138 



THE SKELETON 



at twenty-five they form a single piece, but exhibit, even in advanced life, traces of their original 

 separation. A sternal foramen is usually the result of non-union across the middle line or a 

 defect of ossification. 



The metasternum is always imperfectly ossified, and does not join with the mesosternum 

 till after middle life. The presternum and mesosternum rarely fuse. The dates given above 

 for the various nuclei, and for the union of the various segments, are merely approximate, hence 

 the sternum affords very uncertain data as to age. 



Abnormalities of the Sternum. — The mode of development of the sternum as described 

 above is of importance in connection with some deviations to which it is occasionally subject. 

 In rare instances the two lateral halves fail to unite, giving rise to the anomaly of a completely 

 cleft sternum. The union of the two halves may occur in the region of the manubrium and fail 

 below, whilst in other cases the upper and lower parts have fused but remain separate in the 

 middle. The clefts are in many instances so small as not to be of any moment, and are not 

 even recognised until the skeleton is prepared. In a few individuals, however, they have been 

 so extensive as to allow the pulsation of the heart to be perceptible to the hand, and even to 

 the eye, through the skin covering the defect in the bone. 



A common variation in the sternum is asymmetry of the costal cartilages. Instead of cor- 

 responding, the cartilages may articulate with the sternum in an alternating manner. The 

 cause of this asymmetry is not known. 



THE THORAX AS A WHOLE 



The bony thorax (fig. 166) is somewhat conical in shape, deeper behind than in front 

 and compressed antero-posteriorly, so that in the adult it measures less in the sagittal than in 

 the transverse axis. The posterior wall, formed by the thoracic vertebrae and the ribs as far 



Fig. 167. — The Thorax. (Posterior view.) The scapulae are drawn from an X-ray 

 photograph of a man 33 years old. 



outward as their angles, is convex from above downward, and the backward curve of :the ribs 

 produces on each side of the vertebra' a (l(!ep furrow, the costo-vertebral groove, in which the 

 sacro-Hpinalis {erector spina;) niu.sclc and its subdivisions are lo(lg(!(l. The anterior wall is formed 

 by the sternum and costal cartilages. It is slightly convex and inclined forward in its lower 

 part, forming an angle of about 20° with the v(;rtical piano. The lateral walls are formed by 

 the ribs from the angles to t lu; costal cartilages. The top of the thorax presents an ellip- 

 tical aperture, the superior thoracic aperture, wliich measures on an average 12.5 centimetres (5 



