92 ANATOMY FOR NURSES [Chap. \II 



Exiensibility of a li\'ing muscle means that it can be stretchetl 

 or extended, and elasticity means that it readily returns to its orig- 

 inal form. Normally, the skeletal muscles are in a condition of 

 slight tension, being stretched from bone to bone. This condition 

 is of importance in two ways : (1) smoothness of movement is 

 dependent upon it ; (2) a stretched muscle will contract more 

 cjuickly than one that is relaxed. To understand the first state- 

 ment it is important to remember that skeletal muscles are usually 

 arranged in antagonistic groups, one of which opposes another. 

 Thus the muscles located on the anterior surface of the arm and 

 forearm are called flexors, and those located on the posterior sur- 

 face are called extensors. The action of the flexors is to bend the 

 arm, the action of the extensors is to extend or straighten the arm. 

 When stimulated, either group of muscles must overcome the resist- 

 ance of the opposing group. Therefore contraction takes place 

 more slowly and evenly, and smoothness of movement is the result. 



Under normal conditions there is a constant and insensible 

 tendency to contract called tonicity. It is really a mild, sustained 

 contraction, and though it may vary in degree, it is rarely absent 

 altogether. Tone in the skeletal muscles gives them a certain 

 firmness and maintains a .slight steady pull upon their attach- 

 ments. It is not likely to result in movement on account of the 

 action of an antagonistic muscle. In fractures the over-riding of 

 the broken ends of a bone is often due to the contraction of the 

 muscle, that is the result of its tonicity. This property is of 

 importance in several ways: (1) in connection with elasticity 

 it promotes a quicker response to stimulation, (2) it assists in 

 maintaining the circulation of the blood and hinph, and (3) it 

 assists in the regulation of nutrition and body heat. As previously 

 staled tonicity may vary in degree. Increase in the degree may 

 follow (1) muscular exercise, (2) mental exercise, and (3) lower- 

 ing of the surrounding temperature. The effect of a cold bath is 

 a familiar example of the last. 



Blood-vessels and nerves. — All varieties of muscular tissue 

 are well supplied with blood-vessels and nerves. The blood- 

 vessels that supply blood to the muscles are supported and carried 

 by the connective tissue. They do not penetrate into the cells, 

 but each cell is bathed in lymph which exudes from the blood- 

 vessels. The cells take from the lymph the materials they need 



