ii.] THE SPINAL SKELETON. 35 



The bone is flat. Of nearly equal width for the greats- 

 part of its length, it broadens out above and narrows greatly 

 at its lower end. 



The broad upper part to which the first rib is annexed is 

 called the manubrium or pre-sternum. 1 - 



The narrow lower end, which projects freely and remains 

 cartilaginous till late in life, is called the xiphoid 2 process. 



17. The RIBS are long, slender, curved bones, which extend 

 from the spine, and some of them join the breastbone or 

 sternum. They are twelve in number on each side. The 

 seven upper ribs on each side join the sternum by cartilages, 

 and are termed " true ribs." The five lower ribs do not join 

 the sternum, and are called "false ribs." The first rib is 

 much stouter and shorter than the others. 

 . Each rib, or costa (except the last two on each side), has a 

 double attachment to the backbone. At its hinder end is a 

 rounded "head," which articulates with the capitular surface 

 of the dorsal vertebrae. A little distance from this there is 

 on the outer side a rounded articular prominence called the 

 " tubercle? which joins the articular surface on the anterior 

 side of each transverse process of the dorsal vertebrae. 

 Between the head and tubercle is a narrower interval called 



Each rib ends at its ventral termination in an elongated 

 cartilage called " costal? Those costal cartilages which are 

 attached to the true ribs, have somewhat pointed inner ends, 

 and these join the sides of the sternum. Those of the false 

 ribs either (as those of the upper three) blend with the lower 

 border of the costal cartilage next above, or else end freely in 

 a blunt point. The backbone and breastbone, with the ribs, 

 form together a sort of bony cage, called the skeleton of the 

 thorax? which is narrow above, broaj and widely open 

 below, and wider at its greatest breadth than it is deep at its 

 greatest depth from before backwards. 



This variation in its dimension from above downwards is 

 produced by the corresponding variation in the length of the 

 ribs, which increases from the first to the eighth, and then 

 gradually decreases. 



1 8. The DEVELOPMENT of the skeleton of the trunk, or 

 spinal endoskeleton, is briefly as follows : 



From each side of the primitive groove, mentioned in the 

 First Lesson, a longitudinal fold extends up on each side 



1 A handle ; maims, a hand. 2 From ?/i/>o, a sword ; eiioc, like. 



3 From 6>tpa, a breastplate. 



D 2 



