The Microscope. Its History, Construction, and Application 15th ed. / Being a familiar introduction to the use of the instrument, and the study of microscopical science

by Jabez Hogg

CHAPTER V.

Vertebrata.

The most complicated condition in which matter exists is where, under the influence of life, it forms bodies with a structure of tubes and cavities in which fluids are incessantly in motion, and producing continuous changes. These have been rightly designated “organised bodies,” because of the various organs they contain. The two principal classes into which organised bodies have been divided are recognised as vegetable and animal. It was Bichat who taught that our animal life is double, while our organic life is single. In organic life, to stop is to die; and the life we have in common with vegetables never sleeps, and if the circulation of the fluids within the animal body ceases for a few seconds, it ceases for ever. In the vertebrate body, however, the combination of organs attains to the highest development, in striking contrast with that of the class we have previously considered, the Invertebrata, the animal kingdom being divided into Vertebrates and Invertebrates.

The Vertebrata are distinguished from all other animals by the circumstance that a transverse and a vertical section of the body exhibits two cavities completely separated from one another by a partition. A still more characteristic feature separates the one from the other; it is the specialisation of the chief nervous centres, and their peculiar relation to the other systems of the body.

The dorsal cavity of the body contains the cerebro-spinal nervous system, the ventral, the alimentary canal, the heart, and usually a double chain of ganglia; these pass under the name of the sympathetic system. It is very probable that this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. In any case, the central parts of the cerebro-spinal nervous system—i.e., the brain and the spinal cord—would appear to be unrepresented among invertebrate animals. Likewise, in the partition between the cerebro-spinal and visceral tubes, certain structures which are not represented in Invertebrates are contained. During the embryonic condition of all Vertebrates, the centre of the partition is occupied by an elongated cellular cylindrical mass, the notochord, or chorda dorsalis. This structure persists throughout the life in some Vertebrata, but in most it is more or less completely replaced by a jointed, partly fibrous, cartilaginous, and bony vertical column. All vertebrate animals have a complete vascular system. In the thorax and abdomen, in place of a single perivisceral cavity, in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior, recalling in the latter case the atrial cavities of the Mollusca. In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess a hepatic portal system, the blood of the alimentary canal never being wholly returned directly to the heart by the ordinary veins, but being more or less completely collected into a trunk (the portal vein), which ramifies through and supplies the liver.

With reference to one other point of importance, the development of the ova of Vertebrates, these have the same primary composition as those of other animals, consisting of a germinal vesicle containing one or more germinal nuclei, and included within a vitellus. But as this forms a part of general anatomy, and as my object is simply the investigation of the fundamental and microscopical structure of animal organisms, I shall not further pursue the morphological part of the subject, especially as so many excellent text-books are within reach of the student who desires to fully acquaint himself with precise information.

Notwithstanding, then, the apparent diversity in the structure of the vertebrate and the invertebrate and the various tissues of which animals and vegetables are constituted, microscopical research has satisfactorily demonstrated that all textures have their origin in cells; in fact, when the formative process is complete, the animal cell is seen to consist of the same parts and almost the same chemical constituents as the typical cell of the plant—namely, a definite cell-wall enclosing cell contents, of which the nature may be diverse, but the cell nucleus is precisely the same and is the actual seat and origin of all formative activity. The cell and nucleus grow by assimilation or intersusception, that is, by inflowing of nutrition among all parts, the new replacing the old, yet maintaining its original structure and composition. That which was once thought special to animals is now found to be common to both plants and animals: they are found to be alike fundamentally in internal structure, and in the discharge of the mysterious processes of reproduction and of nutrition, although the latter forms a convenient line of separation. Life in plants goes on indefinitely; cuttings may be taken without injury to their vigour and duration of life. The same may be said of some of the lower forms of invertebrate life; for example, the hydra, the anemone, and some other well-known animals, may be cut up, divided into several parts, each one of which will form a new animal, provided a nucleus be included in the section. Nevertheless, the organisation of the amœba and the hydra is as complete for its purpose as that of man for his, and the evidence of continuity forbids the drawing of hard and fast lines, as was formerly done between the two kingdoms, the animal and vegetable. The amount of similarity or agreement in the organisation of animals is various. Animals indeed differ from each other in slight points only, for the discovery of which the microscope must be brought into requisition. Living matter in its earliest stage and simplest form appears to the naked eye as a homogeneous structure, but when placed under the highest powers of the microscope, it is seen not to be so.

But perhaps the most marked feature of the age has been the increasing attention given to the study of the lower forms of life, using their simpler structures and more diffuse phenomena to elucidate the more general properties of living matter. To understand life we must understand protoplasm. Of this there can be no doubt, as we have seen in a previous chapter that a whole family, the Monera, consists of this simple living, microscopic, jelly-like substance, which has not even begun to be differentiated, as in the amœba, which has as yet no special organs, and every speck becomes a mouth or a stomach, and which can be turned inside out and shoot out tongues of jelly to move and feel with. “Reproduction is the faculty most characteristic of life, and sharply distinguishes the organic from the inorganic.” It is, then, the corpuscles of protoplasm, called cells (cellulæ), which have so much interest for the physiologist, and these, like the cytods, may form independent organisms, which are then termed unicellular. Again, cells form other cells, and a multicellular organism results, and goes on increasing in geometrical progression. In the Vertebrata the cell retains its characteristic spheroidal shape, as seen in [Fig. 423], and undergoes division by virtue of its living protoplasmic mass.

Fig. 423.