472 MUSCLE-NERVE PHYSIOLOGY 



to its normal condition. In the living body the muscles are always stretched 

 somewhat beyond their natural length, they are always in a condition of 

 slight tension: an arrangement which enables the whole force of the con- 

 traction to be utilized in approximating the points of attachment. If the ex- 

 tensibility of a given muscle be measured by adding to it equal increments 

 of weight, it will be found that the extension or stretching is considerable at 

 first, but that the amount decreases with each additional weight. If the 

 figures obtained be plotted on co-ordinate paper, a curve approaching a parab- 

 ola is obtained, whereas a steel spring is perfectly elastic and gives a straight 

 line. When the weights are removed from a stretched muscle, one by one, 

 the muscle regains its original length, though slowly. Extreme fatigue 

 greatly decreases the elasticity, while an increase of temperature increases it. 



Cardiac muscle and smooth muscle both manifest elasticity in the same 

 manner as skeletal muscle. In fact, the elasticity of the arterioles is chiefly 

 due to the smooth muscle in their walls, a fact that is of great importance in 

 the adaptability of the circulatory apparatus. The flexibility of the stom- 

 ach, the urinary bladder, etc., is traceable to the same property of their 

 muscular walls. 



Contractility and Irritability of Muscle. 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 in- 

 directly to them through the medium of their motor nerves. The property 

 of the muscle which enables it to respond to a stimulus is called its irritability. 

 This property, although commonly brought into action through the nervous 

 system, is inherent in the muscular tissue. This is proven: i. By the fact 

 that contractility is manifested in a muscle which is isolated from the influ- 

 ence of the nervous system by division of the nerves supplying it so long as 

 the natural tissue of the muscle is duly nourished. 2. It is manifested in a 

 portion of muscular fiber in which, under the microscope, no nerve fiber can 

 be traced. 3. Substances such as curara, which paralyze the nerve endings 

 in muscles, do not at all diminish the irritability of the muscle itself. 4. 

 When a muscle is fatigued, a local stimulation is followed by a contraction 

 of a small part of the fiber in the immediate vicinity, without any regard to 

 the distribution of nerve fibers. 



Forms of Stimuli for Muscle or Nerve. The power of contraction 

 in voluntary muscles is normally called forth in the body by nerve impulses 

 which reach the muscles over the motor nerves. But a muscle will respond 

 to stimuli of various kinds, and these stimuli may be applied directly to the 

 muscle or indirectly to the nerve supplying it. There are distinct advantages, 

 however, in applying the stimulus to the nerve, as it is more convenient as 

 well as more potent. The stimuli which will produce contraction in a 

 muscle are: 



i. Mechanical Stimuli. A blow, pinch, prick of the muscle or its nerve 



