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

  • Thomas M. Markoe
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.

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    to permit the movements of whicli the miiscuhar system is ca- pable ; and 3. A power of increase and change which will adapt it to the increasing size or changing shape of the animal. These mechanical requirements are, in the lowest classes of animals, easily answered by a silicious or calcareous covering, which increases in extent as the animal inhabiting it increases in dimensions. This increase, however, it must be noted, is one merely of siiperaddition to the edges or surfaces of the original shell, which, once hardened, is no longer under the in- fluence of vital action in any such sense that it can undergo any change either in its consistence or in its size. It must be evident, however, that such an arrangement can only be effi- cient in the simplest shapes that animal life assumes, and that the moment complexity of form is assumed, and variety of locomotive action exhibited, a new element is added to the problem of the skeleton, and that this new element is the power, not merely to increase, but to change its form, in obe- dience to the changes which each part of the more complex animal is liable to present, as each part grows to a greater size, and usually to a greater power. Thus, in the cell-like protozoa, already alluded to, we can readily conceive that the rounded or elongated form, partly roofed in by a rounded or elongated shell, may, as it increases in size, be still covered by a simple increase of size of its protecting case, and the diffi- culty is not much increased in the Radiata, whose form is merely a repetition of simple elements round a common centre. But, among the Articulata, as the Crustacea and the Insecta, the growth of the animal in all the segments of its body, and in all the complex subdivisions of limbs and antennae, must be accommodated by a provision for something more than a mere superaddition at the edges of the original-simple formed shell. This part of the problem is differently solved in different classes of animals. In the gasteropod Mollusca, for example, it is evaded rather than solved. As the animal grows it leaves the narrow quarters of its original dwelling, and finds better accommodation in the larger segment of shell which, as it grows, it adds to the old homestead, from which it usually shuts itself off by a partition-wall, which completely isolates the
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