CHAPTER VI 

 THE STRUCTURE OF THE MOTOR ORGANS 



Motion in Animals. Motion is produced in animals by various 

 sorts of motor tissues (p. 32), but in all the underlying mechanical 

 principle is the same, namely, the forcible contraction of some ele- 

 ment or elements. Various means of making these contractions 

 effective exist in nature. The most familiar is that already cited 

 (p. 37) of causing the contractile structure to pull across a movable 

 joint. In some situations, the human stomach, for example, a 

 hollow organ is completely surrounded by contractile tissues, whose 

 contractions diminish, and whose relaxations permit increase of 

 the capacity of the organ. Still another form of motion is that of 

 the cilia previously mentioned (p. 33). 



The Muscles. These are the main motor organs; their general 

 appearance is well known to every one in the lean of butcher's 

 meat. The majority of them being fixed to the skeleton can, by 

 alterations in their form, bring about changes in the form and posi- 

 tion of nearly all parts of the Body.- With the skeleton and joints, 

 they constitute preeminently the organs of motion and locomotion, 

 and are governed by the nervous system which regulates their ac- 

 tivity. In fact skeleton, muscles, and nervous system are cor- 

 related parts: the degree of usefulness of any one of them largely 

 depends upon the more or less complete development of the others. 

 Man's highly endowed senses and his powers of reflection and rea- 

 son would be of little use to him, were his muscles less fitted to carry 

 out the dictates of his will or his joints less numerous or mobile. 

 All the muscles are under the control of the nervous system, but 

 all are not governed by it with the cooperation of will or con- 

 sciousness; some move without our having any direct knowledge 

 of the fact. This is especially the case with certain muscles which 

 are not fixed to the skeleton but surround cavities or tubes in the 

 Body, as the blood-vessels and the alimentary canal, and by their 

 movements control the passage of substances through them. The 



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