trols. The first neurone as it enters the central organ breaks tip into a large number of delicate branches which are in physiological continuity 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 neurones. 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 sensory or motor constituents. The simplest conceivable reflex mechanism, 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 mechanism 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 adjuster 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 probably 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 physiologically continuous thread of protoplasm must connect the two extremes. It is, therefore, conceivable that a reflex mechanism might