Animal physiology / by John G. M'Kendrick.

  • John Gray McKendrick
Date:
1877
Catalogue details

Licence: Public Domain Mark

Credit: Animal physiology / by John G. M'Kendrick. Source: Wellcome Collection.

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    (4.) The movements, often in a definite direction, of the molecules of the yolk of the egg after fecundation. (5.) The movements of pigment molecules in certain cells of the skin of the frog, under the influence of light, as described by Mr Lister, and which account for the changes of colour in that animal with different intensities of light. When the animal is dark in colour, the molecules of dark pigment are diffused through the cell; but on exposure to light, or by section of the nerve supplying the part, or on the sudden death of the animal, the molecules crowd into the centre of the cell, and the skin becomes pale. Here we have an example of molecular movements under the influence of a physical agent, light, and also apparently affected by a vital agent, nerve force. A similar explana- tion probably accounts for the changes in colour of the chameleon, and of various fishes. CELLULAR ELEMENTS. 26. General Description of a CelL—A cell (fig. 27) is a microscopically small body, consisting of a soft substance termed protoplasm, in which there may be imbedded a body called a nucleus^ in which there may be a still smaller body, known as the nucleohis. Sometimes the external boundary of the cell may become a hardened stratum, a, which is then called the cell membrane or cell wall, A cell is more than a molecule in a physiological sense ; it is the smallest physi- ological apparatus, to a certain extent complete and inde- pendent. It is also a generally received doctrine that all tissues originate in cells. Not unfrequently several nuclei may be seen in the same cell or mass of protoplasm, as in fig. 30 (p. 33), a representing a cell or mass of protoplasm from the marrow of bone. 27. Size of Cells.—They vary in size from the -^ixV^rth Fig. 27.—Two Cells of round or oval form : a, border of the cell; b, b, cell body; c, c, nuclei with nucleoli, d.
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