ORIGIN OF THE NERVOUS SYSTEM 57 



trols. The first neurone as it enters the central organ breaks up into 

 a large number of delicate branches which are in physiological con- 

 tinuity with similar branches from the second neurone. It is over these 

 delicate branches that the nerve-impulse passes from one neurone to 

 the other and it is the structure of this system of branches that has 

 been a matter of so much discussion within recent years. Anatomically, 

 then, this simplest form of central nervous organ consists of motor 

 cell-bodies and fibrillations from these bodies and from sensory neu- 

 rones. Of course most central organs include additional neurones, 

 such for instance as association neurones, which connect one part of 

 the central organ with another and do not participate directly as sen- 

 sory or motor constituents. The simplest conceivable reflex mech- 

 anism, however, does not include these, but only the sensory and the 

 motor neurone as described. Such a chain reaching from the periphery 

 of the animal through its central nervous organ to and including its 

 muscles is usually regarded as the primary type of neuromuscular 

 mechanism. 



From a physiological standpoint this simplest type of reflex mech- 

 anism falls into three parts. The first of these is the sense organ or 

 receptor, which, as its name implies, receives the external stimulus; 

 the receptor is also the seat of the production of the nerve-impulse. 

 The second is the central nervous organ or, as it may be called, the 

 adjuster, which is concerned with directing the impulse toward the 

 appropriate end-organ and with modifying it in accordance with the 

 particular reaction to be obtained. The third and last is the effector 

 or organ brought into action by the impulse, such as a muscle or gland. 

 Thus a simple reflex may be said to involve at least three special classes 

 of mechanisms : receptors, adjusters and effectors. These mechanisms, 

 however, do not correspond exactly to the three histological elements 

 already named, for, though the receptive function is an activity limited 

 entirely to the first neurone in such an animal as the earthworm, and 

 the effector is the muscle-fiber, the adjustor is a part of the first as well 

 as of the second neurone and is made up of at least the fine fibrillar 

 material contributed by these two neurones to the central nervous organ. 

 The neuromuscular mechanism even in this its simplest type has prob- 

 ably not sprung into being fully formed, but it has had without doubt 

 a slow and gradual growth. It is one of the objects of these articles 

 to trace as far as possible the steps in this growth. 



It is to be noted that every reflex mechanism is in the nature of a 

 physiologically continuous span of living substance which reaches from 

 the receptive surface on the one hand to the effector organ on the other. 

 At no point in this span can there be a real interruption, for a physi- 

 ologically continuous thread of protoplasm must connect the two ex- 

 tremes. It is, therefore, conceivable that a reflex mechanism might 



