents the vertical prominence 

 corresponding to the narrow cerebellum, which 

 is separated by a venous foramen and furrow 

 (Q,fig. 126) from a broad muscular depression 

 on either side ; below these are the large occi- 

 pital foramen, (1, jig. 126); the hemispheric 

 tubercle, which unites the head to the atlas ; 

 and on either side of this tubercle a smaller 

 muscular depression, separated by a transverse 

 ridge from the larger one above, and per- 

 forated by the pneumogastric and hypoglossal 

 nerves ; these depressions are bounded laterally 

 by the mastoid processes. (10, W,fg. 126.) 



The inferior facet or base of the skull joins 

 the posterior and lateral facets almost at a 

 right angle. It is bounded anteriorly and at 

 the sides by the lower jaw, which, on account 

 of the compressed form and divarication of the 

 rami, scarcely intercepts any part of the view 

 of this very complicated surface. The occipital 

 condyle, with the muscular depressions on 

 either side and the mastoid processes, may be 

 considered in some, and more especially in the 

 Struthious birds, as forming part of the base of 

 the skull. Anterior to the basilar portion of 

 the occiput comes the body of the sphenoid, 

 which in the Struthionida sends outwards and 

 forwards two rounded processes O.K-%- 126 ) 

 to abut against the flattened pterygoid bones. 

 Between the origins of these, and anchylosed 

 to the spine of the sphenoid, the vomer extends 

 forwards to a distance varying in different birds. 

 The tympanic bones are seen on either side of 

 the body of the sphenoid, and external to these 

 the zygomatic processes of the temporal ; the 

 space circumscribed by these bones, with the 

 mastoid processes behind, forms the expanded 

 external passage of the ear, which is closed in 

 the recent state by the large convex membra/no, 

 tympani, (8, 8, fig. 126.) Anterior to the 

 tympanic bones the pterygoid processes (i i, 

 fig. 126) extend forwards and inwards to join 

 the palatine bones; which are then continued 

 forwards to the superior maxillary, leaving 

 between them the large posterior nasal fissure 

 divided longitudinally by the vomer. These 



fissures are commonly continuous with the 

 anterior palatal fissure, (7, 7, fig. 126,) but in 

 the full grown Struthious and some Gallinaceous 

 birds, the palatine and maxillary bones unite 

 with the vomer and separate the two fissures, 

 thus increasing the bony floor of the nasal 

 cavities. External to the rami of the lower 

 jaw, the malar or zygomatic bones may in ge- 

 neral be seen converging from the tympanic to 

 the superior maxillary bones, the elongated 

 triangular space between these bones and the 

 pterygoid and palatine leads directly from below 

 into the large orbits. 



The two lateral facets present posteriorly the 

 tympanic or auditory cavity, (8, Jig- 128,) ante- 

 rior to which is the tympanic bone, with the 

 malar and inferior maxillary bones extending 

 forwards from its lower extremity. Above the 

 tympanic bone is the zygomatic process of the 

 temporal, (d, Jig. 128,) arching over it in the 

 Struthious and Psittaceous birds, as if to effect 

 its normal connection with the malar bone. 

 Between the zygomatic and post-orbital pro- 

 cesses is the crotaphyte depression, (g )t /zg.l28,) 

 always well-marked, but bounded by ridges 

 more or less developed in different birds. At 

 the lower part of this depression may be per- 

 ceived the, large foramen common to the supe- 

 rior and inferior maxillary divisions of the trifa- 

 cial nerve. Then come the spacious rounded 

 orbits, bounded above by the supra-orbital 

 lamella, behind by the sphenoid and frontal ex- 

 pansions, which form, at the same time, the an- 

 terior walls of the cranium ; separated from each 

 other, but always more or less incompletely, by 

 the thin sphenoidal and ethmoidal plates, the 

 deficiencies of which are supplied in the recent 

 state by aponeurotic membranes, and defended 

 anteriorly by the largely developed lachrymal 

 bones and the ethmoidal alee, between which 

 there are always present apertures varying in 

 size. The pterygoid and palatine bones, with 

 the styliform malar bone, form a very incom- 

 plete floor of the orbit. 



Anterior to the orbits the sides of the skull 

 become gradually narrower to the end of the 

 beak ; between the lachrymal and the superior 

 maxillary bones a large triangular or rounded 

 space is left, (11, fig. 128,) which conducts to 

 the nasal cavity. A second vacancy occurs, 

 anterior to this, bounded by the nasal, superior 

 maxillary, and intermaxillary bones, forming 

 the osseous boundary of the wide external 

 nostrils. (12,/g.s. 127, 128.) 



The superior surface of the cranium is gene- 

 rally convex in relation to and indicative 

 of the development of the brain ; it is round- 

 ed posteriorly, where it is generally widest. 

 Here on each side is seen the temporal de- 

 pression : the interorbital space in the Gulls, 

 Petrels, Albatrosses, Penguins, and other sea- 

 birds, presents also two depressions, scarcely 

 less marked, of a semilunar form, the convexi- 

 ties meeting in the mesial line, and lodging a 

 gland, whose secretion is carried into the nose 

 to lubricate the pituitary membrane. Slight 

 traces of these glandular depressions may be 

 seen at 13, fig. 127, in the Ostrich. In other 

 birds the interorbital space is moderately con- 



