46 STRUCTURE AND ECONOMY OF THE HORSE. 



a form well adapted for passing through the air with the least" 

 resistance ; and, at the same time, more favourable to the flexion 

 and extension of the shoulders and the fore legs than any other. 

 The upper part of the chest is formed by the spine or back bone, 

 the sides by the ribs, and the lower and front part by the ster- 

 num, or breast bone. The number of ribs varies in different 

 animals ; in man there are twelve, in the elephant eighteen, 

 but in the horse there are sixteen pair. Each rib possesses two 

 heads or protuberances, each of which is connected by a joint 

 with two vertebras or bones of the back, and to the breast bone 

 by means of cartilage. The Sternum, or breast bone, in young 

 animals is chiefly cartilaginous, and may be separated into eight 

 pieces ; it afterwards becomes divisible into four only, and, with 

 age, is consolidated into one. The ribs are externally convex, 

 and are divided into the true and the false ; the former being 

 situated anterior to the others, and immediately connected with 

 the sternum ; whilst the latter are implanted into each other at 

 their cartilaginous extremities, and are only connected with the 

 breast bone by means of the true ribs. Their connection with 

 the spine, by means of a double joint, aflfords to the ribs a motion 

 backwards and forwards, by which means the cavity of the chest 

 is enlarged or diminished. This motion, however, is consi- 

 derably less in quadrupeds than in man, for in the latter the 

 rising and falling of the chest is seen in common respiration, 

 whilst in the former it is not perceived, unless the breathing be 

 embarrassed. The ribs are connected together by fleshy sub- 

 stance, termed the intercostal muscles, which are disposed in an 

 oblique course, by which means their length considerably exceeds 

 that of the space between one rib and anothei', so that a con- 

 traction of one-third their leno;th will bring the ribs together, 

 which could not be the case if the muscles took the shortest 

 course from one rib to another. 



The chest is separated from the abdomen or belly by a very 

 singular and Important muscle, called the Diaphragm, or midriff, 

 which is convex towards the chest when in a state of rest. This 

 muscle is shaped somewhat like a fan, and is attached to the 

 inferior extremities of the ribs and to the spine, by which means 

 its position is rendered oblique, and its development more ex- 

 tended, and its action greater than it would otherwise have 

 been. The diaphragm, unlike every other muscle, is fleshy at 

 its circumference and tendinous at its centre. The reason of 

 this peculiar construction may be thus explained. The central 

 part of the diaphragm is pierced with two holes for the passage 

 of the oesophagus, the tube which conveys food to the stomach, 

 and the vein which conveys the blood to the liver for the secre- 

 tion of bile. Now, if these important vessels Avere surrounded 

 with muscular substance, they would be forcibly compressed 



