The shape of the sternum, correlated to the faculty of flight (or of swimming; apropos of which we may cite the penguin, of which the rudimentary wings resemble fins, and perform their functions only), or the absence of this faculty, has furnished the division of birds into two groups. In one are included, under the name Carinates (carina, keel), those in which the sternum is provided with a keel; in the other division are those in which the sternum is not furnished with one. These latter, on account of their unique mode of progression, are more nearly allied to the mammals.
The keel is developed in flying mammals (bats).
Ribs and Costal Cartilages.—There are on each side of the thorax as many ribs as there are dorsal vertebræ. In animals, as in man, the ribs which articulate with the sternum by their cartilages are called true, or sternal ribs; those whose cartilages do not articulate with the sternum are called false, or asternal. The longer ribs are those situated in the middle region of the thorax.
The ribs are directed obliquely downwards and backwards, and this obliquity is more marked in the posterior ones than in the anterior. They are, however, less oblique than in the human being; what proves this is that the first rib in man is oblique, while in quadrupeds it is vertical.
The curvature of the ribs is less pronounced in quadrupeds than in the human being, but this is not equal in all animals. The ribs of the bear are more curved than those of the dog; the latter has ribs more curved than those of the horse.
Each rib, at its vertebral extremity, presents, from within outwards, a wedge-shaped head for articulation with two dorsal vertebræ, a neck, and a tuberosity. External to the tubercle are found some rough impressions, for muscular attachments, which correspond to the angle of the human rib.
In the following table, we give the number and classification of the ribs of some animals:
Number of the Ribs on Each Side of the Thorax.
| Sternal. | Asternal. | |||||
| Bear | 14 | divided | into | 9 | and | 5 |
| Dog | 13 | „ | „ | 9 | „ | 4 |
| Cat | 13 | „ | „ | 9 | „ | 4 |
| Rabbit | 12 | „ | „ | 7 | „ | 5 |
| Pig | 14 | „ | „ | 7 | „ | 7 |
| Horse | 18 | „ | „ | 8 | „ | 10 |
| Camel | 12 | „ | „ | 8 | „ | 4 |
| Ox | 13 | „ | „ | 8 | „ | 5 |
| Sheep | 13 | „ | „ | 8 | „ | 5 |
The costal cartilages, by which the first ribs are united to the sternum (sternal ribs), whilst the latter are united one to the other without being directly connected with the sternum (asternal ribs), are, as a rule, in quadrupeds, directed obliquely downwards, forwards, and inwards; each forms, with the rib to which it belongs, an obtuse angle more or less open anteriorly. Their length is proportionate to that of the ribs. The cartilages, which are continued from the asternal ribs, unite and form the borders, directed obliquely downwards and forwards, of the fossa which is found at the inferior and posterior part of the thorax, and which forms the lateral limits of the epigastric region. In the dog and cat the ribs are thick and almost cylindrical; the costal cartilages are thicker at the margin of the sternum than at their costal extremity. In the ox, the ribs are flattened laterally and are very broad, the more so as we examine a portion further from the vertebral column. From the second to the twelfth they are quadrangular in the superior fourth, and thicker than in the rest of their extent. The first costal cartilage is vertical; the following ones are progressively more oblique in a direction downwards and forwards. The four or five cartilages which succeed the first unite with slight obliquity to the sternum; their union with that bone gives the impression of a very strong, well-knit apparatus. The costal cartilages which unite with the sternum are flattened laterally in the portions next the ribs, and flattened from front to back in the rest of their extent.