Note.—The thyroid gland no longer secretes anything into the alimentary canal, and its duct disappears at an early age. If, however, it become diseased or is surgically removed, the distressing symptoms of goitre supervene. Such a patient may be completely cured by grafting a thyroid, excised from another animal, anywhere in his body. Doctors usually, however, give the patient extract of sheep’s thyroid either in pills or injections.
ESSAY III.
THE MECHANICS AND PHYSICS OF THE BODY.
I.
In the preceding essay we regarded protoplasm as a chemical factor in the universe.
We have seen how it is always changing, always taking in food, always giving off waste materials. We have seen, too, that it grows and that it does work, and that in a large mass the cells which compose it share the labour instead of each component cell performing all the vital functions. We have now to consider the work which protoplasm does—in a word, the mechanical effect of the chemical actions just described.
The simplest movement of protoplasm is to be seen by the aid of the microscope in certain vegetable cells, where granules seem always streaming about in different directions. A step higher, and we find this streaming movement converted into movements of the whole cell. In the simplest unicellular animals the fluid protoplasm is contained in a membrane, or denser bounding layer, to which are attached fine filaments springing from a minute body known as the centrosome. These centrosomes—for there are sometimes several in a cell—seem to control the mechanical department, just as the nucleus does the chemical. Along the fibrils at intervals are minute globules, and by watching the distance between them it is seen that the fibrils undergo changes in length, pulling in the membrane when they shorten, and letting the cell flow out in any direction when they relax. By adjusting these two movements to balance one another, the cell can move in any direction, surround and engulf particles of food, and assume a strange variety of shapes. ([See Diagram 1.])
Diagram 15.—Cell Division.
In some cells, probably in all, the centrosome presides over division. Cells, however, do not always divide in the same way. Some simply lengthen, the nucleus also lengthening inside, become constricted in the middle like a dumb-bell, and separate. ([See Diagram 15.])