280 MUSCLES 



Such a simple experiment also enables us to understand 

 how a desire formulated in the brain may be transmitted 

 as a nerve impulse to the muscle cells and thus produce 

 movement on the part of the muscle which receives such 

 impulse. Every muscle in our body is supplied with a 

 nerve connection, the ultimate ends of the nerves being 

 embedded in the muscle fibers while the main trunks are 

 collected into the great trunk called the spinal cord, and 

 through that are in connection with the brain. The brain 

 or cord, therefore, is the starting point of the impulse that 

 causes the contractions of the muscle fibers, and this 

 arrangement puts all the complicated movements of the 

 body under their control. 



The power of cells to respond to stimulus is called irri- 

 tability. By irritabilty in general we mean the power of 

 a cell when stimulated to exhibit that particular form of 

 activity for which it has been specialized. Muscle cells, 

 for example, have lost many of the characteristics of a free 

 moving amoeba but have developed the power of con- 

 traction to a high degree. A secreting cell, on the other 

 hand, has little power of contraction but a highly developed 

 power of secretion. Hence, the irritability of the muscle 

 cell shows itself in contraction when stimulated; the 

 gland cell, on the other hand, manifests increased secretory 

 power when stimulated. In other words, each possesses 

 irritability, but the expression of that irritability depend^ 

 upon the manner in which the cell is specialized. Any 

 impulse which can produce an expression of the cell's 

 specialized power is said to excite its irritability. 



If we plunge the ends of the electric wires directly into 

 the muscle substance the mass will contract and relax as 

 before, but not to so great an extent. It is evident from 



