MUSCULAR ACTIVITY. 21 



ing in nerve cells the structural results (cellular collapse, &c.) of fatigue, // 

 and that in such diverse types as bee, frog, bird, and dog. . ' 



A'luscular Activity. 



The movements of a unicellular animal are due to the 

 contractility of the living matter, or of special parts of the 

 cell such as cilia (see p. 106). In sponges, there are often 

 specially contractile cells ; in most higher animals such cells 

 are aggregated to fonn the muscles on whose activity all 

 movement depends. 



In many of the lower animals, e.g., sea-anemones and sea- 

 squirts, the contractile strands consist of long spindle-shaped 

 cells which appear almost homogeneous ; these are called 

 smooth muscle fibres. They occur in certain parts of the 

 body in higher Vertebrates, e.g., on the wall of the urinary 

 bladder. A more specialised kind of muscle, prevaiUng in 

 active animals, consists of fibres which show alternate 

 light and dark cross bands ; these are called striped muscle 

 fibres. The two kinds, unstriped and striped, may be seen 

 to pass into one another in the same animal, and in a 

 general way one may think of the former as slowly con- 

 tracting, the latter as rapidly contracting. 



A piece of living muscle consists of fine transparent tubes 

 or fibres, each invested by a sheath or sarcolemma, and the 

 whole muscle is surrounded by connective tissue. It 

 usually runs from one part of the skeleton to another and 

 is fastened to the skeleton by tendons or sinews. It is 

 stimulated by motor nerves, and is richly supplied with 

 blood. 



When a muscle contracts, usually under a stimulus 

 propagated along a motor nerve, there is of course a change 

 of shape — it becomes shorter and broader. The source of 

 the energy expended in work done is the "chemical ex- 

 plosion" which occurs in the fibres, for the o.xygen stored 

 up (intramolecularly) in the muscle enters into rapid union 

 with a carbohydrate. Heat, COo, and water are produced 

 as the result of this combustion, and lactic acid is also 

 formed as a bye-product. Besides the chemical change and 

 the change of shape, there are also changes of electrical 

 potential associated with each contraction. 



