126 MECHANISM OF THE MUSCLES. 



every two ribs on each side of the body there are 

 muscles to pull these ribs together, which is one 

 part of the operation of breathing. It is necessary, 

 you perceive, the motion should be capable of being, 

 performed very quickly, because we sometimes have 

 occasion to breathe very quickly. How is this mo- 

 tion best effected ? 



B. One would say, by having the muscles, that 

 pull the ribs together,, pass as straight across, as 

 possible. 



T. So it might seem but it is just the reverse, 

 in principle an^ fact. The muscles do not go direct- 

 ly across from bone to bone ; that is, the strands or 

 fibres of the muscles slope very much from one 

 bone to the other. Can you explain it ? If so, you 

 will know something of a curious principle in me- 

 chanics and of the wisdom with which you are made 

 at the same time. 



A. The door seems to illustrate it very plainly. 

 If the door of a room be thrown back agaunst the 

 partition, and a person, pulling it with a string, stand 

 close to the door post/ in this situation, the line 

 will slope along in the same direction very nearly 

 with the door. Shortening the cord a foot or two 

 now, he will perceive this sloping position of the 

 line, produces a much quicker motion in the door 

 than pulling in the same quantity, when the door 

 has come to, so that the line is no longer flat with 

 the door, but nearly perpendicular to it. Whoever 

 tries the experiment will find it to be so. Now, the 



