Stimuli are necessary to arouse the activity of the organism. Without any stimulus whatever, it seems likely that the animal would relapse into total inactivity. It should be said, however, that stimuli, such as that of hunger, may arise within the organism itself. The stimulus may be external or internal, but some stimulus is necessary in order to release the stored energy.
In general, then, a reaction consists in the release by a stimulus of some of the stored energy of an animal, and the direction of that energy by the animal's own internal mechanism of nerves and muscles (and, we may add, bones and sinews) into the form of some definite response.
EXERCISES
1. Outline of the chapter, being at the same time a "completion test". Complete the following outline by filling in the blank spaces (usually a single word will fill the blank, but sometimes two words will be better):
A. Definition: A reaction is a response to a ___________. The stimulus energy stored in the organism, and the __________ has a definite form determined by the organism's own machinery of ________ and ______.
B. Among very prompt reactions are the reflex and the "simple reaction". The reflex differs from the "simple reaction" in that:
(1) It usually takes less________.
(2) It requires no___________,
(3) The machinery for it is ________in the organism.
C. The machinery for a reflex consists of:
(1) a________ organ.
(2) a ________nerve.
(3) a nerve ________,
(4) a _________nerve.
(5) a muscle or _________.
D. The sensory and motor nerves consist of ________ which are branches of ______. The cells for the motor nerves lie in the ________, and those for the sensory nerves lie in two cases in the _________, and in all other cases in bunches located close beside the _________or ________,
E. The neurone is the _______ of which the nervous ______ is composed. It consists of a ________ and of two sorts of branches, the ________ and the ________. Internally, the neurone shows a peculiar structure of ________ and ________.
F. Communication from one neurone to another occurs across a _____ called the synapse. The _________of an axon here comes into close contact with the ______or with the _________of another neurone. The communication takes place from the ________of the first neurone to the ___________ of the second.
G. The "nerve current" in a reflex therefore runs the following course: from the sense organ into a ________ axon, along this to its _________ in a nerve, and across a _________ there into the _________ of a neurone, and thence [{43}] out along the _______of this neurone to the ________or _________ that executes the reflex. This is a two-neurone _________, but often there is a third, ________neurone between the _________ and the _____________.
H. Coördination is effected by the ________ of the axons of the sensory and ________ neurones, by which means the nerve current is ______ to a team of ________ and so to a team of _________.