76 THE HUMAN BODY 



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

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

 meat. While amoeboid cells can only move themselves, and (at 

 least in the Human Body) ciliated cells the layer of liquid with 

 which they may happen to be in contact, the majority of the 

 muscles, being fixed to the skeleton, can, by alterations in their 

 form, bring about changes in the form and position of nearly all 

 parts of the Body. With the skeleton and joints, they constitute 

 preeminently the organs of motion and locomotion, and are gov- 

 erned by the nervous system which regulates their activity. In 

 fact skeleton, muscles, and nervous system are correlated 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 reason 

 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 

 former group, or skeletal muscles, are also from their microscopic 

 characters known as striped muscles, while the latter, or visceral 

 muscles, are called unstriped or smooth muscles. The skeletal 

 muscles being generally more or less subject to the control of the 

 will (as for example those moving the limbs) are frequently spoken 

 of as voluntary, and the visceral muscles, which change their form 

 independently of the will, as involuntary. The heart muscle forms 

 a sort of intermediate link ; it is not directly attached to the skele- 

 ton, but forms a hollow bag which drives on the blood contained 

 in it and that quite involuntarily; but in its microscopic struc- 

 ture it resembles somewhat the skeletal voluntary muscles. The 

 muscles of respiration are striped skeletal muscles and, as we all 

 know, are to a certain extent subject to the will; any one can draw 

 a deep breath when he chooses. But in ordinary quiet breathing 

 we are quite unconscious of their working, and even when attention 

 is turned to them the power of control is limited; no one can 



