224 THE CHEST: 



It is called the ensiform cartilage, although it bears little resemblance to a 

 sword. It is flat and flexible, yet strong, and serves as the commencement of 

 the floor or support of the abdomen. It also gives insertion to some of the 

 abdominal muscles, and more conveniently than it could have been obtained 

 from the body of the sternum. 



The intercostal muscles. — The borders of the ribs are anteriorly concave, thin 

 and sharp — posteriorly rounded, and presenting underneath a longitudinal 

 depression or channel, in which run both blood-vessels and nerves. The space 

 between them is occupied by muscular substance firmly attached to the borders 

 of the ribs. These muscles are singularly distributed ; their fibres cross each 

 each other in the form of an X. There is a manifest advantage in this. If the 

 fibres ran straight across from rib to rib, they might act powerfully, but their 

 action would be exceedingly limited. A short muscle can contract but a little 

 way, and only a slight change of form or dimension can be produced. By 

 running diagonally from rib to rib, these muscles are double the length they 

 could otherwise have been. It is a general rule with regard to muscular action, 

 that the power of the muscle depends on its bulk, and the extent of its action on 

 its length. 



The ribs, while they protect the important viscera of the thorax from injury, 

 are powerful agents in extending and contracting the chest in the alternate 

 inspiration and expiration of air. In what proportion they discharge the labour 

 of respiration is a disputed question, and into the consideration of which we 

 cannot enter until something is known of the grand respiratory muscle, the dia- 

 phragm. Thus far, however, may be said, that they are not inactive in natural 

 respiration, although they certainly act only a secondary part ; but in hurried 

 respiration, and when the demand for arterialised blood is increased by violent 

 exertion, they are valuable and powerful auxiliaries. 



This leads to a very important consideration, the most advantageous form of 

 the chest for the proper discharge of the natural or extraordinary functions of 

 the thoracic viscera. The contents of the chest are the lungs and the heart : — 

 the first, to render the blood nutrient and stimulating, and to give or restore to 

 it that vitality which will enable it to support every part of the frame in the 

 discharge of its function, and devoid of which the complicated and beautiful 

 machine is inert and dead ; and the second, to convey this purified arterialised 

 blood to every part of the frame. 



In order to produce and to convey to the various parts a sufficient quantity of 

 blood, these organs must be large. If it amounts not to hypertrophy, the larger 

 the heart and the larger the lungs, the more rapid the process of nutrition, and 

 the more perfect the discharge of every animal function. 



Then it might be imagined that, as a circle is a figure which contains more 

 than any other of equal girth and admeasurement, a circular form of the chest 

 would be most advantageous. Not exactly so ; for the contents of the chest are 

 alternately expanding and contracting. The circular chest could not expand, 

 but every change of form would be a diminution of capacity. 



That form of chest which approaches nearest to a circle, while it admits of 

 sufficient expansion and contraction, is the best — certainly for some animals, 

 and for all under peculiar circumstances, and with reference to the discharge of 

 certain functions. This was the grand principle on which Mr. Bakewell pro- 

 ceeded, and on which all our improvements in the breeding of cattle were 

 founded. 



The principle holds good with regard to some breeds of horses. We value 

 the heavy draught-horse not only on account of his simple muscular power, but 

 the weight which, by means of that power, he is able to throw into the collar. 

 A light horse may be preferable for light draught, but we must oppose weight 



