CONTRACTION OF MUSCLE Zg 



interesting to notice, as a triumph of histological technique, that 

 Hodge, Gustav Mann, and others have succeeded in demonstrating in 

 nerve cells the structural results (cellular collapse, etc.) of fatigue, and 

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



Muscular 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 lashes or cilia. In 

 sponges specially contractile cells begin to appear ; in most 

 higher animals such cells are aggregated to form the muscles. 



There are two distinct types of muscle, with some inter- 

 mediate forms. The most highly developed is the cross- 

 striated or skeletal muscle, which typically consists of 

 numerous fine transparent tubes or fibres, each invested 

 by a sheath or sarcolemma, while 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 controlled by motor 

 nerves, which may bring about a sharp " twitch " con- 

 traction, a powerful maintained contraction (tetanus), or a 

 steady *' postural " state of tension (tonus). The fibres 

 of smooth or involuntary muscle are of an attenuated 

 spindle shape ; their contractions are more sluggish, and 

 they frequently encircle hollow organs without being 

 bound to the hard parts of the body. They are not under 

 voluntary control, though they have their motor nerves 

 and take part in reflexes ; but they have some inde- 

 pendence of the nervous system, and can maintain a tonic 

 tautness or even carry on a rhythmic series of contractions 

 automatically. Muscle displays to some extent the 

 phenomena of the all-or-none law and the refractory 

 period. 



When a muscle contracts, usually under a stimulus 

 propagated along a motor nerve, it and each of its fibres 

 becomes shorter and broader. The contraction of the 

 fibres is itself a physical rather than a chemical pheno- 

 menon, like a change in the state of a spiral spring. In the 

 actual contraction there is no using up of oxygen or output 

 of CO2, but lactic acid is set free from glycogen within the 

 muscle, and this acts by its acidity on the fibres, causing 

 them to contract. Relaxation takes place when the lactic 

 acid is neutralised, chiefly by bases set free from combina- 



