REPARATIVE POWERS OF HIGHER ANIMALS. 325 requiring a higher degree of the stimulus of Heat, than does their ordinary nutrition. In Lizards, an imperfect reproduction of the tail takes place when part of it has been broken off; but the newly-developed portion contains no perfect vertebræ, its centre being occupied by a cartilaginous column like that of the lowest fishes.-In the warm-blooded Vertebrata generally, the power of reproduction after loss or injury seems much more limited. We do not find that entire parts or members once destroyed, are completely renewed; though very extensive breaches of substance are often filled up. The tissues most readily reproduced are Bone, the Simple Fibres (§ 22), and the Membranes (such as the Skin, the Mucous and Serous membranes), of which these tissues form the basis. As a general rule, losses of substance in Glandular tissue, Muscle, and other parts of comparatively high organization, do not seem to be reproduced; but there is a curious exception to this in the case of Nervous tissue, which, with Blood-vessels, is very readily re-formed in the new growths by which losses of substance are repaired, as we often see in the rapid skinning-over of a large superficial wound. One of the most remarkable manifestations of reparative power in the Human body, is the re-formation of an entire bone, when the original one has been destroyed by disease. The new bony matter is thrown-out, sometimes within and sometimes around the dead shaft; and when the latter has been removed, the new structure gradually assumes the regular form, and all the attachments of muscles, ligaments, &c., become as complete as before. A much greater variety and complexity of actions are involved in this process, than in the reproduction of whole parts in the simpler animals; though its effects do not appear so striking. It appears that, in some individuals, this regenerating power is retained to a much greater degree than it is by the species at large; thus, there is a well-authenticated instance, in which a supernumerary thumb on a boy's hand was twice reproduced, after having been removed from the joint. In many cases in which the crystalline lens of the eye has been removed, in the operation for cataract, it has been afterwards regenerated; and there is evidence that, during embryonic life, the regeneration of lost parts may take place in a degree to which we have scarcely any parallel after birth; attempts being sometimes made at the re-formation of entire limbs, in 326 REPAIR OF LOSSES OF SUBSTANCE. place of such as are lost during the earlier periods of development. 391. When an entirely new structure is to be formed,—as for the closure of a wound, the union of a broken bone, or the repair of any other injury,—the process is of a kind very much resembling the first development of the entire fabric. The neighbouring vessels pour out their liquor sanguinis, which is known to the Surgeon under the name of coagulable lymph; this fills up the open space, and forms a connecting medium between the separated parts. If this intervening layer be thin, the two sides of the wound may adhere so closely as to grow together without any perceptible interposition of new substance; this is what is called "healing by the first intention." But if the loss of substance has been too great to allow of such adhesion, the vacant space is filled by the gradual organization of the coagulable lymph; and this may take place in one of two very different modes, the determination being chiefly dependent on the condition of the wound as to seclusion from air or exposure to it. 392. The former of these conditions is by far the more favourable of the two; for the reparative material is usually developed gradually but surely into fibrous tissue, without any loss, and with very little irritation either in the part itself or in the system at large. This process seems to take place naturally in cold-blooded animals, even in open wounds; the contact of air not having that disturbing influence in them, which it exerts in warm-blooded animals. And Nature frequently endeavours to bring it about in the superficial wounds of warm-blooded animals, by the formation of a large scab, which protects the exposed surface; but this happens much less frequently in the Human subject than it does among the lower animals, the unnatural conditions in which a large proportion of the so-called civilised races habitually live (especially deficient purity of the air, continual excess in diet, and the frequent abuse of stimulants) being unfavourable to it. The performance of many operations which formerly left open wounds, in such a manner that the air may be effectually excluded by a valvular fold of skin, is one of the greatest improvements in modern Surgery. 393. In an open wound, on the other hand, which is healing by the process termed "granulation," the reparative HEALING OF OPEN WOUNDS. 327 material is rapidly developed into cells, amongst which bloodvessels speedily extend themselves. The formation of new blood-vessels, in this and other cases, seems to commence in the giving-way of the walls of some of the previously-existing capillary loops, at particular spots, and in the escape of blood corpuscles in rows or files into the soft substance that surrounds them; thus channels or passages are excavated, which come into connexion with each other; and these channels, after a time, acquire proper walls, and become continuous with the vessels from which they originated,-to be in their turn the originators of a new series. The vitality of this new “granulation-tissue," however, is very low; and the part exposed to the air passes into the condition of pus (the yellow creamy fluid discharged from an open wound), which contains the same materials in a decomposing state. Thus there is a constant waste of organizable substance, the amount of which, in the case of an extensive wound, becomes a serious drain upon the system; at the same time, there is a much greater irritative disturbance both in the part itself and in the system generally; and the new tissue that is formed is of such low vitality that it subsequently wastes away, so as by its disappearance to leave a contracted cicatrix or scar.-The difference between the two modes of reparation now described is often one of life and death, especially in the case of large burns of the body in children. CHAPTER IX. ON THE EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY BY ANIMALS. Animal Luminousness 394. A large proportion of the lower classes of aquatic Animals possess, in a greater or less degree, the power of emitting light. The phosphorescence of the sea, which has been observed in every zone, but more remarkably between the tropics, is due to this cause. When a vessel ploughs the ocean during the night, the waves-especially those in her wake, or those which have beaten against her sides—exhibit a diffused lustre, interspersed here and there by stars or ribands of more intense brilliancy. The uniform diffused 328 LUMINOSITY OF MARINE ANIMALS. light is chiefly emitted by innumerable minute animals, which abound in the waters of the surface; whilst the stars and ribands are due to larger animals, whose forms are thus brilliantly displayed. This interesting phenomenon, when it occurs on our own coasts, is chiefly produced by incalculable multitudes of a small creature, termed the Noctiluca, having a nearly globular form, and a size about equal to that of the head of a minute pin. When these cover the water, and a boat is rowed among them, every stroke of the oars produces a flash of light; and the ripple of the water upon the shore is marked by a brilliant line. If a person walk over sands that the tide has left, his footsteps will seem as if they had been impressed by some glowing body. And if a small quantity of the water be taken up and rubbed between the hands, they will remain luminous for some time. The transparency of the little animals to which these beautiful appearances are due, might cause them to be overlooked if they are not attentively sought; they somewhat resemble grains of boiled sago in their aspect, but are much softer. In the general simplicity of their structure, the Voctiluca appear to correspond rather with the Rhizopoda (§ 130) than with any other group; but they are distinguished by some remarkable peculiarities. 395. Of the larger luminous forms which are seen to float in the ocean-waters, a great proportion belong to the class Acalephæ. The light emitted by these appears to be due to the peculiar chemical nature of the mucus secreted from their bodies; for this, when removed from them, retains its properties for some time, and may communicate them to water or milk, rendering those fluids luminous for some hours, particularly when they are warmed and agitated. It is probably from this source, that the diffused luminosity of the sea is partly derived. The secretion appears to be increased in amount, by anything that irritates or alarms the animals; and it is from this cause that the dashing of the waves against each other, the side of a ship, or the shore,—or the tread of the foot upon the sand,—or the compression of the animals between the fingers, occasions a greater emission of light. But some of these causes may act, by bringing a fresh quantity of the phosphorescent secretion into contact with air, which seems necessary to maintain the kind of slow combustion on which the light depends. LUMINOSITY OF MARINE ANIMALS AND INSECTS. 329 396. But the Noctiluce and Acalephæ are by no means the only luminous animals which tenant the deep. Many Zoophytes appear to have this property in an inferior degree, and also some of the Echinodermata. Of the lowest class of Mollusks, the Tunicata, a very large proportion are luminous, especially those which float freely through the ocean, and which abound in the Mediterranean and tropical seas; the brilliancy of some of these can scarcely be surpassed. Among some of the Conchifera, also, the phenomenon has been observed; as well as in certain marine Annelida. Other marine animals of higher classes are possessed of similar properties; thus, many Crustacea, especially the minuter species, are known to emit light in brilliant jets; and the same may be said of a few Fishes: but it is probable that the luminosity attributed to many of the latter is due to the disturbance they make in the surrounding water, which excites its phosphorescence in the manner just explained. In all these, the general phenomena are analogous,-the luminous matter appearing to be a secretion from the surface of the animals, and to undergo a sort of slow combustion by combination with oxygen. Wherever it is presented by these animals, it is always most brilliant upon the surfaces concerned in respiration. The light continues for some days after death; but ceases at the commencement of putrefaction.1 397. In the class of INSECTS, there are several species which have considerable luminous power; and in these the emission of light is for the most part confined to a small part of the surface of the body, from which it issues with great brilliancy. The luminous Insects are most numerous among the Beetle tribe, and are nearly restricted to two families, the Elateride, and the Lampyride. The former contains about 30 luminous species, which are known as fire-flies; these are all natives of the warmer parts of the New World. Their light proceeds from two minute but brilliant points, which are situated one on each side of the front of the thorax; and from another 1 There are certain cases, however, in which the production of Light, like that of Electricity (§ 423), appears to be a peculiar manifestation of Nervous power. There is strong reason to believe that Nerve-force may be directly metamorphosed (as it were) into these or other forms of physical and vital force, according to the principle of "Correlation" now generally admitted as regards the Physical Forces. |