42 THE HUMAN BODY. 



to diminish the shocks and the pressure to which they are 

 subjected from the weight of the parts of the trunk above 

 them. They sink down and become thinner while the body 

 is erect, so that there is a difference in the height between 

 morning and evening of about "02 or '03 of a millimetre; 

 but repose in bed restores to the fibrous disks their primi- 

 tive thickness. 



Between the vertebral plates stretch the yellow ligaments, 

 remarkable for being formed of an elastic tissue which yields 

 to the movements of the spinal column. Other inextensible 

 ligaments envelop the spine at every point, and give great 

 solidity to the whole. The spinal column has three curves 

 two backward in the cervical and lumbar regions, and one 

 forward in the dorsal region. The ligaments which unite its 

 layers permit only a slight degree of flexibility in the upper 

 dorsal region, but this is a little more extended at the neck 

 and loins, and powerful muscles give it at need great rigidity. 

 And lastly, to its curves, and to the complicated mechanism 

 of its articulations, it owes its great power of vertical re- 

 sistance. 



The head is balanced upon the first cervical vertebra, 

 which is called the atlas; the manner of its articulation with 

 the spinal column permits great extent and freedom of move- 

 ment, while powerful ligaments and muscles give it great 

 strength. 



Thorax. The ribs are attached by the transverse pro- 

 cesses to the dorsal vertebrae, and in front by cartilages to 

 the sternum or breast-bone. They are twelve in number on 

 each side. The interstices of this bony cage are filled with 

 muscles ; they cover and form with it the walls of the chest, 

 called also the thorax or thoracic cavity, which contains the 

 lungs and heart. The flexibility of the costal cartilages, and 

 the mobility of the articulations of the ribs with the spinal 

 column, allow the thorax to expand and to -contract in re- 

 spiration. 



Upper limb. Near the apex of the cone formed by the 

 chest the upper or thoracic limbs are attached. They are 

 composed of four parts the shoulder, arm, fore-arm, and 

 hand. The two bones of the shoulder are the scapula^ which 



