Any particular part of the body may be caused to do its work upon receiving impulses from the central nervous system. Its efficiency can be considerably altered by the absence of substances normally present in the animal ’s food—those known as vitamins. Since such chemical substances have marked effects, it is not surprising to find that the body itself produces others which affect the metabolism of cells. These are produced by glands which pour the secretion directly into the blood, and not along a duct, and are therefore termed ductless glands, the secretion being a. Thus these chemical messengers are distributed to all parts. The ductless glands are themselves made to work in one of two ways—either by nervous impulses from the central nervous system, like all glands, or by receiving a from another gland, so that the initial stimulation really comes from the nervous system. Each gland, although present at birth, does not function until a definite stage is reached during the animal ’s life. Sometimes a gland fails to. develop at the proper time or may be over active, in which cases very marked effects become apparent. The outstanding features relating to these glands are given in the facing table. You are not expected to learn such facts, but they are given to show how important ductless glands are ; how, in spite of the fact that their products are distributed to all parts in the blood, each hormone affects only certain parts ; how our health and normal development are dependent upon them. Our outlook on life can be considerably changed if the ductless glands do not function properly—we are, in fact, very much ’creatures of our glands. ’ that only suitable ones reach their destination is called co-ordination. This becomes easier every time such sorting out is repeated. Our movements, for example, when learning to ride a bicycle are clumsy and jerky at first, become smooth later, and finally are carried out seemingly of their own accord, so that we can converse or admire the scenery when riding, until something unforeseen forces us to concentrate our attention on controlling the machine. Actually what happens is that the conscious part of the brain is involved at first, but once co-ordination has been effected the control is gradually taken over by the cerebellum. Thus we ’learn by experience ’and become ’creatures of habit. ’
The part played byas chemical messengers must not be neglected in this connection, but it must also be remembered that either directly or indirectly the glands producing them are under the control of the central nervous system, so that it is this system which has the fundamental control of the whole organism.
Till now we have considered nervous activity as that initiated by stimuli acting on or in the body and producing responses which are either reflex actions or which are experienced as sensations. Many of our activities are, however, not due to impulses set up by receptors, but arise in the conscious part of the brain itself. Such activities are of the voluntary type. Those muscles which are controlled at will are therefore termed voluntary muscles, having striped or striated fibres forming most of that part of an animal termed the ’meat ’as distinct from ’offal. ’ We have no voluntary control over plain or unstriped muscle like that forming the walls of the gut and blood vessels which carry out their activities in a purely reflex fashion, e.g. we cannot by deciding to do so make the heart beat more quickly, stop ourselves from blushing, alter the size of the pupil of the eye or of artery walls. The muscles involved in such actions are therefore termed involuntary muscles. The converse, however, is not true—our voluntary muscles do not only carry out voluntary actions but involuntary ones as well—the example of riding a bicycle given above should make this abundantly clear.
Experiment 74—To show the Effect of the Secretion of the Thyroid Gland on the Metamorphosis of the Frog
Two sets of tadpoles just showing the commencement of the hind limb buds should be kept in separate 2-lb. jam jars in water and supplied with water weeds for food and aeration of the water. One or two 1-gr. tablets of thyroid gland are crushed and sprinkled on a few scraps of raw meat. These are dropped on to the surface of the water in the second jar only. After a week it will be found that the tadpoles in this jar will have metamorphosed, showing fore limbs, narrowed ’waist ’and greatly diminished tails ; whereas the others will not have changed to any marked degree. If the desired result is not obtained in a week, the dose may be repeated, but care must be taken not to overdo the dose or the tadpoles will die. The tadpoles in the first jar which act as a control should be given pieces of untreated raw meat.