GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 71 



If the stimulation be still further increased in frequency, the individual 

 contractions become fused together and the curve described by the lever 

 becomes a continuous line. (See Fig. 33.) Notwithstanding the fact 

 that the individual contractions are no longer visible, it can be shown by 

 other methods that the muscle is undergoing a series of slight alternate con- 

 tractions and relaxations or vibrations at least. After a varying length of 

 time the muscle becomes fatigued, relaxes, and returns to its natural con- 

 dition even though the stimulation be continued. The number of stimuli 

 per second necessary to develop complete tetanus will depend under normal 

 circumstances on the period of duration of the individual contractions. 

 The longer this period, the less the number of stimuli required, and the 

 reverse. Hence the number of stimuli will vary for different classes of animals 

 and for different muscles in the same animal, e.g., 2 or 3 for the muscles of 

 the tortoise, 10 for the muscles of the rabbit, 15 to 20 for the frog, 70 to 

 80 for birds, 330 to 340 for insects. 



An effect which follows frequent stimulation of a muscle, e.g., 50 to 

 60 times per minute, and especially when the muscle is somewhat fatigued 

 or cold is shown in Fig. 34. It is evidently a combination of contracture and 

 fatigue. It will be observed that at the beginning of the stimulation there 

 is a staircase effect, a-b, combined with diminished relaxation. This in 

 turn is followed by a decline in the height of the contractions, b-c, and a fall 

 of the base line which may be attributed to fatigue conditions. After a short 

 time there is a second rise of the base line, d, and a rapid development of 

 contracture. The muscle at this period is in a condition of incomplete 

 tetanus which gradually passes into complete tetanus attended by fatigue. 



The tetani of muscles may be classified in accordance with their causes 

 as follows: 



. , . f Volitional. 

 . i. Physiologic { Reflex _ 



2. Experimental. 



3. Pharmacologic. 



0^1 f Bacterial. 



4. Pathologic ( Reflex 



i. Physiologic Tetanus, i. Volitional. Because of the fact that 

 during the continuance of a volitional movement the muscle is in a state of 

 continuous contraction, it may be accepted that volitional contractions are 

 states of tetanus, more or less complete; for the shortest possible volitional 

 contraction, however quickly it takes place, has always a longer duration 

 than a single contraction caused by an induced electric current. As the 

 volitional contraction is similar to that observed when a muscle or its related 

 nerve is stimulated by rapidly repeated induced currents, it is assumed that 

 the nerve-cells in the spinal cord are discharging in a rhythmic manner a 

 certain number of nerve impulses per second in consequence of the arrival 

 of nerve impulses coming from the cerebral cortex, the result of volitional 

 acts. In other words the volitional tetanus is the result of a discontinuous 

 stimulation. The number of stimuli transmitted to a muscle during a 

 volitional tetanus has been estimated by the employment of the graphic 

 method at from 8 to 13 per second, 10 being about the average. When a 



