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an inch long, rising froin the edge of the opening of the ventricle which projects into the artery.*

These different structures, so very unlike one another, and bearing no resemblance to the mechanism of the same parts in quadrupeds, make it probable, that the circulation through the gills is impeded by the external pressure of the water, in different degrees according to the depth of the fish from the surface: therefore in those fishes which frequent great depths, as the Squalus Maximus and all the shark tribe, there is a muscular structure in the coats of the branchial artery, which, when the fish is deep in the water, by its contraction diminishes the area of the vessel, and makes the valves perform their office; but when the fish is near the surface, this muscular structure, by its relaxation, renders the area of the artery so wide, that regurgitation of the blood takes place into the ventricle, and prevents the small vessels of the gills from being too much loaded. +

In fishes that swim deep, and do not come to the surface, as the wolf fish, the regurgitation does not take place into the ventricle; but the relaxation of this muscular portion of the artery allows it to dilate, and form a reservoir, and the valves remain closed, so as to prevent more blood leaving the ventricle. In fishes residing at moderate depths, as the turbot, elasticity is employed as a substitute for muscular power, there being less variation in the pressure made upon the gills; but in the Lophius Piscatorius, which probably never descends into

* This structure, which it is difficult to describe, will be better understood by a reference to the annexed drawing. (Pl. XXII.)

+ That such regurgitation takes place when the muscle is relaxed, is ascertained by the ventricle being readily injected after death with common wax injection from the artery, the valves allowing it to pass. MDCCCXIII,

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water of much depth, the ventricle is so weak, that the supply of blood to the gills is regulated by the contraction and relaxation of a muscular valve.

As water, according to the degree of pressure upon it, is capable of containing a greater quantity of atmospherical air, than under ordinary circumstances, such a supersaturated state of the water might compensate, with respect to the respiration of fishes, for the difficulties, which occur at great depths, of forcing the blood through the vessels of the gills: I enquired what evidence could be produced, of the water at great depths containing a more than ordinary quantity of air; my philosophical friends, to whom I proposed this question, said, that it was a point that had not been considered. I therefore resolved to put it to the test of experiment, and as I knew there was a well at Mr. Coutts's in the Strand, which more than twentyfive years ago had been sunk four hundred feet below the surface of the water in the Thames, I requested permission to make the experiment in that well. A cylindrical vessel, with a valve above and below, was let down to the depth of one hundred and eighty-six feet, which is now the depth of the well; it came up full of clear water, which Mr. W. BRANDE ascertained to contain no greater proportion of atmospherical air, than is met with in common river water. It therefore

appears that the supply of air to fishes is nearly the same at whatever depth they are from the surface.

Besides the guards which have been mentioned, to prevent the circulation through the gills from being improperly carried on, in particular tribes of fishes, the ventricle itself is so formed, and is so situated with respect to the auricle, that the blood received is first impelled in a direction nearly at right

angles to that of the artery, and then another set of muscles is employed to force it into the artery.

This mechanism, common to most fishes, and so different from what is met with in land animals, where the blood is forced by the action of the left ventricle into the different parts of the body, appears to be intended to prevent too great a force being employed, at any one time, in impelling the blood through the gills.

Having traced in these different gradations the guards upon the circulation through the gills in fishes, I am led to extend my remarks to the hearts of a lower class of animals. In the Mollusca there is no reason for putting guards upon the action of the heart, as in fishes, because in them the blood first supplies the different parts of the body, and only in its return passes through the gills; but there is a regulation of another kind, by means of which the circulation is increased or diminished, according to the activity or torpor of the animal. In the teredines, where a boring engine, requiring a great muscular power to work it, is almost constantly employed, the heart consists of two auricles and two ventricles; the auricles strong, cylindrical, and having valves between them and the ventricles; the ventricles themselves very strong, so that in fact there is a heart, composed of two auricles and two ventricles, both acting at the same time, and the blood is hurried on by a double power to supply the muscles of the boring shells, and, in this part of its course, has a bright red colour.

In the Oyster, on the other hand, the heart consists of only. one auricle and one ventricle, both very weak in their muscular power, when compared with those of the teredines, although belonging to an animal of larger dimensions; but the heart is

laterally connected to the great muscle that shuts the shell, so that whenever that muscle is actively employed, its lateral swell presses against the heart, and forces out the blood, and by this means gives activity to the circulation, at the only time in which an increased circulation is particularly required.

In the Muscle, the heart can hardly be said to be distinctly divided into auricle and ventricle ; but to consist of an oval bag, through the middle of which the lower portion of the intestine

passes. Two veins from the gills open into the heart, one on each side, these may be considered as the auricles.

The coats of the ventricle are very thin, so as to have little power of propelling the blood; but this weakness appears to be compensated by the effect of the contraction and relaxation of the intestine contained in their cavity. This circumstance renders it probable, that the circulation goes on with activity while the processes connected with digestion are employed ; but when the intestines are empty, there being no supply of nourishment, the circulation is not only very languid, but may possibly be entirely stopped.

In the caterpillar, the blood cannot be said to circulate, but is carried from one end of the animal to the other, in a tube which may be either called heart or artery, by a species of peristaltic motion. This ebbing and flowing of the blood, if such a term can be admitted, is greatly increased by the pressure upon the different portions of this tube, produced by the action of muscles employed in the progressive motion of the animal, so that the supply of blood to the different parts of the body is proportioned to the demand, which arises out of the bodily exertions.

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