CHAPTER XV

TIRING THE NERVE CELL

Several years ago the restless little bird that is a trial everywhere, the bird that every city feels obliged to destroy the small, brown English sparrow sparrow-gave

science quite a lift through its brain cells.

Dr. Hodge chose birds for his investigations because, taking size and weight into account, they probably do more work in a single day, are more active, than any other vertebrate on the face of the earth; and he wished to see whether the cell part of a neuron is changed at all when a bird works so hard.

He made two examinations every day: one in the morning, to see how the neurons looked after a long night of rest; another in the evening, to see how the neurons looked after a long day of work. By condensing a good deal, I make two extracts from the record which Dr. Hodge kept.

Experiment I. Made at the close of a cold, blustering snowstorm in December. Sparrows under cover during the storm; at its close, out in force hunting for food. Spinal ganglia examined at seven in the morning, after a night of rest, and again at 5.30 P.M., after a day of food hunting. Shrinkage 54.3 per cent.

This means, of course, that the cells of the neurons that formed the ganglia were very much smaller after the work was done than before it was begun.

Experiment II. February 17, a rainy day. Birds had had no exercise; had stayed close "in the dense cover of the pine trees over my window"; had "spent the day scolding and chattering at a great rate.” No flying done. The microscope showed no sign of fatigue in the spinal ganglia, but clear signs of fatigue in the nuclei of the cortex of the cerebrum, "as though, while confined by rain, the little birds had kept up a deal of thinking."

Swallows also made their contributions to these important investigations. It was well to test them, for no other bird is more active than the swallow. As Dr. Hodge says:

Its food consists of insects taken entirely on the wing. Quick, vigorous, purposeful, careful in all its actions, it must require an enormous amount of nervous energy to coördinate its countless movements for a long summer's day. There is nothing lazy or stupid about the swallows. When their work is done they play games and fly races; and, with all the energy required for flying, they have enough left to do no end of talking. At one hundred miles an hour for ten hours, — and I have observed them as early as five o'clock in the morning and as late as eight at night, a swallow might cover a distance of one thousand miles in a single day, and day after day.

These swallows were tested morning and evening, and Dr. Hodge was not surprised to find that, instead of cells that were tired in the cerebrum, it was now cells in the cerebellum that were changed. We have no difficulty in understanding this, for we know that the cerebellum helps the cerebrum in the matter of making muscles pull together, in walking and running, for example, in dancing and swimming. Naturally, then, flying is helped too. "And where could be sought an instance of more delicate manipulation of muscles than must be required to drive the wing of a swallow as it flits and whirls and balances and wheels and darts

A SWALLOW

"It flits and whirls and balances and
wheels and darts the whole
day long."-HODGE

the whole day long!" No wonder the cerebellum of the swallow showed the traces of its work! The protoplasm is often vacuolated, that is, filled with clear spaces, as though particles here and there had been dissolved out.

In each one of these examinations the principal change was in the innermost center of each cell - in the nucleus itself.

But what creature is ever busier than a bee in honey season! His brain, therefore, should teach quite a lesson, as indeed it did. Dr. Hodge made the test and found.

that each cell which was taken from a bee's brain in the morning had a plump, round nucleus, whereas each cell taken from some other bee at evening — after a hard day spent in carrying loads of honey-was much smaller, much more jagged and irregular.

Now that which happens to the neuron of one creature is accepted by scientists as a sign of what happens to all of us who are supplied with neurons; for, whether covered with feathers, fur, or silken garments, we are the kindred of each other. Some of us do surely have more neurons than others; some of us also think we are more intelligent than our feathered and furred fellowcreatures; but the same treatment affects their neurons and ours in the same way. Through their neurons they feel pain, through them also they move their muscles, while fatigue affects them and us in similar ways.

Dr. Hodge decided that, for us all, it is the cell and not the fiber of each neuron that gets tired; and that the greatest change is in the nucleus at the center of each cell. The case for one and all can be stated in two sentences:

1. Before exertion the nucleus of the cell is large and round, smooth and regular.

2. After prolonged exertion the nucleus is small, jagged, irregular. It has lost substance and become crumpled. Dr. Hodge also learned, by other experiments, that when tired cells have a chance to rest,

the nucleus begins at once to grow larger, rounder, and more regular again.

These different facts throw a flood of light on all the laws of fatigue and rest. Through them we come to the

CELLS FROM THE CORTEX OF A PIGEON

A, cells examined at the close of the day (notice the outline about the nucleus, it is somewhat irregular and shrunken); B, cells examined in the morning (notice the smooth, full outline about the nucleus)

conclusion that when the African quails were exhausted from muscular work, and when the professors in Turin were tired out from mental work, each one had tired nuclei at the center of certain sets of neurons. We see then why fatigue and good work do not go together.