34 



THE INDIVIDUAL ORGANISM 



sternum, and the cartilages that connect the ends of the ribs with the 

 sternum. Each rib at its basal end is attached to a vertebra by means of 

 two movable articulations. The curved ribs not only project forward but 

 also slant downward, and their convexities also tilt somewhat downward. 

 Their attachments are such that contraction of the thoracic muscles lifts 

 the outer ends of the ribs and also rotates each rib slightly so that its 

 convexity extends more nearly at right angles to the median axis of the 

 body. The sternum is lifted by the flexible cartilaginous connections with 



clavicle 



coracoid process 



Fig. 3.5. The pectoral girdle and ribs. 



the ribs and pushed somewhat outward. The result is that the depth and 

 breadth of the thoracic cavity are increased. At the same time the up- 

 domed muscular diaphragm that forms the floor of the cavity contracts 

 and pulls downward. These movements increase the cubic capacity of 

 the thorax and cause inspiration of air into the lungs. 



The appendicular skeleton. The bony framework of the arms and 

 legs, and the girdles that attach these appendages to the axial skeleton, 

 constitute the appendicular skeleton. Comparison will show that the two 



