158 Dynamic Theory. 



there are six fontanelles or spaces between the edges of the skull bones 

 covered by cartilaginous membranes instead of bone 1 , and the innomina- 

 tum, or hip bone, is then in three pieces, the ilium, ischium and pubis. 

 The fontanelles close up with . bone during infancy, and during child- 

 hood the three bones of the innominatum are consolidated into one. 

 The sternum in youth is also in three pieces, which are consolidated in 

 advanced life. 



The coccyx, or tail bones, four in number, are separate from each 

 other in infancy but generally consolidate with age. As age advances, 

 the bones become harder and less elastic, and ossification is liable to ex- 

 tend to cartilages, especially in the ribs and larynx. 



The whole development of cartilage consists in a gradual packing and 

 hardening of tissue, brought about by mechanical compression under 

 stimulus and work, the habit of growth of each age tending to perpetu- 

 ate itself by hereditary transmission. 



The first cartilaginous formation in the body is that of the seven neck 

 or cervical vertebrae. But the first ossification in the body is that of 

 the clavicle, or collar bone. 



FIG. 82. Human Pectoral Arch. 



A Sternum (breast bone). 



B Scapula (shoulder blade). 



C Clavicle (collar bone). 



D Upper part of back bone. 



R,-Ribs. 



lAcromion process of shoulder blade articulating with clavicle. 



1Glenoid Cavity. Place where the arm is attached. 



3Coracoid process of the shoulder blade (or raven's beak). 



1 The skull of Selachian fishes retains the fontanelles during life. 



