AVES. 



274 



energetic respiratory and circulating functions 

 occasion so rapid an evolution of the osseous 

 system, that the bones of the cranium become 

 at an early period anchylosed into one piece, 

 with a total obliteration of the original har- 

 monic; it is necessary, therefore, to examine 

 the skull of the Bird at an early period of ex- 

 istence, and to compare it with the foetal con- 

 dition of the skull of the Mammal, when it will 

 be found to be ossified from analogous centres, 

 which, in their expansion and subsequent 

 union, obey the same laws of, as it were, 

 elective attraction. 



The occipital bone is originally composed 

 of four pieces: the basilar, below, (a, fig. 126,) 

 the two cond>/loid, laterally, (b, b,) and the 

 expanded spinons process, or supra-occipital 

 piece above (c). These fulfil the usual 

 functions of the occipital bone, protecting the 

 cerebellum and medulla oblongata, and form- 

 ing the medium of connection between the 

 cranial and cervical vertebrae. 



The head is articulated to the spine by 

 means of a single hemispherical tubercle (x, 

 fig. 126,) which plays in a corresponding 

 cavity of the atlas. In most birds the tubercle 

 is formed exclusively by the basilar piece of 

 the occipital bone, but in the Ostrich and 

 Penguin the condyloid portions also contribute 

 to its formation, which is an approximation to 

 the structure of the occipital condyle in the 

 Chelonian reptiles. In all birds, however, the 

 articulation is such as to allow of a much greater 

 extent and freedom of motion to the head than 

 exists in the Mammalia. 



The temporal bone consists of tt\e petrous por- 

 tion, the squunwus portion, (d, d,fig. 126, 127) 

 and the ti/mpanic bone, or os quadratum (e.) 

 The petrous bone includes the complex parts 

 of the internal ear, and is soon anchylosed to 

 the condyloid portions of the occipital bone, 

 which fulfil the functions of the mastoid pro- 

 cesses. The squamous, or, as it may be termed, 

 the zygomatic portion of the temporal bone (d) 

 remains for a longer time separate; it forms 

 the lateral boundary of the cranial cavity, as in 

 quadrupeds, and the tympanic element is move- 

 ably articulated to its inferior part. 



The parietal bones (f, f, fig. 127) retain 

 their separated condition till after the union of 

 Ihc occipital pieces, they then unite and protect 

 the posterior part of the cerebral hemispheres. 



The sphciwld In me is composed of a basilar 

 portion, (g,fg. 1 26,) two orbital plates, (h,fg. 

 127,) forming the floor and part of the septum 

 of the orbits, and which rapidly anchylose with 

 the preceding; two cranial portions, or ata ma- 

 jores, (g, fig. 127, 128,) which remain longer 

 separate, and form the posterior part of the or- 

 bits, and two pten/goid portions ('interarticular' 

 or ' omoid ' bones), (i, i, fig- 126,) which, in 

 birds, abut against the tympanic or quadrate 

 bones. The great alac of the sphenoid join the 

 parietal, and separate the temporal from the 

 frontal bones. 



The./hw/w/ bone, (k, fig. 127) continues for 

 a longer period than the parietal to be sepa- 

 rated into two lateral halves by the continuation 

 of llic -a^inul -uiUirc through its whole length. 



Fig. 127. 



The ant-orbital 

 processes C13, 

 Jig. 127) are 

 elongated and 

 pointed, ex- 

 tending for- 

 wards to join 

 the lachrymal 

 bones, (o, o, 

 fig. 127,) con- 

 siderably be- 

 yond the ori- 

 gins of the nasal 

 bones, and are 

 separated from 

 each other by 

 these and by a 

 process of the 

 ethmoid bone. 

 The post-orbi- 

 tal processes 

 are most de- 

 veloped in the 

 Parrots and 

 Maccaws, in 

 the latter of 

 which they join 

 Skull of a yowuj Ostrich. the lachrymal 

 bones, and complete the bony circumference 

 of the orbits, (fig. 128.) In the Emeu they 

 remain for a long time distinct bones, as in the 

 reptiles. The frontal bone thus forms the 

 whole of the superior, and, more or less, of the 

 outer boundary of the orbits, and protects the 

 anterior part of the cerebrum. It supports 

 the horn-like prominences which are seen upon 

 the heads of the Cassowary, Pintado, and Cu- 

 rassow, the bony bases of which commence by 

 distinct ossifications.* 



A small part of the ethmoidul bone (I, 

 fig. 127) is seen, in the Ostrich, on the ex- 

 'terior of the cranium lodged between the 

 ant-orbital processes and nasal bones (n, n.) 

 The ethmoid separates, as usual, the orbits from 

 the cavity of the nose, and forms a great part of 

 the inter-orbital septum where this exists, as 

 in the parrots, (in, fig. 128.) 



In the mature bird the whole of the prece- 

 ding bones, with the exception of the tym- 

 panic elements of the temporal bone, are 

 usually found anchylosed into one piece. 



The internal surface of the cranium exhibits 

 a well-marked transverse ridge, which divides 

 the cavity into two principal depressions. 

 In the anterior division the hemispheres of 



* A most remarkable sexual difference appears 

 in the skull of the crested Hens: in these the frontal 

 portion of the cranium is dilated into an immense 

 cavity, on which the crest of feathers is placed. 

 This 'degeneracy of the formative impulse, which 

 is propagated to the offspring, is quite unparalleled 

 in the whole animal kingdom : I have lately ex- 

 amined several heads of such hens in a fresh state, 

 and have found that this peculiar dilatation of the 

 cranium is filled hy the hemispheres of the cere- 

 brum, and is separated from the posterior part 

 which holds the cerebellum, as in the common 

 hen, by an intermediate contracted portion.' Law- 

 rence'i iilnmnibnfli'.s &/>. Anut. p. 61. 



