320 PHYSIOLOGY. 



circulated, and the iris could not guard the eye against the 

 admission of too much light, which would speedily destroy 

 the vision. 



5. The shape of muscles is various/some are round, some 

 flat, and the fibres of which they are composed, are con- 

 nected by means of cellular membrane. Some are penni- 

 form, or made of bundles of fibres, diverging from a central 

 line, like the feathers of a quilly Muscles compose a large 

 part of the bulk of the body ; and when they contract, the 

 fibres shorten and become harder, as may easily be perceived 

 by placing one hand on the middle of the arm and bending 

 the elbow, or on the temple and closing firmly the lower 

 jaw. 



6. The force with which a muscle contracts^aepends on 

 the physical condition of the muscle and the entergy of the 

 brain^ When the fibres of the muscle are large and firm, 

 they will contract with more force than when they are small, 

 soft, and delicate. We see some persons, who labour under 

 great mental excitement, perform astonishing feats of 

 strength, although, perhaps, they may not have muscles of 

 the ordinary size. 



7. |lhe knee-pan has often been split in two by the con- 

 traction of the muscles of the leg ; a horse has been known 

 to break its under jaw by biting a piece of iron. Men have 

 been known to lift eight and nine hundred pounds weight ; 

 to break ropes two inches in circumference, and to bend a 

 round piece of iron, a yard long, and three inches in circum- 

 ference, to a right angle, by striking it across the left arm, 

 between the shoulder and the wrist, with the right handy* 



8. The force of muscular contraction is greatly increased 

 \by exercise.} The strength of an active man labouring to 



the greatest possible advantage, is estimatedfto be sufficient 

 to raise ten pounds, ten feet in a second, for ten hours in a 

 dayV or to raise one hundred pounds, one foot in a second, 



* Sec Dunglison's Physiology. 



