482 



FISH. 



must not be supposed that the process, even in the 

 mechanical application of the water to the gills, con 

 sists of a mere current, running without intermission 

 in at the mouth, and out at the gill openings ; for, as 

 the separation of oxygen from nitrogen in the free air 

 received into the lungs of land animals, requires some 

 time for its performance, we may naturally conclude 

 that more time will be required for performing the 

 double operation of separating the air from the water, 

 and then the oxygen from the nitrogen. This is also 

 an operation which the fish can suspend at pleasure 

 for a short time ; but in the generality of fishes this 

 time is very short. There is also rather a complicated 

 operation of the mouth, which aids the current of the 

 water when breathing. There is a sort of double 

 mouth, the inner portion of which (which in many 

 fishes is pretty far back) shuts, and shuts close on the 

 anterior edges of the gill arches, so that when the gill- 

 lid, or flap, or whatever it may be, is pressed down, 

 the water, which has been wholly exhausted of its air, 

 and is, therefore, unfit for the purpose of respiration, 

 is wholly expelled from the gills, and as soon as this 

 inner gate of the mouth is opened, and the gills 

 expanded, fresh and wholesome water is admitted. 



In ordinary fishes, there is no return by the mouth 

 of the water which has once been received by the gills, 

 and any thing which prevents the discharge through 

 their openings, is just as fatal to the fish as if the sup- 

 ply by the mouth were excluded. If the ejection of 

 the water from the gills of a fish be prevented, by the 

 gill-cover being tied down with a string, it soon 

 expires in convulsions. Similar fatal consequences 

 follow when fish are placed in water previously de- 

 prived of its atmospheric air by boiling or freezing : 

 and when fish are kept in a small pond, whose surface 

 is frozen over, and where the water in that case can- 

 not obtain a fresh supply of air, they speedily perish. 

 If a small opening be made in the ice, before it be too 

 late, the fishes will come near it for a fresh supply. 

 In this manner fishes are frequently taken during 

 winter in ponds and lakes. The extent of surface 

 presented by the gills of a fish, to enable the blood to 

 come in contact with the air in the water, is much 

 greater than one would, without attentive considera- 

 tion, be led to suppose. Dr. Monro calculated, that 

 the whole gills of a large skate presented a surface equal 

 to two thousand two hundred and fifty square inches, 

 or equal to the whole external surface of the human 

 body. The process of respiration for the supply of 

 the gills is carried dn even during sleep. The number 

 of respirations in a minute is seldom above thirty, or 

 below twenty. In the same individual it is liable to 

 considerable variation, depending on the will of the 

 animal. The blood, after being renovated in the gills, 

 is re-absorbed by a multitude of minute vessels, which 

 unite together ; but, instead of returning the blood to 

 the heart again, to be afterwards distributed through 

 the body, this aorta exercises that function, and de- 

 scends along the inferior side of the spine, in a canal 

 fitted for its reception, giving off arteries during its 

 course, to the adjacent parts. The blood is absorbed 

 again by veins, which have extremely thin coats. 

 These are much larger in their course than at their 

 termination ; and besides form, in different parts of 

 their course, considerable receptacles for blood. Any 

 injury received by the gills of fishes is attended with 

 much pain, and a considerable effusion of blood. Some 

 fishermen seem to be well aware of this last circum- 

 stance, and cut the gills with a knife as soon as the 



fish is taken. A copious bleeding takes place ; and 

 they find that a fish so killed will keep much longer 

 in a fresh state than one on which this operation of 

 bleeding has not been performed. There are some 

 other circumstances connected with the respiration 

 and circulation of fishes, of which it is necessary to 

 take notice, in order to have a full and distinct view 

 of the economy of those most useful and interesting 

 animals. 



The heat of all animals is found to be in exact pro- 

 portion to the energy with which their system works ; 

 and the best indications which we have of this energy 

 are, the quantity of air required in respiration, and the 

 velocity in the pulse of the circulating blood. These 

 do not keep time with each other exactly, because 

 time is only one element in the amount of either, and 

 quantity is the other element, so that we must hike 

 our estimate from these two jointly. But respiration 

 admits of much more variation as to quantity, in the 

 same time, than circulation does ; and, therefore, there 

 may be many changes in the rapidity of breathing, 

 arising from temporary causes, without any effect upon 

 the pulse. If, however, we take the general habit of 

 any animal, we shall find that there is, upon the whole, 

 a correspondence between the two ; and that the heat 

 is in proportion to the intensity, taken jointly. This, 

 by the way, is the only rational view of the source of 

 animal heat, a subject upon which so much ingenuity 

 has been wasted, and so many theories have been pro- 

 posed and abandoned. Sensible heat, notwithstand- 

 ing all that may be said about it, is not a thing, it is a 

 mere accompaniment, the measure, or at all events 

 the indication, of a certain degree of intensity of action. 

 Nor does it appear to signify much of what kind this 

 action may be, whether chemical, mechanical, living, 

 or any thing else ; for we have not only every reason 

 in argument for concluding, but we have the evidence 

 of facts to prove, that any one of those modes of action 

 of any kind which we can name, might be brought up 

 to such a degree of concentration and intensity, 

 as to produce combustion in inflammable sub- 

 stances, or, as we say, kindle fire. We have proofs of 

 this in the action 1 of the animal system, in those cases 

 of spontaneous combustion, which are too well and 

 too minutely authenticated for leaving any room to 

 doubt their truth, though, for the details of these, we 

 must refer to the article COMBUSTION (SPONTANEOUS) 

 in this work. 



But whenever we would approach the subject of 

 animal life with any thing like knowledge, it is neces- 

 sary that we should always refresh our memories on 

 this same subject of animal heat ; because there is no 

 one subject in the whole wide field of physiology in 

 which the ignorant are so much at a loss for the way, 

 and the learned so deeply bcmired in their own 

 fancies. 



As the temperature of the element in which fishes 

 ive is never high, and varies but little, as they have 

 nothing to do but move about, and eat, and propagate, 

 without any burden, even of their own weight, to bear ; 

 as they have but little blood, and that blood has a slow 

 and limited circulation, one might naturally infer that 

 they would have a low degree of temperature, and 

 observation shows that it is in some cases, at least, 

 lower than the temperature of the water in which they 

 reside. The deposit on the great bank of Newfound- 

 land, which affords food for such inexhaustible shoals 

 of cod, is usually, and we think with reason, under- 

 stood to be made by the warm current which comes 



