302 ELEMENTARY PHYSIOLOGY 



applied to the nerve, the muscle responds by an equally 

 rapid series of contractions, each of which takes place 

 before the preceding one is over ; the whole series is 

 thus added together, and the muscle remains in a state of 

 continued contraction as long as the stimuli are continued, 

 until exhaustion sets in. A prolonged contraction made 

 up of such a series of single contractions supei'added to 

 each other is called a tetanic Contraction. The acidity 

 and heat which are developed at a single contraction 

 become much more obvious during a continued tetanic 

 contraction. 



The voluntary contractions by which we execute the 

 various movements of our body are in reality, in at 

 all events nearly all cases, tetanic contractions, how- 

 ever short they may appear to be. Thus when we 

 contract one of our muscles by an effort of the will it 

 appears that a series of impulses is sent out in rapid 

 succession from the spinal cord, perhaps at the rate of 

 twelve or more in a second, to throw the muscle into 

 prolonged contraction. By this means our control of the 

 resulting movement is far greater than it would be if we 

 were only able to execute single, short and sudden con- • 

 tractions suct> as result from sending a single impulse 

 along the nerve going to tlie muscle. 



8. The Various Kinds of Muscles. —Muscles may be 

 conveniently divided into two groups, according to the 

 manner in which the ends of their fibres are fastened ; 

 into muscles not attached to solid levers, and muscles 

 attached to solid levers. 



Muscles not attached to solid Levers. — Under this 

 head come the muscles which are appropriately called 

 hollow muscles, inasnmch as they inclose a cavity or 

 surround a space ; and their contraction lessens the 

 capacity of that cavity, or the extent of that space. 



The muscular fibres of the heart, of the blood-vessels, 

 of the lymphatic vessels, of the alimentary canal, of 

 the urinary bladder, of the ducts of the glands, of 



