222 THE CHEST. 



diaphragm, being its posterior. It is contracted in front, broad and deep towards 

 the central boundary, and again contracted posteriorly. It encloses the heart 

 and the lungs, the origin of the arterial, and the termination of the venous 

 trunks and the collected vessels of the absorbents. The windpipe penetrates 

 into it, and the oesophagus traverses its whole extent. 



A cavity whose contents are thus important should be securely defended. 

 The roof is not composed of one unyielding prolongation of bone, which might 

 possibly have been strong enough, yet would have subjected it to a thousand 

 rude and dangerous shocks ; but there is a curiously-contrived series of bones, 

 knit together by strong ligaments and dense cartilaginous substance, forming so 

 many joints, each possessed but of little individual motion, but the whole united 

 and constituting a column of such exquisitely-contrived flexibility and strength, 

 that all concussion is avoided, and no external violence or weight can injure 

 that which it protects. It is supported chiefly by the anterior extremities, and 

 beautiful are the contrivances adopted to prevent injurious connexion. There is 

 no inflexible bony union between the shoulders and the chest ; but while the 

 spine is formed to neutralise much of the concussion that might be received- 

 while the elastic connexions between the vertebrae of the back, alternately 

 affording a yielding resistance to the shock, and regaining their natural situa- 

 tion when the external force is removed, go far, by this playful motion, to 

 render harmless the rudest motion there is a provision made by the attachment 

 of the shoulder-blade to the chest calculated to prevent the possibility of any 

 rude concussion reaching the thorax*. 



At the shoulder is a muscle of immense strength, and tendinous elastic com- 

 position, the serratus major, spreading over the internal surface of the shoulder- 

 blade and a portion of the chest. A spring of easier play could not have been 

 attached to the carriage of any invalid. It is a carriage hung by springs 

 between the scapulae, and a delightful one it is for easy travelling ; while there 

 is combined with it, and the union is not a little difficult, strength enough to 

 resist the jolting of the roughest road and the most rapid pace. 



Laterally there is sufficient defence against all common injury by the expan- 

 sion of the shoulder over the chest from between the first and second to the 

 seventh rib ; and behind and below that there is the bony structure of the ribs, of 

 no little strength ; and their arched form, although a flattened arch ; and the 

 yielding motion at the base of each rib, resulting from its jointed connexion 

 with the spine above and its cartilaginous union with the sternum below. 



A still more important consideration with regard to the parietes of the thorax 

 is the manner in which they can adapt themselves to the changing bulk of the 

 contents of the cavity. The capacity of the chest is little affected by the 

 external contraction and dilatation of the heart, for when its ventricles are 

 collapsed its auricles are distended, and when its auricles are compressed its 

 ventricles expand ; but with regard to the lungs it is a very different affair. In 

 their state of collapse and expansion they vary in comparative bulk, one-sixth 

 part or more, and, in either state, it is necessary for the proper discharge of the 

 function of respiration that the parietes of the chest should be in contact with 

 them. 



The ribs are eighteen in number on either side. Nine of them are perfect, 



* "Had," says Mr. Percivall, "the entire have compressed the organs of respiration and 

 rib been one solid piece of bone, a violent circulation to that degree that could not but 

 blow might have broken it to pieces. On the have ended in suffocation and death of the 

 other hand, had the ribs been composed from animal. It was only the judicious and well- 

 end to end of cartilage only, the form of the arranged combination of bone and gristle in 

 arch could not have been sustained, but, sooner the construction of the chest that could answer 

 or later, it must have bent inward, and so have the ends an all-wise Providence had in view." 

 encroached upon the cavity of the chest as to Veterinarian, vol. xv. p. 184, 



