VII VARIOUS KINDS OF MUSCLES 303 



the iris of the eye, are so arranged as to form hollow 

 muscles. 



In the heart the muscular fibres which, though peculiar 

 are striated, are arranged in an exceedingly complex 

 manner round the several cavities, and they contract, 

 as we'have seen, in a definite order. 



The u'is of the eye is like a curtain, in the middle of 

 which is a circular hole. The muscular fibres are of the 

 smooth or unstriated kind (see p. 288), and they are 

 disposed in two sets : one set radiating from the edges of 

 the hole to the circumference of the cux'tain ; and the 

 other set arranged in circles, concentrically with the aper- 

 ture. The muscular fibres of each set contract suddenly 

 and together, the radiating fibres necessarily enlarging the 

 hole, the circular fibres diminishing it. 



In the alimentary canal the muscular fibres are also of 

 the unstriated kind, and they are disposed in two layers ; 

 one set of fibres being arranged parallel with the length of 

 the intestines, while the others are disposed circularly, or 

 rather at right angles to the former. 



As has been stated above (p. 254), the contraction of 

 these muscular fibres is successive ; that is to say, all the 

 muscular fibres, in a given length of the intestines, do 

 not contract at once, but those at one end contract first, 

 and the others follow them until the whole series have 

 contracted. As the order of contraction is, naturally, 

 always the same, from the upper towards the lower end, 

 the efiect of this peristaltic contraction is, as we have seen, 

 to force any matter contained in the alimentary canal, from 

 its upper towards its lower extremity. The muscles of 

 the walls of the ducts of the glands have a substantially 

 similar arrangement. In these cases the contraction of 

 each fibre is less sudden and lasts longer than in tlie case 

 of the heart. 



Muscles attached to definite levers. — The great 

 majority of the muscles in the body are attached to 

 distinct levers, formed by the bones. In such bones as 



