406 HANDBOOK OF PHYSIOLOGY. 



been above described are called either natural muscle currents or cur- 

 rents of rest, according as they are looked upon as always existing in 

 muscle or as developed when a part of the muscle is subjected to injury; 

 in either case, up to a certain point, it is agreed that the strength of the 

 currents is in direct proportion to the injury. 



B. Activity. 



The property of muscular tissue, by which its peculiar functions 

 are exercised, is its Contractility, which is excited by all kinds of 

 stimuli applied either directly to the muscles, or indirectly to them 

 through the medium of their motor nerves. This property, although 

 commonly brought into action through the nervous system, is inherent 

 in the muscular tissue. For (1.) it may be manifested in a muscle 

 which is isolated from the influence of the nervous system by division 

 of the nerves supplying it, so long as the natural tissue of the muscle is 

 duly nourished; and (2.) it is manifest in a portion of muscular fibre, in 

 which, under the microscope, no nerve-fibre can be traced. (3.) Sub- 

 stances such as urari, which paralyze the nerve-endings in muscles, do 

 not at all diminish the irritability of the muscle. (4.) When a muscle 

 is fatigued, a local stimulation is followed by a contraction of a small 

 part of the fibre in the immediate vicinity without any regard to the dis- 

 tribution of nerve-fibres. 



If the removal of nervous influence be long continued, as by division 

 of the nerves supplying a muscle, or in cases of paralysis of long-stand- 

 ing, the irritability, i. e., the power of both perceiving and responding 

 to a stimulus, may be lost; but probably this is chiefly due to the im- 

 paired nutrition of the muscular tissue, which ensues through its inac- 

 tion. The irritability of muscles is also of course soon lost, unless a 

 supply of arterial blood to them is kept up. Thus, after ligature of the 

 main arterial trunk of a limb, the power of moving the muscles is par- 

 tially or wholly lost, until the collateral circulation is established; and 

 when, in animals, the abdominal aorta is tied, the hind legs are ren- 

 dered almost powerless. 



The same fact may be readily shown by compressing the abdominal 

 aorta in a rabbit for about 10 minutes; if the pressure be released and 

 the animal be placed on the ground, it will work itself along with its 

 front legs, while the hind legs sprawl helplessly behind. Gradually the 

 muscles recover their power and become quite as efficient as before. 



So, also, it is to the imperfect supply of arterial blood to the muscu- 

 lar tissue of the heart, that the cessation of the action of this organ in 

 asphyxia is in some measure due. 



Besides the property of contractility, the muscles, especially the 

 striated, possess Sensibility by means of the sensory nerve-fibres distrib- 



