THE MUSCLES. 539 



BLOOD-SUPPLY. The blood-supply to unstriped muscle is very 

 free, but not nearly so liberal as that to voluntary muscle. The nerve- 

 supply is from the sympathetic system, and comprises both medullated 

 and nonmedullated fibers. The fibers form a main plexus, lying in 

 the connective tissue of the perimysium. From this plexus of fibers 

 there come off numerous fibrils, which traverse the fiber and nucleus. 



Irritability of Muscle. Contractility, elasticity, tonicity, and 

 irritability are terms used to designate various properties of muscles. 



Thus, contractility is the property the muscle possesses of short- 

 ening and of giving a contraction when it is excited. 



Elasticity is the general property, common to muscles and many 

 other bodies, of stretching under the influence of a weight and of 

 then returning, more or less perfectly, to the first shape. 



Tonicity is the state midway between extreme contraction and 

 relaxation. It is a condition depending upon the central nervous 

 system. 



In addition, muscle possesses a property that is common to all 

 live tissues and which is of fundamental importance in general physi- 

 ology. It is irritability. By irritability is meant that property of a 

 living element to act according to its nature under the stimulus of 

 an excitant. 



Paralyses have been observed which have lasted for several 

 months or even several years and, although the nerves were abso- 

 lutely unexcitable, yet the muscles had retained their irritability. 

 This may be readily demonstrated in cases of paralysis of the seventh 

 pair of nerves. 



The independence of muscle irritability is formally demonstrated 

 by experiment in which the known action of the drug, curare, upon 

 muscles is taken advantage of. A watery extract of this drug, when 

 injected into the blood of an animal or introduced beneath its skin, 

 acts chiefly upon the motor nerve-endings. It does not, however, 

 affect muscular contractility. Curare is an agent which separates the 

 muscle-element from the nerve-element by a physiological dissection 

 much superior to the coarse anatomical dissections which we could 

 make. 



When a few milligrams of this drug are injected into the dorsal 

 lymph-sac of a frog, the poison is absorbed within a few minutes. 

 The animal soon ceases to support itself, but lies in any position in 

 which it may be placed by the experimenter. It is paralyzed, produc- 

 ing neither voluntary nor reflex movements. Now, should the brain 

 be destroyed, the skin removed, and the sciatic nerve stimulated by 



