The rib cage, which forms the chest wall, is an important volume. Shaped somewhat like a cone, it is created by the individual ribs connecting to the spine above and to the sternum below. Its gets smaller toward the front and, in four legged animals, is flattened side-to-side. The back end is wide and open. The rib cage is wider at the top (near the spine) and narrower below (at the sternum).
The individual ribs lie approximately parallel to each other and are directed downward and slightly backward. The lower end of each rib attaches to a flexible costal cartilage. This junction may occasionally be seen on the surface in carnivores. The ribs and their cartilages at the front portion of the rib cage attach directly to the sternum, and are called the true ribs. The costal cartilages of the ribs in the rear portion of the rib cage angle forward to overlap the costal cartilages in front of them. Because they don't connect directly to the sternum, these ribs are called the false ribs. The cartilages of the last ribs do not attach to the cartilages of the ribs in front of them, and are referred to as the floating ribs. Carnivore ribs are slender; those of the ox are wide.
The sternum is an elongated, segmented bone located on the midline of the bottom of the thorax (front of the chest). The costal cartilages of the true ribs articulate directly with it. The sternum in dogs and cats is roughly cylindrical. In the horse, it is keel-like and flattened side-to-side in its front two-thirds. The midline of the sternum, whether flat or raised into a ridge, may be subcutaneous between the pectoralis chest muscles originating on either side of the midline. The projection at the rear end of the sternum, the xiphoid process, does not affect surface form.
The front end (or top) of the sternum is called the manubrium. It may be bony or predominantly cartilaginous (called the cariniform cartilage). The first pair of ribs articulates with it. In four legged animals, it is either pointed or flattened side to side, and may project forward beyond the articulation with the first ribs. In primates, the manubrium articulates with well developed clavicles; it, as well as the rest of the sternum, is wide, flat, and faces forward. Some of the neck muscles descending from the head attach to the front (or top) of the manubrium (or its cartilage). This junction becomes the point at which the front profile of the neck meets the profile of the chest. Called the point of the chest, it remains fixed in space regardless of the position of the head and neck, as long as the trunk remains stationary. This point is approximately level with the point of the shoulder in horses. The depression found above the manubrium and between the descending muscles on either side of the midline at the base of the neck is called the pit of the neck.
The scapula, or shoulder blade, is a large, flat, roughly triangular bone which lies on the side of the rib cage; its upper end leans inward toward the spine in four-legged animals. The scapula of the horse is elongate; that of the cat, rounded. A bony ridge running its length on its flat surface, called the spine, has a bony expansion, the tubercle, near its center. The trapezius muscle attaches to this tubercle. The lower end of the spine of the scapula expands into the acromion—well developed in carnivores and especially in primates, subtle in most hoofed animals, and virtually absent in horses. In life, this spine may show up as a raised ridge or as a depression located between bulging muscles on either side.
In the horse and the ox, the top portion of the scapula closest to the spine is made up of flattened cartilage, called the scapular cartilage. Its upper edge is rounded, and its back end projects rearward, beyond the back edge of the bony scapula. The upper border of the scapula (including the scapular cartilage) is approximately level with the tips of the thoracic vertebrae in the ox and dog, higher in the cat, and lower in the horse.
A striking difference between four-legged animals and primates is the shape, orientation, and location of the scapula. In four-legged animals, the scapula is long, narrow, oblique, and located on the side of the chest. In primates, it is triangular and located on the back of the chest, with the edge closest to the vertebral column basically parallel to it. In four-legged animals with no clavicle, the scapula does not make a bony connection with the rib cage, but it is connected to it by muscle only. The serratus ventralis thoracis muscle, attaching to the inner upper edge of the scapula and the side of the rib cage, forms a sling that supports the weight of the body.
The clavicle, or collar bone, is well developed in climbing animals such as primates and squirrels, and in digging animals such as the giant anteater and moles. It forms a strong, direct, bony connection between the scapula and the sternum of the rib cage. In many four-legged animals, the clavicles are reduced or absent. These animals use their forelimbs for support and locomotion, and a clavicle would transmit strong forces onto the rib cage and be either too fragile or would interfere with function, especially when a large, heavy, running animal landed on its front limbs. Therefore, they are absent in horses and cows. In carnivores, they are small and vestigial, persisting as small thin bones or cartilage embedded in neck muscle; they do not connect the shoulder to the rib cage.
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