PRACTICAL CONSIDERATIONS : THE THORAX. 167 



PRACTICAL CONSIDERATIONS. 



The bony and cartilaginous thorax is made up of the ribs, sternum, costal car- 

 tilages, and thoracic vertebrae, and varies in shape as a result of several influences. 

 The slightly larger circumference of the right side of the chest as compared with 

 the left side is probably due to the greater use of the right upper limb, and may 

 be accepted as physiological. Increased circumference of the left side, therefore (in 

 a right-handed person), should indicate careful examination of the spine (for lateral 

 curvature) and of the thoracic viscera. 



\n pigeon-breast the sternum protrudes together with the costal cartilages, while 

 the line of the costo-chondral junction becomes a deep groove. The sides of the 

 chest are flattened, and a transverse section would be almost triangular in shape. 

 There are three modes of production of this very common deformity : 



1. In rickety children it is favored by the softening of the bones and cartilages, 

 which are thus of diminished resiliency, the actual exciting cause being often some 

 form of respiratory obstruction, — e.g., enlarged pharyngeal and faucial tonsils, 

 bronchitis, nasal obstructions, etc. In ordinary breathing, on inspiration, air enters 

 the chest freely to prevent the production of a vacuum, and at the end of the act 

 the external atmospheric pressure is balanced by the pressure within. If an impedi- 

 ment to the free ingress of air exists, the external pressure during at least part of 

 the act is in excess, and in young children, particularly rickety children, this is 

 followed by the bending inward along the weakest part of the thorax (the costo- 

 chondral line) and the relative projection of the sternum. 



2. The lowest five costal cartilages form an especially weak portion of the chest- 

 wall. They are the most distant from the fulcrum (the spine) on which the ribs 

 move in respiration, and hence the expansive forces act with the greatest disadvan- 

 tage of leverage (Humphry). At the same time the diaphragm, during its contrac- 

 tion, tends to draw them inward. If, however, its central arch cannot descend 

 during inspiration on account of an engorged liver, enlarged abdominal lymphatics, 

 persistent flatulence, etc. (as in a poorly nourished child), it becomes the fixed point, 

 and the lateral walls are pulled in and the sternum correspondingly protruded. 



3. Some cases of " pigeon -breast " are seen at or soon after birth in otherwise 

 healthy children. It is probable that these are cases of arrest of development. The 

 so-called " keeled chest " (in which the antero-posterior diameter is increased at the 

 expense of the transverse diameter) is characteristic of the quadrupedal class of 

 mammals, and is necessitated by, and correlated with, the backward and forward 

 swing of the anterior limbs in walking.' In the foetus the antero-posterior diameter 

 is relatively greater than in the adult. 



Attention has already been called (page 164) to the varying ratio between the 

 antero-posterior and transverse diameters of the chest, the transverse diameter in 

 the adult exceeding the anterior in the proportion of 2.5 to i. If this change stops 

 short of full completion, a greater or less degree of relative prominence of the ster- 

 num results. 



The " bellows chest "Ms found among mammals almost exclusively in the bats, 

 the anthropoid apes, and man, that have in common simply the disuse of the anterior 

 limbs as a means of support. In them the chief movements of these limbs tend to 

 pull the sternum towards the vertebral column. The exaggeration of this type results 

 in the so-called "flat chest," which is, however, within proper limits, the type of 

 vigor, as it results from the full contraction of normal muscles. 



Emphysema produces a rotund configuration of the chest-walls, affecting chiefly 

 the upper portion, throwing out the ribs, effacing the intercostal spaces, and making 

 the thorax ' ' barrel-shaped. ' ' 



Old age, owing to an increased bowing of the thoracic spine under the weight 

 of the head and shoulders and to a slipping forward of the shoulder-girdle with its 

 mass of muscles, often causes a depression of the sternum and its approximation to 

 the spine, — a common form of flat chest. 



^ Woods Hutchinson : Journal of the American Medical Association, vol. xxix., 1897. 

 ' Ibid. 



