912 CENTRAL NERVOUS SYSTEM 



but also to the cessation of activity on the part of many fibers as their 

 thresholds rise above the level of the exciting stimulus. 



Smooth Muscle 



The smooth muscles of the mammalian body differ markedly in their 

 histological appearance from the skeletal muscles. Their origin in de- 

 velopment is also different as they arise from the mesenchyme, and not 

 from the mesodermic somites as the skeletal muscles do. It is not sur- 

 prising consequently to find that the properties of smooth muscle differ 

 markedly from those of the skeletal muscles. 



The contraction of smooth muscle is sluggish and never develops very 

 great tension. In this respect it resembles closely the tonic contraction 

 of skeletal muscle, as contrasted with the tetanic contraction. The most 

 outstanding feature is the ability of this tissue to alter its tonic condi- 

 tion so that it may exist now at one length, now at another, under equal 

 degrees of tension. This is seen to be a most important property when 

 it is considered that smooth muscle forms the walls of the various hollow 

 viscera and that these organs must constantly alter their capacity to 

 fit the varying volume of their contents. In the stomach and urinary 

 bladder, especially, the muscular walls must be capable of relaxing as 

 the organ becomes filled, so that the tension exerted on the contents 

 may not increase unduly. The pressure in the urinary bladder is much 

 the same whether its contents be 50 or 150 c.c. of urine. Similarly after 

 injecting 400 c.c. of water into the stomach of a dog the pressure within it 

 returns almost immediately to its original level through a change in the 

 condition of the gastric musculature. The conditions in the blood vessels 

 differ only in degree, for here the smooth muscles adjust their tone to 

 the requirements of maintaining a uniform pressure of the blood, and 

 differ from the smooth muscles of the abdominal viscera only in the 

 relatively high degree of tension which they can maintain. 



Like the tonic contraction of the skeletal muscles, the contraction of 

 smooth muscles is maintained with very little expenditure of energy and 

 consequently without fatigue. The sphincters of the gastrointestinal tract 

 and bladder remain closed almost continuously and the muscles of the 

 arterioles support the relatively great pressure of the blood unremit- 

 tingly and without becoming exhausted in the least. The smooth mus- 

 cles which hold the shells of the mollusca closed can support great weights. 

 The muscle of the fresh water clam, Anadonta, can support a weight of 3 

 kilos for three hours without consuming any oxygen in excess of its re- 

 quirements when at rest. The economy of this is seen when it is considered 

 that the gastrocnemius of the cat consumes when supporting a similar load 

 by tetanic contraction 4500 times as much oxygen as does the Anadonta 

 muscle. 



