4 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 



to which the original forms and relations of the component bones 

 have been obscured by anchylosis. 



Fowl's Skull.— On plate II are given five views of bird skulls ; 

 fio-ures 1 and 2 being slightly enlarged lower and upper views 

 of the skull of the common fowl. I propose first to take a 

 general survey of the bones shown in these two last-named figures, 

 and then to notice a few respects in which some of these bones 

 are modified in other groups of birds. 



Commencing with the hinder part of the skull, we find that this 

 portion is articulated to the back-bone or vertebral column (pi. I. 

 c.v.) by a single hemispherical condyle* (pi. II. en.,) 

 which is called the occipital condyle, and which is formed 

 entirely from the basi-occipital^ bone (b.o.), which occupies 

 the hinder part of the base of the skull. The presence 

 of this single occipital condyle serves at once to distinguish 

 the skull of a bird from that of a mammal, in which there are 

 always present two condyles, which are formed in great part by 

 the ex-occipital bones (e.o.) ReptilesJ on the other hand, agree 

 with birds in having but one occipital condyle. The hinder part 

 of the skull of the adult bird consists of an apparently single 

 bone, which surrounds the foramen § for the passage of the spinal 

 chord. In the young bird, however, as in the young and fre- 

 quently in the adult mammal, this apparently single bone 

 consists of four completely distinct elements. || In the under 

 view of the fowl-skull (fig. 1) the general position of these four 

 bones is indicated. Below the spinal foramen, we have the basi- 

 occipital (b.o.), extending as far forward as the transverse 

 suturelT which runs across the base of the skull, the two ex-ocei- 

 pitals (e.o.) laterally, and the single supra-occipital (s.o.) 

 above. Occasionally, as in the skull of the common Goose, 

 vacuities occur in the occipital bone above the foramen magnum, 



* Condyle, from KOVOvKoa, a knuckle ; applied in Anatomy to any articular bony 

 surface of a more or less spherical form. The occipital condyle" is so called from being 

 placed on the " occiput." 



f Basi-occipital. The term "os occipitalis," or occipital bone, was originally 

 applied in human anatomy to the large bone, pierced for the passage of the spinal 

 chord, which forms the base of the skull (occiput.) It was subsquently found that 

 this bone really consisted of four anchyloaed bones. Two of them form a pair on 

 either side, and were hence called ex-occipitals {ex, from) : another which forms 

 the top of the occiput was termed supra- occipital (supra, above,) : while a fourth 

 which occupied the base was termed the hasi-occipital {basis, base of) 



J The term reptile in this sketch is used in its scientific meaning, as being equi- 

 valent to the class Eeptiua, and as excluding the frogs and newts, (Amphibia) 

 which are popularly classed as reptiles. 



§ The term for*amen, meaning an aperture, is applied in osteology to any aperture 

 in a bone which serves to transmit a nerve or blood-vessel. The foramen referred to 

 above is \h& foramen magnum, or great foramen, which transmits the spinal chord and 

 vessels to the brain. 



II The term element in osteology is applied to any bone which is formed from a 

 single primitive point of ossification. 



^ Suture, from sua, I sew, is the line of junction of two bones. 



