EICKETS. 57 



Atmospheric pressure, acting during inspiration, may drive in 

 the softened ribs in the neighbourhood of the costal cartilages 

 causing a lateral sulcus with curved projection of the sternum and 

 pigeon breast. The action of the diaphragm on the lower six 

 costal cartilages, to which it is attached, is believed to produce 

 constriction of the lower thoracic zone, and at the same time the 

 costal margin is everted by the mass of the enlarged liver, and 

 also by the enlarged spleen. 



An accumulation of imperfectly ossified spongy membrane 

 bone at the centres of ossification of the parietal and frontal 

 bones causes an exaggeration of the frontal and parietal emi- 

 nences. In the parietal bones, and the vertical plate of the 

 occipital bone, a process of atrophy known as cranio-tabes may 

 occur, so that the skull in these regions becomes thin and flexible 

 or is even perforated. The weight of the head, resting on the 

 pillow, may produce flattening and deformity of the bone thus 

 softened. The process of dentition is delayed by rickets, the 

 proper development of the teeth being interfered with. In 

 addition certain deformities of the jaws are induced. The 

 mandible is stunted in its growth, and the alveolar margins of 

 the jaws may become inverted by pressure of the masseters and 

 muscles which surround the oral aperture. In infants the gentle 

 continuous curve of the spine becomes exaggerated, so that it is 

 much more convex backwards than is normal. This is the result 

 of the weight of the head and the relaxation of the supporting 

 muscles and ligaments. The curve usually rights itself when the 

 child is supported by the arms, but occasionally there appears to 

 be some rigidity in the lumbar region. Later on lateral curvature 

 may also show itself. 



Greenstick fractures are common in the long bones and 

 especially in the clavicles. The softened bones yield readily to 

 slight violence, but the pulpy periosteum retains the ends in 

 apposition. 



