334 STRUCTURE OF TttE SHEEP. 



sheep than in the horse and many other quadrupeds. 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 

 sternum or breast-bone. The number of ribs varies in dif- 

 ferent animals ; in man there are twelve, in the horse eigh- 

 teen, but in "the sheep there are only thirteen pair. Each 

 rib possesses two heads or protuberances, each of which is 

 connected by a joint with two vertebrae 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 be- 

 comes divisible into four only, and with age is consolidated 

 into one. The ribs are , externally convex, and are divided 

 into the true and false ; the former being situated anterior to 

 the others, and immediately connected with the sternum, 

 virhilst the latter are implanted into each other at their carti- 

 laginous extremities, and are only connected with the breast 

 bone by means of the frue ribs. Their connexion with the 

 spine, by means of a double joint, affords to the ribs a mo- 

 tion backwards and forwards, by which means the cavity of 

 the chest is enlarged or diminished. This motion, however, 

 is considerably less in quadrupeds than in man, for in the lat- 

 ter the rising and falling of the chest is geen in common 

 respiration, whilst in the former it is not perceived, unless 

 the breathing be embarrassed. The ribs are connected to- 

 gether by fleshy substance, 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 another, so that a contraction of one-third their 

 length will bring the ribs together, which could not be the 

 case if the muscles took the shortest course from one rih to 

 another. 



The chest is separated from the abdomen or belly by a 

 very singular and important muscle, called the diaphragm or 

 midrifi", 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 the spine, 

 by which means its position is rendered oblique, its develop- 

 ment more extended, 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 



