280 ADVANCED PHYSIOLOGY 



fastened by tendons at the ends and generally extend between 

 two bones, moving one upon the other when they contract. 

 The muscles are generally arranged in pairs, one muscle of 

 the pair acting in opposition to the other. This is necessi- 

 tated by the fact that muscles cannot push the bones, their 

 sole power being that of contraction. 



Muscles are usually so attached to bones that a short con- 

 traction of the muscle will produce a much larger movement 

 of the bone. For example, it has been seen (Fig. 137) that 

 the biceps, by shortening an inch, will lift the hand several 

 inches. This gives great freedom and quickness of motion. 

 A few muscles, however, are fastened in such a way that the 

 muscle contracts through a greater distance than that through 

 which the weight is moved; this gives greater strength, but less 

 movement. An example of this type of arrangement is seen in 

 the "calf" muscle of the leg, when one rises on the toes; Fig. 2. 



STRUCTURE OF UNSTRIPED MUSCLE 



Unstriped muscles include all those in the walls of the 

 oesophagus, stomach and intestine, those in the arteries and 

 veins and in the contractile parts of the kidneys, ureters and 

 bladder. Unstriped or plain muscles are not attached to 

 bones and are always found in the walls of hollow organs. 

 Although, like striped muscles, they appear to act only when 

 stimulated by the nerves, they cannot be controlled through 

 the will and are, therefore, called involuntary muscles. Since 

 they are unattached and in sheets, e. g. those surrounding or 

 running lengthwise of the oesophagus, they have no tendons. 



Plain muscle is composed of very small cells or fibres each 

 with a single nucleus; Fig. 8, page 16. One of the greatest 

 differences between plain and striped muscles is in their modes 

 of contraction. Smooth, or plain muscles contract very 

 slowly, several seconds often being required for a single con- 

 traction, and they may remain contracted for some time. 

 Voluntary muscles, on the other hand ; act very quickly both 



