RESPIRATION. 123 



hoops surround a barrel. But their course is not horizontal. 

 They incline downward from the back-bone to the breast- 

 bone in front ; consequently, the diameter of the chest is so 

 much lessened by this obliquity of position ; but when the 

 ribs are raised to a horizontal position, at right angles with 

 the axis of the chest, this diameter is increased, and the 

 capacity of this cavity is enlarged. This is p 



easily shown by the experiment of putting 

 a large hoop obliquely upon a barrel of 

 smaller diameter. The hoop, a, c, in its ob- 

 lique position, touches the barrel ; but, if the 

 hoop be raised horizontally to 6, it would ex- 

 tend beyond the cask, and allow it a much 

 greater expansion. 



CHAPTER II. 



Movements of Ribs Diaphragm. Expansion of Chest in Inspira. 

 tion. Contraction in Expiration. Size of expanded and con. 

 tracted Chest. 



271. THE posterior ends of the ribs are attached to the 

 back-bone, and fixed. The motions are all made with the 

 anterior ends, which are free. They are joined to the spine 

 in such a manner that they can only move upward and down- 

 ward, not from side to side. The spine being the pillar 

 upon which the frame of the chest rests, it is fixed and im- 

 movable in breathing ; but all the movements of the ribs and 

 breast-bone are made upon it. These are lifted up and fall 

 down at every respiration. 



272. The first or upper rib is fixed and motionless ; the 

 second has very little motion; the third has more motion 

 than the second ; and the fourth more than the third. This 

 motion goes on increasing to the eleventh and twelfth, which 

 move very freely. There are several muscles which are at- 



