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Updated: June 22, 2025
These corresponding parts are called, in the technical language of anatomy, "homologous parts." The ring of the third division is the "homologue" of the ring of the fifth, the appendage of the former is the homologue of the appendage of the latter. And, as each division exhibits corresponding parts in corresponding places, we say that all the divisions are constructed upon the same plan.
Hence the same names can be given to the homologous bones in widely different animals.
But in reference to M. Pichot's specimen, an expert anatomist remarked to me that it would far exceed the skill, whether of the peasant who owned the vineyard or of the dealer above mentioned, to put together in their true position all the thirty-eight bones of the hand and fingers, or the sixteen of the wrist, without making any mistake, and especially without mixing those of the right with the homologous bones of the left hand, assuming that they had brought bones, from some other spot, and then artificially introduced them into a mixture of volcanic tuff and plaster of Paris.
In the great class of molluscs, though we can homologise the parts of one species with those of other and distinct species, we can indicate but few serial homologies; that is, we are seldom enabled to say that one part or organ is homologous with another in the same individual.
The several parts of the body which are homologous, and which, at an early embryonic period, are alike, seem liable to vary in an allied manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed to be homologous with the limbs.
They are called homologous because they have similar parts in similar relations. This can be seen in Figs. 47 and 48, where it will be seen that each has the same bones, although in the bird's wing some of the bones have been fused together and others lost.
Other similar cases could be given; for instance in plants, the very curious contrivance of a mass of pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently the same in Orchis and Asclepias, genera almost as remote as is possible among flowering plants; but here again the parts are not homologous.
But the locomotive is not formed from the mine engine; it is made new, and is a distinct type. And yet, the same principles are seen in both. Even so it is with the genera of animals. The whale and the elephant both have backbones, jointed limbs, warm blood, and a hundred homologous organs. They are both mammals, both are sagacious, and are gifted with acute senses.
Thus in man and other mammals, nothing of the kind is externally visible, and we have to penetrate to his skeleton to find such a series of homologous parts. There, indeed, we discover a number of pairs of bones, each pair so obviously resembling the others, that they all receive a common name the ribs.
There are many kinds of homology, but it is only necessary to consider the three kinds above enumerated. The term "homologous" may be applied to parts in two individual animals of different kinds, or to different parts of the same individual. Thus "the right and left hands," or "joints of the backbone," or "the teeth of the two jaws," are homologous parts of the same individual.
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