A treatise on diseases of the bones / By Thomas M. Markoe.

  • Markoe, Thomas M. (Thomas Masters), 1819-1901.
Date:
1872
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on diseases of the bones / By Thomas M. Markoe. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

    18/436 (page 4)
    old cliaiubers from the new. In this way many of our most beautiful shells are ]tru(luced, their form and size depending ou the Bueeessive a<lditions and alterations which the increas- in(^ size, and often the chanj^ing form, of the animal has obliged it to make to its original ccjustruction. In the oyster, each degree of growth of the animal is provided for by an entire new layer of shell-growth, which, being internal and larger than tiie preceding one, takes its })lace; and thus we have produced the peculiar lamination and the very thick, heavy shell by which these valuable animals are protected. In many of the Crustacea, some of whom, as the crab and the lobster, are entirely encased in a calcareous envelope, the dithculty is met by a process of throwing otf the skeleton entirely, and providing a new and larger one, proportionate to the increase in size of the animal. A similar action is ob- served in some of the changes which take ]>lace, during the development of several species of the Insecta. This process of the periodical shedding of the shell is a very curious and interesting one, and it may enable us to appreciate somewhat better the difficulty of the problem we are now studying, to watch the tedious, difficult, and one would think painfid ex- ertions which these animals have to undergo, in order to free themselves from a covering which has simply become too small for them, and has by this clumsy process to be got rid of, to make way for a larger one. The imperfection, if we may so speak^ of the mode is still more strikingly shown by the un- protected and helpless condition in which the unhap)>v animal is left after casting otf its old coat, and before the new one is firm enough to be available. In some of these species, and also in some of the Yertebrata, which have a partial external skeleton, another arrangement is sometimes found, by which the growth of the excrementi- tious skeleton is provided for. It is composetl of numerous plates, fitting more <»r less closely together, but sejiarated from one another by a portion of the foundation membrane of the shell, which is not calcified, and which therefore allows a cer- tain degree of mobility among the plates. This ]m»vidi>s, to pome extent, for the inercasing size (»f the animal, but the change is still further accompanied by an actual incre;use in