GENERAL CHARACTERS. BIRDS. GENERAL CHARACTERS. 



231 



chest in birds is arranged, so as to give it the firmness 

 necessary to bear the strain thrown upon it in the 

 action of flight, we may now proceed to the considera- 

 tion of the structure of the wings, and the mode in 

 which they are supported. From the articular sur- 

 faces already mentioned as existing on the anterior 

 part of the sternum, there spring two large and strong 

 bones, which are directed upwards and forwards, at the 

 same time diverging more or less from each other. 

 These are the coracoid bones, which, in the Mammalia, 

 with the exception of the monotremata, are reduced to 

 a rudimentary condition, and anchylosed to the upper 

 part of the shoulder-blade. The superior part of each 

 coracoid bone is furnished with an articular surface, 

 which assists in the formation of the shoulder joint. 

 The remainder of this socket is formed by the ex- 

 tremity of the scapula or shoulder-blade, which rests 

 against that of the coracoid bone. The scapula is 

 usually elongated and rather slender, and is applied 

 upon the dorsal surface of the ribs, where it lies nearly 

 parallel to the vertical column. From the position 

 and strength of the coracoid bones, they are evidently 

 well adapted to furnish firm points of support for the 

 wings ; but they are further assisted in this office by 

 the clavicles or collar bones, which are usually anchy- 

 losed to each other in the median line, so as to form a 

 single V-shaped bone called the furculum* The 

 other extremities of the clavicles are articulated to 

 the inner surfaces of the superior extremity of the 

 caracoid bones, to which they serve as supporting 

 buttresses, and thus assist materially in resisting the 

 action of the powerful muscles of the wings. The 

 bones of the wings themselves are easily recognized as 

 corresponding with those of the anterior members in 

 the Mammalia. The humerus, or arm-bone, is a long 

 cylindrical bone articulating with the shoulder-blade 

 and coracoid by a ball and socket joint, and presenting 

 at its lower extremity a double articular surface for the 

 reception of the two bones of the fore-arm. The latter 

 are usually longer than the humerus, cylindrical in 

 form, and thickened at the two extremities ; but one 

 of them the ulna is always much stouter than the 

 other the radius which is generally very slender. 

 The carpal bones forming the wrist, are two in num- 

 ber, small, and rounded. They are followed by two 

 elongated metacnrpal bones of unequal thickness, 

 which are completely anchylosed together at both 

 ends. At the base of these, on the outer edge, we find 

 another small bone, which is sometimes free and 

 sometimes anchylosed to the metacarpal (fig. 137). 

 This is the rudiment of a thumb, and gives support to 

 a few feathers, forming what is called the pinion or 

 bastard-wing. The metacarpal bones are followed by 

 the phalanges, which usually represent two fingers, 

 one of which is composed of two or three joints, the 

 other only of one. These phalanges are endowed with 

 but little mobility, so that the whole wing may be 

 regarded as composed of three joints, united by a more 

 or less hinge-like articulation at the elbow and wrist. 

 By this arrangement the wing is rendered stiff when 

 extended, whilst, at the same time, its three joints lie 



* This is the well-known merrythought, with the ceremony 

 of breaking which most of our readers are probably acquainted. 



nearly parallel to each other when the wing is con- 

 tracted, and thus occupy a very small space. 



The structure of the hind limbs corresponds, in like 

 manner, with that of the same members in the Mam- 

 malia. The femur, or thigh-bone, is usually short and 

 stout, and articulates with the pelvis by a ball and 

 i socket joint, the rounded head being set on, as in the 

 ' mammals, nearly at a right angle to the axis of the bone 

 (see fig. 135, Plate 36). This is followed by the tibia, 

 or shank-bone, usually much longer than the thigh, 

 and accompanied by a slender fibula. This latter bone 

 is, however, commonly attached to the tibia, and 

 it always becomes gradually attenuated towards the 

 apex, and disappears entirely long before reaching the 

 extremity of the shank. The knee-joint is furnished 

 with a small knee-cap, or patella. The tarsus also 

 consists of a single cylindrical bone, often ol great 

 length ; and the foot is terminated by from two to 

 four toes containing a variable number of joints. The 

 usual number of toes, especially in the most typical 

 birds, is four ; and of these one is generally directed 

 backwards. 



Of the muscles by which the apparatus of bones 

 just described is set in motion, we need say but little. 

 They are generally very firm, and of a deep-red 

 colour. The principal mass of the muscles is devoted 

 to the movement of the wings. These, of course, 

 vary in bulk according to the power of flight possessed 

 by the bird, and correspond with the development of 

 the sternal keel. The muscles of the thigh and shank, 

 are also largely developed, and it is in the hinder 

 limbs also that we find the sinews presenting most 

 distinctly those peculiarities which characterize them 

 in birds. They are exceedingly white and glistening, 

 and have a great tendency to become ossified. The 

 long tendons which pass down the back of the tarsus 

 from the flexor muscles of the toes, are especially 

 remarkable in these respects. They are of the greatest 

 importance to the bird in perching; indeed, it is to 

 this peculiar arrangement that the bird is enabled to 

 perch. Passing over the back of the heel, like a cord 

 over a pulley, they are necessarily stretched by the 

 flexion of this joint, caused by the weight of the body 

 in the act of perching. They thus exercise a sort of 

 involuntary action upon the toes, causing them to grasp 

 any object with great firmness, and thus retaining the 

 bird securely in its position even during sleep. 



It is hardly necessary to say that the clothing of 

 the skin in these animals consists of feathers, a circum- 

 stance from which de Blainville proposed to name the 

 class Penniferes. It may be as well, however, to give 

 a few details of the nature of these tegumentary appen- 

 dages, which, although somewhat analogous to the 

 hairs of the Mammalia, are of a far more complicated 

 structure. 



A feather ordinarily consists of two distinct portions 

 a central shaft or stem, and a pair of webs, occupy- 

 ing the two sides of the shaft, and composed of numer- 

 ous flattened fibres or barls, closely applied to each 

 other. The basal portion of the central stem, which 

 is partially inserted into the skin, is a hollow, horny 

 tube, usually transparent, and terminating in a more or 

 less pointed or rounded extremity. The upper portion 



