94 CONTRACTILE ELEMENTS. 



follows the direction of the normal peristaltic contractions, 

 relaxation takes place, while if it goes in a contrary direc- 

 tion, contraction is produced. 



IV. CONTRACTILE CELLS. 



THE different properties of the contractile cells resemble 

 closely those which we have studied in the cells in general, 

 especially the faculty which they possess of changing their 

 form. This property being common to the whole mass of 

 protoplasm, we will here, after speaking of the muscle prop- 

 erly so-called, mention only those contractile cells which are 

 of special use in the system, on account of their contractility 

 or irritability. Now these elements are to be found scarcely 

 anywhere fully developed, except in the arteries, in the 

 smaller arteries especially. Thus, in order to study the func- 

 tions of these embryonic muscular forms, we must examine 

 the small vessels. (See circulation.) 



Among the movements which take place in the cells, we 

 must also mention the movements of the vibratile cells. We 

 shall speak of these in reference to the cylindrical epitheli- 

 ums which are found to have this ciliary covering. 



V. ADJUNCTS OF THE MUSCULAR SYSTEM. 

 (Connective Tissue, Bones, Tendons.) 



General Mechanism of the Muscles. The muscular 

 fibre, in changing its form, plays an important part in the 

 system as the source of labor and of movement. For this 

 purpose it is in close relation with other organs, and exhibits 

 two different tendencies, acting either by compression or by 

 traction. 



In the former case (pressure) the muscular elements are 

 arranged in the form of handles or rings, or even of mem- 

 branous pouches, in such a manner as to compress on all 

 Bides the organs which they enclose. The sphincters, the 

 muscular tubes (pharynx, oesophagus), and the heart, as well 

 as all the hollow contractile organs, are formed according to 

 this plan. Nearly all the muscles of the organic life (smooth 

 muscles) exhibit this arrangement. Their function is, gen- 

 erally, to further the passage of the liquid, or, at all events, 

 softened matters, into the interior of the reservoirs and tubes 

 of which they form the walls, and thev attain their end by 



