MUSCULAR POWER. 



105 



wc frequently find that different portions of the same mus- 

 cle have the. power of contracting independently of the rest, 

 so as to be capable of producing very various effects, accord- 

 ing as they act separately or in combination. This is exem- 

 plified in the muscle of the back, called the Trapezius, repre- 

 sented in Fig. 44. In many instances, the fibres radiate in all 

 directions from a common centre: this is the case with the 

 delicate muscle of the ear-drum, as shown in Fig. 45. In 

 that of the elephant, which is about an inch and a half in 

 diameter, these radiating fibres are very conspicuous, even 

 to the naked eye: and they are also visible in the membrane 

 of the human ear, when viewed with a good microscope.* 



At other times, the muscular fibres run in a circular di- 

 rection, forming what is called an orbicular, or sphincter 

 muscle, of which an example occurs in that which surrounds 

 and closes the eye. (Fig. 4G.) Very frequently these two 

 last modes of arrangement are united in some part, as ap- 

 pears to be the case in the membrane of the eye, called the 

 Iris. (Fig. 47.) The circular fibres of the iris surround the 

 central aperture, or pupil, the size of which they diminish 

 when they contract; while, on the contrary, the radiating 

 fibres, acting on the inner circle, and drawing it nearer to 

 the outer circumference, which is fixed, lessen the breadth 

 of the ring, and consequently enlarge the circular aperture. 



47 



43 



A similar combination of radiating and circular fibres is 

 employed in the construction of flat, or slightly concave 

 muscular disks, which are thus rendered capable of exerting 

 a strong force of adhesion to the surfaces on which they are 

 applied. In these organs the circular fibres are placed at 



Vol. I. 



Home Phil. Trans, for 1800, p. 1. 

 14 



