70 THE BONES. 



is given off from this process, which joins it to a pointed bone or a cartilage 

 in its vicinity. If the hymn on the " Sacrifice of the Horse," in the most 

 ancient collection of Aryan poems, is to be credited, the horses of antiquity 

 in Central Asia had only seventeen pairs of ribs. The mobility of the ribs 

 is scarcely perceptible in the first, but increases until the ninth or tenth is 

 reached, after which it gradually diminishes.) 



THE THORAX IN GENERAL. 



The description of the interior of the thoracic cavity will be referred to 

 when treating of the respiratory apparatus. It is only necessary here to 

 examine the external surface of this bony cage ; for this purpose it is 

 divided into six regions : a superior plane, an inferior plane, two lateral planes, 

 a base, and a summit. 



Planes. The superior plane is separated into two portions by the spinous 

 processes of the dorsal vertebrae ; each forms, with these spinous processes, the 

 costo-vertebral furrow, intended to lodge the majority of the muscles belong- 

 ing to the spinal region of the back and loins. The inferior plane, less 

 extensive than the preceding, offers: 1, On the median line, the cariniform 

 and xiphoid cartilages of the sternum ; 2, On the sides, the chondro-sternal 

 articulations, and the cartilages of prolongment of the true ribs. The 

 lateral planes are convex and wider at their middle part than in front or 

 behind, and exhibit the intercostal spaces. They serve to give support, 

 anteriorly, to the superior rays of the two fore-limbs. 



Base. This is circumscribed by the posterior border of the last rib, and 

 by the cartilages of all the asternal ribs ; it is cut obliquely from above to 

 below, and from before to behind. It gives attachment, by its internal 

 circumference, to the diaphragm, a muscle which separates the thoracic from 

 the abdominal cavity. 



Summit. It occupies the anterior portion of the thorax, and presents an 

 oval opening, elongated vertically,, situated between the two first ribs. This 

 opening constitutes the entrance to the chest, and gives admission to the 

 trachea, the oesophagus, and important vessels and nerves. 



DIFFERENTIAL CHARACTERS OF THE THORAX IN OTHER THAN SOLIPED ANIMALS. 



1. Sternum, 



In all the domesticated animals except solipeds, the sternum is flattened above and 

 below, instead of from side to side. 



RUMINANTS. In ruminants, each piece is developed from two lateral centres of ossifica- 

 tion. The bones which compose it are seven in number ; they are much more compact 

 than those in the sternum of the horse, and at an early period are united to each other, 

 with the exception of the first, which is joined to the second by a diarthrodial articula- 

 tion that permits it to execute lateral movements. There is no cervical prolongation, 

 and the xiphoid cartilage is feebly developed and well detached from the body of the 

 bone. In the sternum of the Goat and Sheep, the two first pieces have no diarthrodial 

 joint, but are simply united by a layer of cartilage which, in old animals, becomes 

 completely ossified. 



PIG. The sternum of this animal presents in its general conformation the essential 

 features of that of large ruminants. It is provided with a well-defined cervical prolonga- 

 tion, and is composed of six pieces which, at least in the four or five last, are each 

 divided into two lateral centres. 



CARNIVORA. The sternum of the Dog and Cat is formed of eight pieces elongated from 

 before to behind, hollowed in their middle part, and thick at their ends formed, indeed, 

 like the last coccygeal vertebrae of the Horse. They are never ossified to each other. 



