The ribs are elongated curved bones, which form the skeleton of the lateral thoracic walls.
They are arranged serially in pairs, which correspond in number to the thoracic vertebrae. Thus there are thirteen pairs of ribs.
Each rib articulates dorsally with two vertebrae and is continued ventrally by a costal cartilage. Those that articulate with the sternum by means of their cartilage, (eight pairs) are termed sternal ribs; the remainder is asternal ribs.
Ribs at the end of the series with free ventral ends not attached to adjacent cartilage are floating ribs. The intervals between ribs are intercostal spaces.
A typical rib is an elongated, flat curved plate. It presents a shaft and two extremities
Shaft
The shaft is most curved at its upper part while its lower part is twisted and inclines inwards.
The lateral surface is convex and is marked by a wide groove in its anterior part.The point at which a rib most curved is termed the angle and here it presents a rough area for the attachment of the longissimus dorsi muscle.
The medial surface is smooth, concave and presents at its posterior part a distinct groove -the costal groove which turns downwards and disappears about the middle of the bone and lodges the intercostal vein.
The borders serve for the attachment of the intercostal muscles.
Upper extremity
The upper vertebral extremity is made up of a head, neck and a tubercle.
The head presents an articular area which is composed of two convex facets -anterior and posterior, separated by a non-articular groove. The facets articulate with the capitular facets of two adjacent dorsal vertebrae and the intervertebral fibrocartilage between them and the groove serves for the attachment of the round conjugal ligament.
The neck joints the head to the shaft and is roughened above and in front for ligamentous attachment.
The tubercle is situated above and behind the head at the junction of the head and neck. Dorsally it presents a transversely placed concave facet for articulation with a tubercular facet of the posterior vertebra of the two with which the head articulates.
The tubercular facet is small, flat or absent in the last two or three ribs.
Lower extremity
The lower sternal extremity is smooth and articulate with its costal cartilage.
The ventral ends of the second to the tenth or eleventh inclusive form diarthrodial joint (movable) with their cartilages.
Special Features of Ribs of Ox
The first rib is the shortest and least curved.The sternal end widens very greatly.The lateral surface and the anterior border present a rough area about their middle for the scalenus ventralis. At the lower part of anterior border is a smooth impression left by the dorsal roots of the brachial plexus.The groove on the lateral surface and the costal groove are absent.The head is divided by an oblique ridge into the two facets. The tubercle is larger than that of any other rib.
The eighth, ninth and tenth ribs are the longest and widest. Both in front and behind these, the length and breadth diminish gradually. The first costal cartilage is the shortest.
The costal cartilages are rods of hyaline cartilage and form diarthrodial joints with ribs (from) -2nd to tenth or eleventh). Those of sternal ribs articulate with the sternum by sternal cartilages. Those of asternal ribs overlap and are attached to each other by elastic tissue to form the costal arch. Except in the case of the first the cartilage does not continue the rib but forms with the later an angle, which is open in front and increases from the second to the last.
Sheep and Goat
There are 13 pairs of ribs but the occurrence of 14 pair is common.
The 13 th rib is often floating. 14 th rib when present is floating.
The ribs are narrower and strongly curved in the anterior part of the series.
The second to eleventh form diarthrodial joints with their cartilage.
The 13 th rib may be more or less rudimentary on one side or both, may be fused with the corresponding vertebra, the latter therefore be ambiguous in character.
Horse
There are 18 pairs of ribs. 8 sternal and 10 asternal, they are shorter, narrower, thicker, regular and more curved.
The neck is shorter.
The length increases upto the 10th and then diminishes.
The anterior borders of the ribs from the 2nd to 8th are sharp and from 9th to 18th rounded and thick.
The lower ends of ribs are s lightly enlarged and roughened at the junction with the costal cartilage.
In regard to dorso-ventral direction, the first rib inclines a little forward the second is about vertical and others slope backwards in increasing degree.
The two first costal cartilage articulate with each other and with sternum.
The upper ends of cartilages form synarthrodial joints (immovable) with ribs.
Pig
There are 14 or 15 pairs of ribs. Of which 7 are sternal and seven or eight are asternal.
They are strongly curved with a distinct angle.
The backward slope of the posterior rib is slight.
The first rib is prismatic, has a large sternal end and a very short cartilage.
The tubercle fuses with the head on the last five or six.
The second to fifth form diarthrodial joints with their cartilages, which are wide and plate-like.
The 15 th rib when present, may be fully developed and its cartilage enter into the formation of the costal arch.
But in most cases it is floating and in some cases it is only about an inch in length.
Dog
There are 13 pairs of ribs. 9 sternal and 4 asternal.
They are strongly curved narrow and thick.
The last rib is usually floating.
The heads of the last two or three ribs articulate with only one vertebra.
The costal cartilages are long and curve ventrally and forward. The length and curvature of the first pair are striking special features.
Rabbit
There are 12 pairs in number. Of which 7 sternal and 5 asternal.
Last pair is floating.
Fowl
There are 7 pairs of ribs.
Each rib is made up of an upper vertebral rib and a lower sternal rib.
The first, second and sometimes seventh do not have sternal ribs: These are floating.
The head of a rib articulates with the capitular cavity of a single dorsal vertebra.
Except the first and seventh, the posterior border of each vertebral rib presents an uncinate process directed backward to overlap the succeeding rib.
Sternal ribs are homologues of mammalian costal cartilage.
