72 TEXT-BOOK OF PHYSIOLOGY. 



volitional contraction is recorded the myogram not infrequently exhibits a 

 series of small wave-like elevations which indicate that the muscle is not in 

 a state of complete tetanus but is undergoing slight alternate contractions 

 and relaxations. Unless the contraction process in human muscle differs 

 from that of frogs it is difficult to see how 10 or even 20 stimuli per second 

 can give rise to even an incomplete tetanus when the single contraction is -^ 

 of a second in duration. 



2. Reflex. A tetanus of muscle, physiologic in character, arises during 

 the performance of many muscle movements in consequence of peripherally 

 acting causes and may therefore be termed a reflex tetanus. The duration 

 of a tetanus thus induced, like the duration of a volitional tetanus, will vary 

 with the duration of the exciting cause. Reflex tetani are presented by the 

 muscles of the lower jaw during mastication, by the intercostal muscles 

 during breathing, by the muscles of the limbs during walking, etc. In these 

 and other instances there are reasons for believing that for a variable period 

 of time the muscles are in a state of continuous contraction from the dis- 

 charge of nerve impulses from the nerve cells in the spinal cord as the result 

 of the arrival of nerve impulses coming from a peripheral surface. 



2. Experimental Tetanus. The tetanus of muscle developed in 

 accordance with the method described in foregoing paragraphs, i.e., by the 

 employment of instrumental procedures, may be termed experimental 

 tetanus. Its mode of development serves to illustrate and explain the 

 method by which individual contractions are summated and continuous 

 contractions made possible for the performance of volitional acts. 



3. Pharmacologic Tetanus. The administration of certain drugs, e.g., 

 strychnin, in sufficient amounts, is followed in a short time by a series of 

 intermittent spasms in which all the muscles of the body are involved. At 

 the beginning of the spasms the muscles are thrown into tonic or complete 

 tetanus, during the continuance of which the muscles are hard and firm. In 

 a short time this tonic state begins to subside, giving way to tremors or a 

 series of irregular contractions resembling incomplete tetanus or clonus. A 

 tetanus of this character may be termed pharmacologic. Though the onset 

 of the tetanus is occasioned largely by peripheral stimulation, the seat of 

 action of strychnin is central and for the most part focalized in the spinal 

 cord. The exact seat of its action is not definitely determined but there are 

 reasons for believing that it is on the end-tufts of afferent nerves in the spinal 

 cord or on the intercalated neuron between them and the nerve-cells in the 

 anterior horns of the gray matter, the irritability of which is raised and the 

 resistance to the transmission of nerve impulses coming from the periphery 

 diminished. As a result the nerve impulses are transmitted to the nerve- 

 cells more readily, not only in a horizontal but also in a longitudinal direction, 

 and the effects they produce enormously increased. 



4. Pathologic Tetanus, i. Bacterial. The introduction of a specific 

 bacillus into a wound in any region of the body is followed after a period of 

 incubation of from three or four days to a week by a tetanus in which nearly 

 all the muscles of the body are involved, characterized by a tonic contraction 

 and clonic exacerbations. A tetanus of this character may be termed 

 pathologic. The persistent tonic contraction is the result of a more or less 

 continuous discharge of nerve impulses from the nerve-cells of the spinal cord 



