lOS 



THE SKELETON 



completely cleft t^ternuni. The union of the two halves may occur in the region of 

 the manul)riuni, but fail below this point; in some instances the upper and lower 

 segments have duly coalesced with the opposite side, but union has failed in the 

 middle segments. The clefts resulting from these failures of coalescence are in 

 manv instances so small as not to be of anv moment, and not even recognised until 



Fii;. llfi.— Two Stages ix the Fokmatiox of the" Cartilaginous Sternum. (After Ruge.) 



CLAVICLE / y 



B 



the skeleton has been prepared. In a few individuals they have been so extensive 

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

 ej'e, through the skin covering the defect in the bone. 



A common variation in the sternum is asymmetr}' of the costal cartilages. 

 Instead of corresponding, the cartilages may articulate with the sternum in an alter- 

 nating mamuT. Tlio cause of this asymmetry is not known. 



THE THORAX 



The thorax is a bony cage of conical shape, formed by the thoracic vertebrae, 

 the ribs with their costal cartilages, and the sternum. The thorax is compressed 

 antero-posteriorly so that it measures less in the sagittal than in the transverse axis; 

 it is also deeper posteriorly than anteriorly. Its jxisterior boundaries are formed 

 by the thoracic vertel)ne and the ril)s as far outward as their angles; the backward 

 curve of the ribs ])ro(luces on each side of the vertebrse a deep furrow, the costo- 

 vertebral groove, in Avhich the erector spinas muscle and its subdivisions are 

 lodged. 



The anterior boundary is formed b}^ the sternum and costal cartilages. This 

 surface is slightly convex, and has a slight inclination forwards in its lower 

 part. 



The lateral boundaries are formed l)y the ribs from their angles to the costal 

 cartilages. 



The top of the thorax presents an elliptical aperture, the thoracic inlet, Avhich 

 measures, on an average, 12-5 cm. (5"; transversely, and 6-2 cm. (2V') in its 

 sagittal axis. 



The lower opening of the thorax is very irregular, and forms two curved lines 



