112 THE HUMAN BODY. 



distinct from the lives of its different tissues, the ciliated cells 

 in the windpipe have been found still at work three weeks 

 after the general death of the animal. 



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 them- 

 selves, 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 skele- 

 ton and joints, they constitute pre-eminently the organs of 

 motion and locomotion, and are governed 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 

 co-operation of will or consciousness; 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 plain 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 skeleton, but forms a hollow bag which drives 

 on the blood contained in it and that quite involuntarily; but 

 in its microscopic structure it resembles somewhat the skeletal 

 voluntary muscles. The muscles of respiration might perhaps 

 be cited as another intermediate group. They are striped 

 skeletal muscles and, as we all know, are to a certain extent 



