OSSEOUS SYSTEM OF MAMMALIA. 61 



tached by their head to the body of one vertebra, and b.y their tubercle 

 to the transverse process of the succeeding vertebra, while the posterior 

 ribs are attached, exclusively, to the extremity of the transverse pro- 

 cesses. In the Monotremata the ribs are solely attached to the bodies 

 of the vertebrae. The mobility of the ribs is much influenced by the 

 modifications of their union with the vertebrae, their freedom being in 

 proportion to the simplicity of the attachment. 



The ribs may thus be considered as part of the spinal column ; not, 

 indeed, as essentials, but as accessories. They vary in number, in form, 

 in length, and in volume, in different Mammalia : in all, however, they 

 describe an arch, and terminate in a cartilaginous, or, sometimes, osseous 

 continuation, by which the anterior (the number being subject to variation) 

 are attached to the sternum, a long, flat, narrow bone, often permanently 

 divided into several parts, and forming the front of the chest. Those 

 ribs which do not join the sternum, their cartilaginous terminations falling 

 short, and ending in the muscular parieties, or sides of the chest, are 

 termed false, in contradistinction to the others, which are termed true. 



The following is a table, which the student may find of some use, 

 and which he may enlarge, according to his own observations. 



