AVES. 



281 



Rasores seven or eight ; in the Struthiones the 

 number of ribs vanes ; in the Ostrich (Stru- 

 thio) we find ten pairs, of which the 3d, 4th, 

 5th, and 6th, are articulated with the sternum ; 

 in the Nandou (Rhea) there are nine pairs, 

 of which only the 3d, 4th, 5th, and 6th, are 

 completed by sternal portions ; in the Emeu 

 (Druniaius) there are nine pairs, the 3d, 4th, 

 5th, 6th, and 7th, being joined to the sternum; 

 in the Cassowary (Casuarius) there are ten 

 pairs, and of these the 4th, 5th, 6th, 7th, 8th, 

 and 9th, have sternal portions. The last pair 

 of ribs in Struthio and Rhea are extremely 

 short, and abut against the expanded iliac bones. 

 Among the Grallatores we find seven pairs of 

 ribs in the Herons (Ardea), and Gigantic 

 Stork (Ciconia Argula) while the Cranes 

 (Grus) have nine, and the Coots and Water- 

 Jlens have ten pairs. In the Natatores, which 

 vary so much in their locomotive powers and 

 habits of life, we find a corresponding variety 

 in the number of ribs ; in the VVillocic ( Uria 

 troilc) there are twelve pairs, and in the Guil- 

 lemots and allied sea-birds eleven; in the 

 Swans eleven ; in the Penguins nine, of which 

 six are articulated with the sternum. 



The true ribs are not joined to the sternum 

 by elastic cartilages, but by straight osseous 

 portions, called sternal ribs, (q, Jig. 125, 

 //, jig, 129,) which are moveably connected at 

 both their extremities. These are the centres 

 upon which the respiratory motions hinge ; the 

 angle between the vertebral and sternal ribs, 

 and between these and the sternum becoming 

 more open in inspiration, and the contrary 

 when the sternum is approximated to the dorsal 

 region in expiration. 



As the ribs are traced backwards, their 

 vertebral extremities are seen to become gra- 

 dually double or bifurcated from the in- 

 creasing development of the part answering 

 to the cervix and head of the rib in Mam- 

 malia. The spurious cervical ribs may be 

 plainly seen to be articulated, like the pos- 

 terior spurious ribs of the Cetacea, by the 

 tubercle only ; and, as they increase in length 

 in the proximity of the thorax, the head of the 

 rib is then seen to be thrown downwards to 

 join a distinct tubercle on the side of the body 

 of the vertebra close to its anterior margin, 

 but without encroaching on the intervertebral 

 space. The comparative immobility of the 

 dorsal vertebrae allows of this mode of articu- 

 lation ; but it is an interesting circumstance 

 that in the Ostrich, where the costal vertebrae 

 preserve their mobility, the heads of the ribs, 

 at least of those of the anterior ones, evidently 

 pass forwards to the intervertebral space. The 

 tubercle of the rib has thus less the character 

 of a subordinate process than in the ribs of 

 mammalia; it is supported on a pedicle, and 

 is articulated by a simple synovial joint with 

 the transverse process of the corresponding ver- 

 tebra. The ribs, below the union of the two 

 articular processes, are thick and strong, but 

 they gradually become flattened, and increase 

 in breadth as they descend towards the sternum. 

 This is especially remarkable in the second, 

 third, and fourth ribs of the Woodpecker. 



The dorsal ribs are not only connected together 

 by muscles and aponeurotic membranes, but 

 cooperate with the anchylosed dorsal vertebrae, 

 in giving stability to the trunk by means of 

 small osseous splints, detached from the pos- 

 terior margin of each true rib, and directed 

 backwards and upwards to the next in suc- 

 cession, to both of which they are united by 

 means of oblique fibrous ligaments. In birds 

 of powerful flight these connecting pieces are, 

 as might be expected, most developed. In 

 the Raptores they extend beyond and overlap 

 the succeeding posterior rib, and in this order 

 they are anchylosed. 



In some of the Struthious birds, as the 

 Ostrich and Rhea, they exist from the third to 

 the fifth rib, while in the Emeu and Cassowary 

 there are only rudimentary traces of them. 

 In the Penguins these accessory processes are 

 remarkable for their breadth, but they are 

 never anchylosed to the ribs, and consequently 

 are apt to be lost if care be not taken in pre- 

 paring the skeleton. 



The sternal ribs (h, h,fg. 129) are of a less 

 flattened form than the vertebral ; they increase 

 in length as they are situated further back ; 

 their costal extremity is simply rounded, while 

 their sternal extremity is extended transversely 

 and divided into two smooth surfaces moveably 

 articulated by two synovial capsules with cor- 

 responding cavities in the sides of the sternum. 

 The first sternal rib is, however, joined by 

 nbro-cartilaginous substance only, while one 

 or two of the posterior pieces are anchylosed 

 with the rib immediately preceding them, and 

 do not reach the sternum. In the Ostrich 

 the last rib abuts against the ilium, to which it 

 is anchylosed. 



In the Peacock, Pintado, and common Fowl, 

 the vertebral and sternal portions of the last 

 pair of ribs are unconnected with each other ; 

 the latter thus representing the ossified ten- 

 dinous intersections of the rectus abdominis 

 muscle, as in the Crocodile. This analogy is 

 still more striking in the Herons, Storks, and 

 Curlews, and in many of the Natatoi^es, in 

 which the sternal portions alone exist, and are 

 remarkably elongated. 



The part of the skeleton which has undergone 

 the most remarkable modifications in relation 

 to the powers and functions of the anterior ex- 

 tremities is the sternum, (r,s, fig. 125 and 129,) 

 which gives origin to their principal muscles. 

 It is so developed, both in length and breadth, 

 as to extend over the whole of the anterior or 

 ventral aspect of the thoracic and of a great 

 part of the abdominal cavities, reaching in 

 some birds of great powers of flight even to 

 the pubic bones, so as to require removal be- 

 fore the abdominal cavity can be examined. 



In order to afford origin to the accumulated 

 fasciculi of the pectoral muscles, which other- 

 wise would become blended together over 

 the middle of the sternum, an osseous crest 

 (s, Jig. 125, a, Jig. 130) is extended down- 

 wards, analogous to the cranial crest which 

 intervenes to the temporal muscles in the 

 carnivorous mammalia ; and as this crest in- 

 dicates in these the powers of the jaw, so the 



