Royal Army Medical Corps training pamphlet / War Office.

Date:
1944
Catalogue details

Licence: Public Domain Mark

Credit: Royal Army Medical Corps training pamphlet / War Office. Source: Wellcome Collection.

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    1S to form nerves, just as metal wires are gathered into bundles to form electric flex or cable. Among other highly specialized cells are those at the back of the eye which register light andcolour, and those in the ear which register sound. In certain animals the eye is no more than a portion of skin which is unusually sensitive to light and shade ; but in man it has developed into a complicated self-adjusting optical instrument, and the effects of light on the sensitive cells at the back of the two eyes enable him to distinguish details of colour and form in objects farand near. In the same way the cilia on certain cells in the ear vibrate in response to sounds of particular wave-length. Through nerves from the skin the brain is informed about the warmth and pressure and wetness of the air or anything else in contact with it; through nerves from the muscles, tendons and joints it is informed about the exact position of every part of the body ; through nerves from the tongue and nose it is informed about tastes in the mouth and smells at a distance ;. through nerves from the ear it is informed about sounds, and through the eye about colours and shapes. Thus, while awake, the brain is continuously getting information about the surroundings or environment of the body, provided by cells which have specialized in picking up particular impulses. 12. Reproduction .—tThe life of cells of almost all kinds is short, and in bone, muscle, brain or skin the process of cell death and replacement goes on so long as the body as a whole remains alive enough to maintain the essential supply services. Replacement normally takes place by division of a cell into two others of the same kind, and in the young and healthy this multiplication makes good—or more than makes good— the loss of cells that have died. Certain cells, however, have specialized in reproduction, and by dividing over and over again can produce not only cells like themselves but cells of all kinds. The female ovary produces small numbers of round cells called ova (egg-cells) ; the male testicle produces immense numbers of very small active cells, with large heads and long tails, called spermatozoa or sperms. A sperm encountering an ovum pierces its cell wall and the two cells fuse into a fertilized ovum. If this happens in favour- able circumstances in the female uterus (womb), the fertilized ovum divides into two cells; each of these divides into two others, and each of these again into two. Connections are established between this growing mass and the blood-vessels in the wall of the uterus; the thousands of cells soon formed begin to take on special characteristics ; and in about nine