PHYSIOLOGY OF ORGANS 150 



observed to contract. Upon cooling again it relaxed. The curve which 

 the lever made on the smoked drum of the kjTnograph agreed in all 

 essentials with that made by a fresh smooth muscle. The explanation 

 of the contraction of this violin string is that the sudden heating of 

 the string caused by the heated coil of wire caused the doubly refractive 

 particles to absorb more water and to change their shape from linear 

 or prismatic to spherical. A diagram of Engelmann's apparatus is 

 shown in Fig. 118. 



While Engelmann's theory of muscular contraction may not be cor- 

 rect, it at least visualizes the process, and properly appeals to chemical 

 or physical processes for an explanation. 



Nervous Control. — There are three means of controlling the mass of 

 cells of which a metazoon is composed. These are (1) the tissues which 

 bind the organs together and the skin which encloses the whole, (2) hor- 

 mones and other substances which act by virtue of their chemical 

 nature, and (3) the nervous system. The first of these modes is 

 largely mechanical, but it is not unimportant for without the mechanical 

 effect of the connective and muscular tissues there could be no unification 

 of the various parts into an individual. The second method of unifica- 

 tion and control (by means of hormones or other chemical substances) 

 has already been illustrated in an earlier part of the chapter. It is 

 exemplified by the discharge of pancreatic secretions in response to 

 stimulation of the pancreas by secretin. This mode of control is auto- 

 matic, that is the various events or circumstances leading to the end 

 result stand in the mutual relation of links in a chain. Control by hor- 

 mones is involuntary, not under control of the will. By this chemical 

 means of regulation many of the metabolic activities of the body are 

 wholly or partly directed. The nervous system plays a role in certain 

 instances of hormone control; but this system also directs a large number 

 of activities with which the organs of internal secretion and their products 

 have nothing to do. The nervous system not only coordinates the 

 activities of many of the internal organs but also controls the reactions 

 of the individual to conditions of the environment. Much of this control 

 is performed automatically, that is, without involving thought on the 

 part of the individual. 



Unit of Structure of Nervous System. — The unit of structure of the 

 nervous system is the neuroji. The neuron is a cell possessing a number 

 of fine projections which sometimes extend to great lengths. These 

 projections are of two kinds distinguished from one another, not by 

 structure, but by their normal functioning. Those which normally 

 conduct impulses toward the body of the neuron are called dendrites: 

 those which convey impulses from the body of the neuron are axons. 

 Which direction the impulse travels, and hence which processes are 

 dendrites or axons, is determined by the manner in which the given 



