GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 75 



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 

 which have been rendered abnormally irritable by the action of a toxin, 

 produced by the bacilli, and having a selective action on these structures. 

 The clonic exacerbations are evoked from time to time by various forms of 

 peripheral stimulation. 



2. Reflex. A tetanus of individual muscles more or less continuous in 

 character is occasionally the result of peripheral irritations of a pathologic 

 character. A tonic contraction of the masseter muscles, for example, firmly 

 closing the jaws for weeks and months at a time is caused in some instances 

 by an impacted wisdom tooth or an ulcerative condition of the mouth. 

 Since the removal of the cause is followed by a relaxation of the muscle, this 

 form of tetanus, known as trismus, may be regarded as pathologic in char- 

 acter and reflex in origin. 



