The first five vertebræ are each furnished with four ribs instead of two; this is quite a unique arrangement in the osteology of fishes. These supernumerary ribs are attached to the sides of the neural-spine (Owen) far above the neural arch. They are somewhat shorter than the true ribs, which are articulated to the bodies of the vertebræ. But they are so curved, that the inferior end of each reaches to its corresponding true rib, and articulates with the latter near its head. The first supernumerary rib is the longest, and the others gradually decrease until the last, which is the shortest.
A dense fibrous membrane lines the spaces which intervene between these ribs, so that there exists, on each side of the dorsal spine, and covered over by the superior muscles of the vertebral column, a long conical cavity, whose apex is directed upwards and backwards, and whose base opens downwards into the branchial cavity. In this cavity is lodged a good deal of the branchial labyrinth of the fish.
The labyrinth of Polyacanthus is not of the complicated nature of that of Anabas, Helossoma, or Osphromenus, but, in the simplicity of its structure, more resembles that of Ophiocephalus; it is composed of but three heliciform lamellæ, which, however, make up in length, what they want in the sub-division of their lamelliform surfaces, and are so long that they cannot be sufficiently protected by the upper portions of the tympano-maxillary and humero-scapular arches.
An organ like the labyrinth of this fish, with such important functions to perforin, could not well be lodged in the trunk, where it would be in the way of powerfully acting muscles, but it is quite securely situated, under the rib-like protection of this kind of thorax, formed by the five pairs of accessory ribs.
No other known Labyrinthoid fish (I have them all in abundance) presents a similar anomaly.
4. Some results of isolated Arterial Injections.
Isolated arterial injections are in many respects very instructive; by "isolated injection" I mean the injection of the minute arteries, not of those supplying an extremity, or other large portion of the body; these latter will never give the same clear idea of the province which belongs to each small arterial branch, whilst the isolated injection of the minute arteries shows the boundaries of the territories, which are irrigated by certain sets of blood-vessels. When an organ receives several arteries, then the isolated injection of each with differently coloured injections, will show, in a most satisfactory manner, what portion of the organ is supplied by each branch. So far as the nervous system is concerned, anatomists have marked out the districts over which the ultimate nervous ramifications spread;
