On the homological relations to one another of the mesial and lateral fins of osseous fishes / by Professor Humphry.

  • George Murray Humphry
Date:
[1870]
Catalogue details

Licence: Public Domain Mark

Credit: On the homological relations to one another of the mesial and lateral fins of osseous fishes / by Professor Humphry. Source: Wellcome Collection.

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    of the two portions having coalesced and occupying a strictly me¬ dian position. It is also more obliquely placed than the anal fin; though the obliquity is in the same direction, viz. slanting from before backwards and downwards. Both these points of differ¬ ence are attributable to the presence of the abdominal viscera and cavity, which keep asunder the laminae of the haemal sep¬ tum in the manner already mentioned, and cause the interspi- nous and muscular and ray elements of the fin to be developed on the sides of, instead of in, the mesial line. So that just as the fibrous septa and the ribs lining the abdominal cavity, serially considered, are the ununited lateral elements which in the caudal region combine to form the haemal spines and the h^mal septum, so the ventral fins, serially considered, are the ununited elements which, behind the abdomen, combine to form the anal fin. It may be observed that the ventral fins, whether they are placed just in front of the anus or more forwards, derive their nerves, just as do the components of the anal fin, from the ventral branches of the spinal nerves near which they lie. The pelvic bones of the ventral-fin I regard, therefore, as the serial homologues of the interspinous bones of the anal-fin; each pelvic bone answering to one or more halves of inter¬ spinous bones which have been kept separate from those of the opposite side, while their deeper ends have been directed forwards, and their anterior edges inclined inwards towards the middle line. The effect of this has been to throw the hinder margin of the ray backward, and the surface, which in the primitive condition was internal, or opposed to that of the opposite side, backward and upward. Hence, to bring the ventral fins into a position corresponding with the elements of the anal fin, it is only necessary to turn the pelvic bones and the ray from their horizontal to a vertical position, bringing the anterior extremities of the pelvic bones upAvards, and their inner edges forwards. The upper surfaces of the ray will now be turned towards each other, or thrown into contact, their inner edges will be directed forwards, and their hinder extre¬ mities will point downwards. The pelvic bones covered by the fin muscles perforate the abdominal wall, their hinder parts, where articulated with the
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