24 HAND-BOOK OF PHYSIOLOGY 



mechanical, chemical or thermal, will render the injured part electrically 

 negative to other points on the muscle. In a frog's heart it has been 

 shown, too, that no currents exist during its inactivity, but that as soon 

 as it is injured in any way currents are developed, the injured part being 

 negative to the rest of the muscle. The currents which have been above 

 described are called either natural muscle currents or currents 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. 



J 



II. 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 mus- 

 cular tissue. For (1). it may be manifested in a muscle which is iso- 

 lated 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). Substances 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 distribution 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-standing, 

 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 impaired 

 nutrition of the muscular tissue, which ensues through its inaction. 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 partially or wholly lost, until 

 the collateral circulation is established; and when, in animals, the abdom- 

 inal aorta is tied, the hind legs are rendered 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 muscular 



