The most marked characteristic of the dorsal vertebræ is furnished by the spinous processes. They are long and narrow. As a rule, the spinous processes of the foremost dorsal vertebræ are the most developed and are directed obliquely upwards and backwards. As we approach the last vertebræ of this region, the processes become shorter and tend to become vertical, and the last ones are even, in some cases, directed upwards and forwards; this disposition is well marked in the dog and the cat. In the cetaceans, on the contrary, the length of the spinous processes increases from the first to the last.
In the horse the spinous processes of the first dorsal vertebræ produce the prominence at the anterior limit of the trunk, where the mane ends, which is known as the withers.
Fig. 4.—Lumbar Vertebræ of a Quadruped (the Horse): Superior Surface.
1, Spinous process; 2, anterior articular process and transverse process of the first lumbar vertebra of the left side; 3, costiform process.
The lumbar vertebræ are thicker than the preceding; they are known by their short and latterly-flattened spinous processes, and still more readily by their transverse processes, which, as they are evidently atrophied ribs, it is more accurate to denominate costiform processes ([Fig. 4]). These are long, flattened from above downwards, and directed outwards and forwards.
The true transverse processes are represented by tubercles situated on the superior borders of the articular processes of each of the vertebræ of the lumbar region. Apropos of these different osseous processes, we are reminded that they are also present in the human skeleton.
In the horse the costiform processes of the fifth and sixth lumbar vertebræ articulate, and are sometimes ankylosed, one with the other; the terminal ones articulate with the base of the sacrum. Sometimes the processes of the fourth and fifth are thus related; this is the case in the figure ([4]) given; here the costiform processes of the fourth and fifth vertebræ articulate, and the two terminal ones have coalesced.
In the ox, the same processes are more developed than in the horse; their summits elevating the skin, produce, especially in animals which have not much flesh, prominences which limit the flanks in the superior aspect. The costiform processes of the last lumbar vertebræ are separate from each other; those of the latter are not in contact with the sacrum.