i68 HUMAN ANATOMY. 



The pulmonary capacity is but roughly indicated by the circumference of the 

 chest, as the vertical diameter is also obviously an important determining factor. 

 Chest measurements, to be of value, should therefore be supplemented by investiga- 

 tion into the amount of air which can be inhaled and exhaled. The resulting 

 information is often of great value as a basis for prognosis and for advice as to exer- 

 cise and hygiene, especially in persons with a predisposition to pulmonary disease. 



In the infant the thorax is relatively smaller than in the adult. In the female 

 the upper portion of the thorax is less compressed from before backward and is 

 more capacious than in the male. The upper aperture is larger and the range of 

 movement between the upper ribs and the sternum and vertebrae is greater. These 

 circumstances account both for the fulness of the upper portion of the chest in the 

 female and for the character of the respiratory movement, which is known as 

 thoracic ; while that of the male, in which the lower ribs and abdominal walls move 

 more freely, is known as the abdominal type of respiration. 



The sternum may be entirely wanting, or may be divided into two portions by 

 a fissure down the middle, the result of developmental failure, which, when it exposes 

 the thoracic cavity and the heart, is known as ectopia cordis. 



Its subcutaneous position makes it the subject of slight but frequent traumatisms, 

 which often serve to localize the bone lesions of syphilis, tuberculosis, and other 

 infections ; and this fact, in conjunction with its cancellous structure, accounts for 

 the frequency with which it is the seat of gummatous periostitis and tuberculous 

 caries. There are sometimes litde circular defects in the body of the sternum, 

 through which an abscess may pass from the mediastinum outward, or infections from 

 without may find their way within the thorax. They are congenital defects due to 

 a failure of the two halves of the body of the sternum to unite. 



The seven depressions on each side of the sternum for the reception of the 

 cartilages of the seven true ribs are so shaped that the upper and anterior edges of 

 each notch are more prominent and larger than the lower and posterior edges. 

 This accounts for the rarity of luxation forward of these cartilages and their ribs by 

 the forces which so constantly pull the ribs upward and forward, as the action of the 

 scaleni and intercostals in violent inspiratory efforts, that of the pectorals in swinging, 

 by the hands or on parallel bars, etc. 



Backward dislocation at the chondro-sternal junction is even rarer ; but this is 

 because, owing to the elastic curves of the ribs, the sternum and the anterior 

 extremities of the ribs move backward together on the application of direct force to 

 the front of the chest. 



As it is thus movable, and is supported on the ends of elastic levers or springs, 

 the sternum is rarely fractured. When the fracture is the result of indirect violence, 

 it is often associated with injuries to the spine, as the extreme extension or extreme 

 flexion, which is the common cause of a sternal fracture, must necessarily put a 

 severe strain on the thoracic spine. 



In extension the sternum is fixed between the sterno-mastoids and sterno- 

 hyoids and thyroids above and the recti and diaphragm below. In flexion the 

 force may be transmitted through the chin. In either case the most common seat 

 of fracture is at or about on a line with the second costal cartilage, because (^) the 

 bone there is narrowest (Fig. 173), and (^) at that level lies the junction between 

 the manubrium and body. As the various portions of the bone are not united 

 until about twenty years of age, fracture is almost unknown before that time. 

 Moreover, during that period the symphysis between the manubrium and the body 

 is so shaped that, together with the natural curve forward of the bone, it increases 

 the elasticity of the sternum and enables it to resist both direct violence and tensile 

 strain. 



The projection ^ at the union between the manubrium and body {aiigiihis 

 Liidovici) is sometimes exceptionally prominent, and when this is noticed for the 

 first time after an accident or an illness, may give rise to the erroneous diagnosis of 

 fracture or of bone disease. This angle is increased in phthisis, owing to the reces- 

 sion of the manubrium ; it is increased in emphysema, as the second ribs carry for- 

 ward the lower border of the manubrium. 



The greater thickness and strength of the layer of fibrous tissue that covers 



^ Angulus sterni. 



