SPINE OF THE VERTEBRATA. 87 



generally twelve in number on each side, though in 

 some instances they are found to be thirteen, and more 

 rarely only eleven. They are distinguished into true 

 and. false ribs. The seven upper ones, which are artic- 

 ulated, or joined to the sternum, c, are called true ribs, 

 while the five lower ones, which are not immediately at- 

 tached to the sternum, or breast bone, but to a cartilage 

 connected with it, are called the false ribs. The other 

 extremities of the ribs are articulated by small heads to 

 the vertebrae, and secured by a ligament, so as to allow 

 of a small motion upwards and downwards, but in no 

 other direction. 



The use of the ribs is to give form to the thorax, to 

 cover and defend the lungs and heart, which are situa- 

 ted within them ; and also to assist in breathing, by their 

 alternate elevation and depression. 



The sternum, c, or breast bone, it is well known is sit- 

 uated in front of the thorax. In young people this bone 

 is in several parts, united by cartilages ; but as we ad- 

 vance in life these cartilages ossify, or are changed into 

 bone. Its shape is oblong, and its external surface con- 

 vex. To its edges are immediately articulated the sev- 

 en upper ribs. 



The clavicle, b, has its name from the Latin clavis, 

 which appears to come from clando, to shut, this bone 

 resembling in shape an ancient key. It is usually called 

 the collar bone, and is placed at the upper part of the 

 breast, or root of the neck, extending across from the 

 tip of the shoulder to the upper part of the sternum. It 

 is a round bone, curved somewhat into the shape of the 

 italic S. It supports, and maintains the shoulder in its 

 proper place, and prevents it from falling forwards to- 

 wards the breast. Its outer end is firmly fixed to the 

 upper part of the scapula, or shoulder blade. Animals 

 which employ their fore feet as hands, are furnished with 

 this bone, as the monkey tribe and squirrels ; while 



What is the number of ribs in the human skeleton 7 How are they 

 distinguished 1 To what part are the seven upper ribs articulated in 

 front 1 How are the anterior ends of the five lower ribs secured 1 How 

 are the posterior ends of all the ribs secured 7 What are the uses of the 

 ribs 1 What is said of the sternum 1 Whence is the name clavicle ? 

 What is the common name of this bone 7 To what parts is the clavicle 

 articulated 1 What animals besides man have this bone 7 



