ARTICLE X.

Observations on the Uses of the Dorsal Vessel, or on the Influence which the Heart exercises in the Organization of articulated Animals, and on the Changes which that Organization experiences when the Heart or the Organ of Circulation ceases to exist. By M. Marcel de Serres.

(Continued from p. 199.)

THIS last series of researches being finished, it remained to determine the cause of the contractions and dilatations of the dorsal vessel, and of what importance that vessel was to the general economy of insects. These contractions and dilatations might depend on various causes, or only upon the peculiar structure of that organ; though it was difficult to adopt such an opinion, because the absence of vascular vessels announced that the humour contained in it did not circulate. To determine the point I began by examining what influence the muscles which surround the dorsal vessel might have upon its contractions.

The mole cricket, being a very lively insect, was chiefly employed by me in these researches. The dorsal vessel being brought in view, I removed the abdominal muscles (and the adipose membranes of necessity) in the middle part of the abdomen, while I left the dorsal muscles quite entire in the upper and lower part of the abdomen. This being done, I remarked that the dorsal vessel always contracted in those parts where the muscles had not been removed, and these contractions were the less lively the nearer they came to the muscles that had been removed. However, by degrees, some contractions took place in the parts of the dorsal vessel from which the muscles had been removed; but they were always weak, and seemed to proceed from portions in which the muscles still existed.

In other cases I removed entirely the dorsal muscles; then the contractions of the dorsal vessel became weaker by degrees, and ceased at last, though after a very considerable interval. Dr. Encontre* was so good, at my request, as to repeat these experiments. He observed that, after removing the dorsal muscles, the contrac tions of the dorsal vessel diminished by degrees in the rings from which these muscles had been removed; while they continued still strong in those rings where the muscles still remained. In a ring in which he had left only some traces of muscle, the dorsal vessel still continued to contract, when its contractions had ceased in all the parts from which the muscles had been removed. I must remark, that the insects subjected to these experiments are not dead, when all the dorsal vessels have been removed; but what is still

He is the son of M. Daniel Encontre, known with eclat as a mathematician. VOL. V. N° V.

2 A

more singular, they continue to live after the dorsal vessel has been entirely removed. I have seen the caterpillar of a sphinx atropos breathe for six hours after I had removed the dorsal vessel. The inspirations and expirations still continued, and air bubbles broke from the ends of the trachea which had been cut at the removal of the dorsal vessel. If this organ were a heart, how could insects possibly live so long after being deprived of it. To be convinced of the difference, let the heart be removed from those animals in which it exists even in the least complicated state. Not one will resist its loss, most of them will be dead before the removal can be completed. I must observe, that after the removal of the dorsal vessel, insects are no longer able to move about; but life notwithstanding still continues.

This intensity of life is found in all animals that have no centre of action: other animals die very speedily. One would think there ought not to be a great difference in this respect between the faucheurs and spiders, or between the scolopendras and scorpions, and yet there is a very great one. The scorpions and spiders die almost immediately after exposing their heart; while insects live often six or seven hours after removing the organs most essential to life. Yet both have a nervous system composed of a series of ganglions; but the insects have no centre of circulation, or, which is the same thing, they have no heart.

We have seen the influence of the dorsal muscles on the contractions of the dorsal vessel, As this influence might not be the only one, I endeavoured to determine those which might belong to the trachea and nerves. Before engaging in this new set of experiments, I wished to determine whether by means of metallic exciters I could render the contractions of the dorsal vessel more frequent and stronger. When the two exciters were brought into contact the insect exhibited symptoms of pain; but the dorsal vessel, instead of increasing in liveliness, beat always more and more slowly. On continuing the contact of the wires of zinc and copper between the membranes of the dorsal vessel, the humour contained in this vessel gradually coagulated, and the contractions ceased by degrees. Thus the galvanic agent in the present case produces effects similar to the chemical, with this difference, that it acts less rapidly.

It might be concluded, à priori, that the trachea have a considerable influence on the contractions of the dorsal vessel, on account of the great number of them which enter it and constitute one of its coats, and on account of the great influence of air on the organs of insects. But great difficulties prevent us from determining that influence; the most insurmountable of which is the impossibility of separating the tracheie from the dorsal vessel: for, without such a separation, we cannot judge of the influence which they exert on its contractions. I have not been able to surmount these difficulties in those insects which have tributary traches. So that I can only depend upon one experiment, which succeeded

very well on a caleoptera lamellicornis. I removed with all possible care, in the ateuchus semi-punctatus, the vesicular tracheæ which surround the dorsal vessel, as well as their ramifications. When they were removed the contractions of the vessel diminished by degrees. I had indeed removed some muscular fibres; but in so small a quantity, that I do not believe that cause had a remarkable influence. These contractions have continually gone on diminishing, and the diminution was much greater than what is the consequence of pain, and of the cessation of certain functions. I believe, therefore, that the contractions of the dorsal vessel are proportional to the quantity of adipose tissue; the energy of the muscular fibres that surround it, and of the number of trachea which enter into it, or of the air which it receives.

Let us see now whether the nerves have not some influence on these contractions, as the dorsal vessel receives a certain number of them. To determine this influence, I removed the spinal marrow with the nerves coming from it, as far as I could distinguish them, and then examining the contractions of the dorsal vessel, I could not perceive that they were sensibly enfeebled. I repeated this experiment on a great number of individuals of different orders, and I always obtained the same result. Hence I conclude, that the nerves have no very sensible influence on the contractions of the dorsal vessel. It is possible, that in all these animals in which the nervous system is very much divided, and the principal centre has little preponderance, the nervous influence is less distinct. This I shall endeavour to determine by future researches.

It remained still to determine whether the contraction of the dorsal vessel does not depend in part on the circulation of the liquid which it contains. We have already observed, and the observation, since the time of Malpighi has been renewed by all anatomists, that the humour of the dorsal vessel has a very irregular motion, and that the fluid often appears to go from the head towards the tail, and at other times to take a contrary direction. Sometimes the dorsal vessel contracts at its two extremities, while these contractions do not take place in the middle of the organ, especially if the dorsal muscles have been removed in this part. So that the liquid in the dorsal vessel appears at one time to move with extreme rapidity, at another very slowly, without any apparent connexion between this irregularity and the state of the insect. But how can this irregular motion be reconciled with the circulation of a fluid analogous to the blood, and how can a circulation take place without vessels? All these facts are in themselves so conclusive, that it is difficult to consider the dorsal vessel as a heart, and, of course, to admit that the contractions are produced by a liquid in circulation. In the second place, if we pierce the heart of any animal whatever, the moment the, blood arrives it rushes out in a quantity proportional to that which enters the heart. It was important, therefore, to determine whether this would happen when the dorsal vessel of insects is punctured. For that purpose I chose

several very lively individuals. I pierced the dorsal vessel in different parts at the time when the liquid had flown back, but I never perceived the least moisture issue out of it. As it is difficult to determine this point in a satisfactory manner, I repeated the experiments with all the precaution possible. The results were constantly the same: no liquid flowed out. If this were the only proof that could be given of the non-circulation of the humour contained in the dorsal vessel, it would not be of much importance; but, joined to those that we have already made known, it acquires a certain force, and even a considerable weight. But it may be said, that the reason why the humour does not exude when the dorsal vessel is punctured, is because it is too thick. I am very much inclined, I own, to that opinion; for this humour has always appeared to me very little liquid, and in the voraceous larvæ it has a remarkable consistence. This consistence, joined to some other particularities, has put us on the way to determine with some precision the functions of the dorsal vessel in those animals that have no other circulation than that of air. The different movements of contraction and dilatation which we remark in the dorsal vessel, can never induce us to consider that organ as a heart; since, in the animals like the Naiades (Nerèis, Lin.) in which there exists only a single organ, that of digestion, we observe pulsations as distinct as those exhibited by the heart of other animals. Therefore the pulsations of the dorsal vessel ought not to make us decide. in any manner on the use of that organ, nor lead us to consider it as a heart.

In all animals which have a system of circulation and respiration, the influence of the one of these on the other has been perceived.* Thus often when the respiration is entire, the circulation is only half so, or when the circulation is complete, the respiration does not operate in a complete manner; so that a demicirculation, multiplied by a complete respiration, or vice versâ, gives always equal products; that is to say, a semioxygenation of the blood. But in mammiferous animals, in which the circulation and respiration are complete, the oxygenation of the blood is equally so; and in birds which have a complete circulation with a double respiration, the oxygenation of the blood is doubled, in consequence of the quantity of air that combines continually with it. †

If the dorsal vessel of insects is an organ of circulation, it ought to experience the influence of the organs of respiration like the heart of animals with vertebræ. This influence ought to be so much the greater, as insects, like birds, have a double respiration. The air penetrates into all the parts of their bodies, as it does in birds. It continually bathes their nutritive humour, or their blood, which has this particularity, that it is not confined in vessels; just as in birds it acts upon the blood in the great circulation, at the

→ See Cuvier's Anatomie Comparée, t. iv. p. 167.

+ These speculations about the oxygenation of the blood are quite vague and certain. T.

same time that the small circulation experiences the action of the air in the lungs. Some insects have even peculiar reservoirs of air in their bodies, reservoirs so numerous, that those which are provided with them ought to have a respiration more than double. These reservoirs of air, called vesicular trache; but which may likewise be called pneumatic pockets, exist only in those insects which have to exert a great muscular force, and which, having great spaces to traverse, ought to have their body specifically light. From what has been said, it is evident that the air ought to have a great influence on the general economy of insects. We shall see hereafter, that it is the only fluid which has a real circulation in them. Now the greater this influence is, the more ought it to exert itself on the organ of circulation, provided such an organ exist. As the dorsal vessel has been considered by different anatomists as coming in place of the heart in insects, let us see whether the influence of the respiratory organ is sensible on this vessel. But to lay open that influence in a more certain manner, let us examine in the first place the organs of respiration themselves, and see whether, when these organs undergo modifications, the dorsal vessel feels their effects.

All the organs of respiration in insects may be reduced to simple trachea. These organs indeed have not all the same composition; we must therefore divide them into two orders: the first, which we shall call tubular trachea, on account of their disposition, are com- › posed of three membranes, one external, one internal, and one. intermediate. The two first are formed of a cellular membrane, pretty thick, and very extensible; while the intermediate one consists of a cartilaginous string, rolled up into a spiral, which may be unrolled with great facility. It is the convolutions of this elastic string which forms those brilliant and silvery looking conduits, destined to hold air, and to transmit it to all parts of the body. These trachea are always kept stretched by means of this cartilaginous string. They form tubes, and have a great elasticity, which enables them to dilate, when the air, continually circulating in them, expands. These trachea are likewise the only ones which are ramified, the multiplied branches of which go to all parts of the body, to communicate the impression of air, the aliment of life as well as of flame.

The second order of trachea, or the vesicular trachea, do not form tubes like the preceding; they present, on the contrary, pouches more or less extended, which communicate with one another by means of ramifications, always single, and branched like those proceeding from the spiral trachea. These vesicular tracheæ are composed of two very white cellular membranes, very supple and very extensible. As these tracheæ have not the spiral string observable in the first order, they never communicate immediately with the air, but always by means of spiral rache, Accordingly, in the species which have need of a great