36 PSYCHOBIOLOGY 



plasm, the apparent anastomosis being produced in the process of prepar- 

 ing the section of tissue for observation. The groups of mesenchymal cells 

 from which smooth and cardiac muscles develop, do, however, show well- 

 marked anastomosis between their branches. 



Mesenchymal cells, in becoming cardiac muscle cells, lose their original 

 power of reproduction. Possibly striated muscle can reproduce. 



THE FUNCTION OF MUSCLE. 



The most important characteristic of the muscle cell is its highly-devel- 

 oped power of contraction. Many other cells are able to contract, or to 

 change their shape in various ways; but in the muscle cell, the contractile 

 function is developed to the highest degree. The beating of the heart, the 

 regulation of the diameters of blood vessels, the inflation of the lungs, the 

 movement of food along the alimentary canal, the erection of the body 

 hairs in ' gooseflesh ', the movements of the limbs and other portions of the 

 body, are produced by the properly timed contractions of myriads of 

 muscle cells. 



In contracting, the muscle cells become thicker and shorter, and undergo 

 internal changes which are not well understood. During this process, 

 toxic substances, having decided effects on both muscular and nervous 

 tissue, are formed. In its resting phase the cell carries on metabolic pro- 

 cesses which maintain its own life, produces material essential to the 

 process of contraction, and possibly produces substances of value to other 

 tissues. 



Under ordinary circumstances a striated muscle contracts only when it is 

 stimulated by some external agency. Normally, the stimulation is an 

 impulse conveyed to the muscle fiber from a ganglion cell through its 

 axon, whose terminal branches are in contact with the sarcoplasm beneath 

 the sarcolemma. Contraction can be produced, however, by mechanical, 

 electrical, chemical, or thermal stimuli applied directly to the muscle. A 

 muscle from the leg of a frog, for example, contracts if pinched by a pair 

 of tweezers, or if an electric current be made or broken through it, or if 

 ammonia or certain salt solutions be applied to it. If the muscle be 

 gradually warmed, it commences to contract at about 34° C. (93° F.), 

 the contraction increasing up to about 45° C. (113° F.), at which point 

 the muscle dies, although the contraction lasts some time longer as rigor 

 ccdoris. 



When a single stimulation is applied to a striated muscle there is a brief 

 latent period after the stimulus, and then the muscle responds with a 

 single short sharp contraction, relaxing immediately. The duration of the 

 latent period, contraction and relaxing vary with the intensity of the stim- 



