526 MOVEMENTS. 



acid. The action of the acetic acid renders the accessory structures of the ligament 

 transparent, and the elastic fibres become very distinct. The same may be accomplished 

 by boiling the tissue for a short time in caustic soda. 



The third variety of elastic tissue can hardly be said to consist of fibres, as their branches 

 are so short and their anastomoses so frequent. This kind of structure is found forming 

 the middle coat of the large arteries, and it has already been de- 

 scribed in connection with the vascular system. The fibres are 

 very large, flat, with numerous short branches, " which unite 

 again with the trunk from which they originate or with adja- 

 cent fibres. In certain situations, the interstices are considera- 

 ble, in proportion to the diameter of the fibres, and the anasto- 

 mosing branches are given off at acute angles, so that they 

 follow pretty closely the direction of the trunks, and the anas- 

 tomoses do not disturb the longitudinal direction and parallelism 

 of the fibres. Indeed, the anastomoses are so numerous, and the 

 intervals so small, proportionally to the fibres, that we should 

 believe we had under observation a reticulated membrane, pre- 

 diameters, senting openings, rounded and oval, some large and others 

 small." (Henle.) These anastomosing fibres, forming the so- 

 called fenestrated membranes, are arranged in layers, and the structure is sometimes 

 called the lamellar elastic tissue. 



The great resistance which the elastic tissue presents to chemical action serves to 

 distinguish it from nearly every other structure in the body. We have already seen that 

 it is not affected by acetic acid or by boiling with caustic soda. It is not softened 

 by prolonged boiling in water, but it is slowly dissolved, without decomposition, by 

 sulphuric, nitric, or hydrochloric acid, the solution not being precipitable by potash. Its 

 organic base is a nitrogenized substance called elasticine, containing carbon, hydrogen, 

 .oxygen, and nitrogen, without sulphur. This is supposed to be identical with the sarco- 

 lemma of the muscular tissue. 



The purely physical property of elasticity plays an important part in many of the 

 animal functions. We have already had an example of this in the action of the large 

 arteries in the circulation and in the resiliency of the parenchyma of the lungs ; and we 

 shall have occasion, in treating of the functions of other parts, to refer again to the uses 

 of elastic membranes and ligaments. The ligamenta subflava and the ligamentum nucha) 

 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 is also necessary to keep the body in a vertical position. 



Muscular Movements. 



Muscular movements are observed only in the higher classes of animals. Low in the 

 scale of animal life, we have the contractions of amorphous substance and ciliary motion ; 

 and, in some vegetables, movements, even attended with locomotion, have been observed. 

 These facts make the absolute distinction between the two kingdoms a question of some 

 difficulty; but in animals only, do we have a distinct muscular system. 



The muscular movements capable of being excited by stimulus of various kinds 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 certain involuntary functions, like the action of the heart or the movements 

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

 muscular tissue, and here we do not find the structure characteristic of the involuntary 

 muscles. With a few exceptions, however, the anatomical division of the muscular tissue 

 into voluntary and involuntary is sufficiently distinct. 



