5* 



9. Describe the action of the muscles concerned during walking, running, 

 and jumping. 



10. Mention the different ways in which a muscle may be made to contract. 



1 1. What are flexor muscles and extensor muscles ? 



12. What is rigor mortis ? How is it caused ? 



.-, LESSON VIII. 



THE STRUCTURE OF MUSCLE. 



THERE is a distinct difference between the structure of voluntary 

 muscles and that of involuntary muscles. But the two kinds 

 resemble each other in that 



(1) They are both composed of fibres. 



(2) The fibres of both have the power of contraction. 



(3) Both produce motion by this contraction. 



We will first study the structure of voluntary muscles. As 

 already stated, we include under 

 this term all those muscles which 

 may be set in motion by an 

 effort of the will. If we remove 

 the skin from the leg of a rabbit, 

 we at once expose several of 

 these voluntary muscles, each 

 of which is connected with two 

 separate bones. Then if we trace 

 one of these muscles to one of 

 its ends its origin or its inser- 

 tionwe notice that it is con- 

 nected with the bone by means 

 of a tendon. Again, we may see 

 that each of the muscles of the 

 leg is surrounded by a sheath, composed of a very thin and trans- 

 parent skin or membrane. 



Now let the leg of the rabbit be boiled till the flesh is easily 

 torn asunder ; and, by means of needle points, we may then ' tease 

 out ' the flesh in such a manner as to show that each muscle is 

 composed of bundles of fibres. 



As a further, and, perhaps, a better illustration of this last 

 point, take a small portion of a very large muscle, such as one of 

 those which form the fleshy portion of a ' leg of beef.' Let this 

 also be boiled till its fibres may easily be separated by needle 

 points. We shall now see distinctly that not only is the muscle 



Fig- 55- A. small Portion of Muscle, 

 consisting of larger and smaller 

 Fasciculi. 



A, natural size ; B, magnified five diameters. 



