464 MOVEMENTS VOICE AND SPEECH. 



The great resistance which the elastic tissue presents to chemical action 

 serves to distinguish it from nearly every other structure in the body. It is 

 not affected by acetic acid or by boiling with sodium hydrate. It is not soft- 

 ened by prolonged boiling in water, but it is slowly dissolved, without decom- 

 position, by sulphuric, nitric or hydrochloric acid, the solution not being 

 precipitable by potassium hydrate. Its organic constituent is a nitrogenized 

 substance called elastine, containing carbon, hydrogen, oxygen and nitrogen, 

 without sulphur. This is supposed to be identical with the sarcolemnui of 

 the muscular tissue. 



The purely physical property of elasticity plays an important part in 

 many of the animal functions. Examples of this are in the action of the 

 large arteries in the circulation, and in the resiliency of the parenchyma of 

 the lungs. The ligamenta subflava and the ligamentum nuchae are important 

 in aiding to maintain the erect position of the body and head and to restore 

 this position when flexion has been produced by muscular action. Still, the 

 contraction of muscles also is necessary to keep the body in a vertical posi- 

 tion. 



MUSCULAR MOVEMENTS. 



The muscular movements are divided into voluntary and involuntary; 

 and generally there is a corresponding division of the muscles as regards 

 their minute anatomy. The latter, however, is not absolute ; for there are cer- 

 tain involuntary actions, like the contractions of the heart or the movements 

 of deglutition, that require the rapid, vigorous contraction characteristic of 

 the voluntary muscular tissue, and here the structure resembles that of the 

 voluntary muscles. With a few exceptions, however, the anatomical division 

 of the muscular tissue into voluntary and involuntary is sufficiently distinc 



Physiological Anatomy of the Involuntary Muscular Tissue. The invol- 

 untary muscular system presents a striking contrast to the voluntary muscles, 

 not only in its minute anatomy and mode of action, but in the arrangement 

 of its fibres. While the voluntary muscles are almost invariably attached by 

 their extremities to movable parts, the involuntary muscles form sheets or 

 membranes in the walls of hollow organs, and by their contraction, they sim- 

 ply modify the capacity of the cavities which they surround. On account of 

 the peculiar structure of the fibres, they have been called muscular fibre-cells, 

 smooth muscular fibres, pale fibres, non-striated fibres, fusiform fibres and con- 

 tractile cells. The distribution of these fibres to parts concerned in the or- 

 ganic functions, as the alimentary canal, has given them the name of organic 

 muscular fibres, or fibres of organic life. In their natural condition, the 

 involuntary muscular fibres are pale, finely granular, flattened, and of an 

 elongated spindle-shape, with a very long, narrow, almost linear nucleus in 

 the centre. The nucleus generally has no distinct nucleolus, and it is some- 

 times curved or shaped like the letter S. The ordinary length of these fibres 

 is about -^ (50 fi) and their breadth, about 4() i 60 of an inch (6 /x). In the 

 gravid uterus they undergo remarkable hypertrophy, measuring here -5*5- to-g^ 

 of an inch (300 to 500 /A) in length, and ^V<F f an i ncn ( 13 /*) i n breadth. 



