230 



GENERAL CHARACTERS. BIRDS.- 



JENERAI, CHARACTERS. 



gence in these animals than in those of the preceding 

 class. The bones forming the skull in birds, become 

 completely united together at a very early period of 

 life, so that the whole of the true skull (cranium) 

 usually appears to be composed of a single bony piece 

 without any of those sutures, which, in the Mammalia, 

 mark out the separate bones of which the skull is com- 

 posed. The facial bones, on the contrary, are gene- 

 rally attached to the skull in such a way as to retain a 

 certain amount of mobility; and this is so great in some 

 species as to" give the upper mandible the appearance 

 of being articulated to the cranium. The jaws are 

 prolonged into a beak of variable form, upon the upper 

 surface of which, near the base, the nostrils are almost 

 invariably situated; the internal partition between these 

 is sometimes deficient, when the nostrils appear to form 

 a narrow horizontal slit or passage, leading from one 

 side of the beak to the other. The jaws bear no teeth, 

 but are covered with horny sheaths, the form of which 

 varies remarkably, according to the nature of the food 

 upon which the creature is destined to subsist. The 

 orbits, which are always placed laterally, are seldom 

 completely closed; the partition separating them is 

 often perforated (fig. 140). Beneath them on each 

 side runs a slender process of the upper jaw, called the 

 jttgal bone, which passes backward until it reaches a 

 small bone springing from the hinder part of the skull 

 on each side close to the ear ; these, which are called 

 the tympanic bones, furnish the points of articulation 

 for the two branches of the lower jaw. The occipital 

 bone, situated at the lower part of the back of the skull, 

 exhibits a large aperture for the passage of the spinal 

 cord, and a single condyle or tubercle for the articula- 

 tion of the skull with the first vertebra of the neck. 

 This condyle is always very convex, and sometimes 

 nearly globular a structure which gives to the heads 

 of these animals a great range and facility of move- 

 ment. 



The neck in birds is of greater average length than 

 in mammals, and even in those species which appa- 

 rently have a short neck when clothed with feathers, 

 the cervical region of the skeleton is usually much 

 elongated. The number of vertebrae is never less than 

 nine ; but most birds have from twelve to fifteen of 

 these bones in the neck, whilst some have upwards of 

 twenty. The neck is longest in some of the aquatic 

 birds, such as the swans and flamingoes; the neck of 

 the swan has twenty-seven vertebrae. The. bodies of 

 the vertebras present a convex surface behind, and a 

 concave one in front, an arrangement which gives great 

 freedom of motion ; and in most birds we find the neck 

 capable of describing very sharp curves, whilst the 

 strong lateral processes of the vertebras furnish suffi- 

 cient points of attachment for the muscles which 

 support the neck in its different positions, and enable 

 the bird to dart its head rapidly forward in order to 

 capture its prey. 



Of the dorsal vertebrae there are usually from eight 

 to ten ; but some birds have only six of these bones, 

 while others have as many as eleven. They are gene- 

 rally short and very firmly attached to each other, so 

 as to form a solid column ; great firmness being requisite 

 in this part of the body, in order to give a proper sup- 



port to the wings. In some cases, indeed, the dorsal 

 vertebra? are anchylosed. They are furnished with 

 spinous processes both above and below; the latter 

 serving to give attachment to the lungs, which, as 

 already stated, are affixed to the inner surface of the 

 chest. They are also provided with transverse pro- 

 cesses for the articulation of the ribs, which present 

 two articulating surfaces one of which is applied to 

 the body of the vertebra, and the other to the trans- 

 verse process. The ribs are flat bones composed oi 

 two parts, united by a movable joint (fig. 135) ; the 

 upper part which articulates with the vertebral column 

 is the true rib ; the lower, which is attached to the edge 

 of the sternum, is analogous to the sternal cartilage 

 in the Mammalia. It is by this arrangement that the 

 movements of the chest, necessary for the inspiration 

 and expiration of air, are performed. From the pos- 

 terior surface of each true rib, at a variable distance 

 from its base, there springs a laminar process of greater 

 or less length, which projects backwards and upwards, 

 so as to overlie the succeeding rib just above its laminar 

 process (fig. 134). The object served by these pro- 

 cesses is that of furnishing an additional bond of union 

 between the different ribs, so that the whole framework 

 of the chest acquires a great degree of firmness. The 

 processes are very large in birds possessing great 

 powers of flight (such as the hawks), whilst in those 

 which are not remaikable for this faculty they are 

 usually of small size, or even rudimentary (see fig. 135). 



The sternum, which completes the bony framework 

 of the chest, is a large, more or less triangular bone, 

 concave internally and usually very convex externally, 

 where it is also furnished with a large crest or keel, 

 serving to give attachment to the enormous pectoral 

 muscles, by which the wings are made to strike down- 

 wards upon the air. In birds noted for a very powerful 

 flight, this keel is of great size, as might be anticipated ; 

 whilst, in the ostrich and some other birds whose wings 

 are so small as to be incapable of raising them into the 

 air, the sternal keel is entirely deficient. The edges 

 of the sternum, as already stated, give attachment to 

 the sternal ribs ; at its anterior part it is provided with 

 articular surfaces for the reception of some bones 

 connected with the wings, to which we shall have to 

 allude further on. 



The lumbar vertebrae, which in the Mammalia 

 always retain a certain amount of mobility, are here 

 completely amalgamated together, and with the sacral 

 vertebrae, form a single piece to which the pelvis is 

 attached. The latter is greatly elongated, advancing 

 so far as often to conceal a portion of the last ribs (see 



i. 135, 136) ; but its inferior arch is not closed, as is 

 the case in the Mammalia. The only known exception 

 to this rule is to be found in the African ostrich. The 

 posterior limbs are articulated to the sides of the pelvis 

 by a ball and socket joint, as in the mammals. The 

 sacral vertebrae are succeeded by those of the short 

 tail, which are of small size, and vary from six to nine 

 in number. The last vertebra is usually larger than 

 the others, and often placed so as to rise perpendi- 

 cularly to the axis of the body ; it is to this that the 

 muscles for moving the tail are attached. 



Having shown above how the framework of the 



