Gambar halaman
PDF
ePub

the insect consists of a chain of ganglia connected by nerves or commissures, while the spinal cord of the fish or man is essentially“ a double and fused series of nerve-centers.” Moreover, if the vertebrate cord is cut through, a section shows that it consists of two kinds of substances or tissues, called the "gray" and “ white" substance. The gray matter is situated in the center, and consists largely of nerve or so-called “ganglion cells,” while the external white matter of the brain or cord is composed of a mass of nerve fibers. Now, in the nervous system of insects there is nothing to compare with these substances, but the ganglia, on the contrary, as we shall see farther on, consist primarily of an external layer of ganglion cells, whose fibers pass in to form a central fibrous mass or net-work, the meshes of which are filled with a fine granulated nerve substance, the nature of which is not clearly understood. Moreover, the entire brain of an insect is white, as are all the ganglia.

A ganglion in its simplest form is a little rounded mass, or nodule, of ganglion cells, with fibers leading from them: such cells are represented by Fig. 3a--3e, on Plate III. Now when the fibers lead in from the sensitive hairs on the crest of the insect, or from the antennæ, or the eyes or ears, and end in separate masses or lobes, which are modified ganglia, such ganglia are regarded as "sensory ganglia," and the nerves leading in from them are called ingoing or "afferent nerves," while the ganglia which give rise to the outgoing or “efferent" nerves, i.e., those going to the muscles of the wings, legs, &c., are called "motor ganglia."

It should be borne in mind as the result of recent studies by several observers, as Leydig, Flögel, Dietl and Newton, that the subæsophageal ganglion, or “ brain," of the insect is much more complex than any other ganglion, consisting more exclusively both of sensory as well as motor ganglia and their nerves. But it should also be understood that the suboesophageal ganglion also receives nerves of special sense, situated possibly on the palpi, and possibly on the tongue, at least the latter is the case with the bee; hence, this ganglion is probably complex, consisting of sensory and motor ganglia. The third thoracic ganglion is also, without doubt, a complex one, as in the locusts the auditory nerves pass into it from the ears, which are situated at the base of the abdomen. But in the green grasshoppers, such as the katydids and their allies, whose ears are situated in their fore legs, the first thoracic ganglion is a complex one. In the cockroach and in the Leptis (Chrysopila), a common fly, the caudal appendages bear what are probably olfactory organs, and as these parts are undoubtedly supplied from the last abdominal ganglion, this is probably composed of sensory and motor ganglia; so that we have in the ganglionated cord of insects a series of brains, as it were, running from head to tail, and thus in a still stronger sense than in vertebrates the entire nervous system, and not the brain alone, is the organ of the mind, or psychological endowments, of the insect.

We will now proceed to examine the brain of the adult Caloptenus spretus, and compare it with that of other insects; then study its development in the embryo, and finally examine the changes it undergoes in the larva and pupal stages before attaining the fully developed structure of the adult locust.

Histological Elements of the Brain.The brain is histologically or structurally divided into two kinds of tissue or cellular elements.

1. An outer, slightly darker, usually pale-grayish white portion is made up of "cortical cells," or " ganglion cells." (Pl. 11., Fig. 3, a, b, c,

This outer loose cellular envelope of the brain consists of large and small ganglion cells. Where the tissue consists of small ganglion cells, it is naturally from the denser arrangement of the smaller cells, which are packed closer together, rather darker than in those regions where the tissue consists of the more loosely disposed, large ganglion cells.

1. The large ganglion cells (Pl. 111., Fig. 3, 3 a, 3 b, 3C, 3 d, e) are oval, and send off usually a single nerve fiber; they have a thin fibrous cell wall, and the contents are finely granular. The nucleus is very large, often one-half the diameter of the entire cell, and is composed of large round refractive granules, usually concealing the nucleolus (the granules are much larger and fewer in number and the nucleolus is less distinct than in the brain of Limulus, the king crab). These large ganglion cells are most abundant and largest on each side of the upper furrow, and in front of the “central body,” also at the bottom of the lower furrow, and along the exterior of the optic and antennal lobes, and along the commissural lobes.

d.)

B. The small ganglion cells apparently differ chiefly in size from the large ones, and are most numerous in the front swelling of each hemisphere; they surround and fill the calices of the mushroom bodies, and they extend along each optic nerve and form a large proportion of each optic ganglion, especially the layer next to the retina of the eye, though they are replaced by large ganglion cells at the junction of the fibrous part of the optic nerve with the dilated granular portion. The brain is surrounded more or less completely by the connective tissue cells belonging to the mesoderm or middle germ layer, and which are sometimes liable to be confounded with the ganglion cells, as they stain the same tint with carmine. It should be borne in mind that the nervous system, ganglia and nerves, originate from the tegumental or exodermal layer.

II. The medullary or inner part of the brain consists of matter which remains white or unstained after the preparation has remained thoroughly exposed to the action of the carmine. It consists of minute granules and interlacing fibers. The latter often forms a fine irregular net-work inclosing masses of finely granulated nerve matter.

In the antennal and commissural lobes is a third kind of matter, in addition to the granular and fibrous substances, which forms irregularly rounded masses, cream-colored in picro-carmine preparations, and which stain dark with osmic acid. This is called by Dietl "marksubstanz," and is described by Newton as “a peculiar arrangement of nervous matter, which appears sometimes as fine fibrillæ, with an axial arrangement, sometimes as a very fine net-work of different thicknesses, and sometimes as thin lamellæ, or altogether homogeneous.”

It is to be noticed that this central unstained portion contains few, if any, ganglion cells, and it is most probable that the fibers of which it is composed originate from the cortical ganglion cells. At one or two points at Fig. 3, Pl. iii, I have seen the fibers passing in from ganglion cells towards the middle of the brain. In the horseshoe crab (Limulus), owing to the simple structure of the brain, it is evident that the optic and ocellar nerves and posterior commissures originate from the large ganglion cells which in this animal are situated in or near the center of the brain. In the last abdominal ganglion also the fibers arising from the peripheral ganglion cells can very plainly be seen passing in towards the center of the ganglion and mingling with the fibers forming it. Hence, in all probability the fibrous mass of the central part of the brain mostly originates from the peripheral or cortical ganglion cells.

To briefly describe the brain of the locust, it is a modified ganglion, but structurally entirely different from and far more complicated than the other ganglia of the nervous system. It possesses a "central body," and in each hemisphere a “mushroom body," optic lobe, and optic ganglion, and olfactory lobe, with their connecting and commissural nerve fibers, not found in the other ganglia. In the succeeding ganglia the lobes are, in general, motor; the fibers composing the esophageal commissures, and which arise from the esophageal commissural lobes, extend not only to the subæsophageal ganglion, but pass along through the succeeding ganglia to the last pair of abdominal nerve centers. Since, then, there is a direct continuity in the fibers forming the two main longitudinal commissures of the nervous cord, and which originate in the brain, it seems to follow that the movements of the body are in large part directed or coördinated by the brain. Still, however, a second brain, so to

1 We have seen that the two great longitudinal commissures pass directly from the the brain into and then pass backward from the suboesophageal ganglion, but beyond that point we have not traced their course, as it is generally supposed that they extend uninterruptedly to the last abdominal ganglia. This has indeed been shown to be the case by Michels, in his admirable treatise on the nervous system of a beetle (Oryctes) in Siebold and Kölliker's Zeitschrift für wissen. Zoologie, Band 34, Hest. 4, 1880. Michels states that each commissure is formed of three parallel bundles of elementary nerve fibers, which pass continuously from one end of the ventral or nervous cord to the other. “The commissures take their origin neither out of a central punctsubstanz (or marksubstanz), nor from the peripheral ganglion cells of the several ganglia, but are mere continuations of the longitudinal fibers which decrease posteriorly in thickness, and extend anteriorly through the commissures forming the csophageal ring to the brain.”

2 The following extract from Newton's paper shows, however, that the insra or subæsophageal ganglion, according to Faivre, has the power of coördinating the movements of the body; still it seems to us that the brain may be primarily con. cerned in the exercise of this power, as the nerves from the subæsophageal ganglion supply only the mouth parts. The physiological experiments of Faivre, in 1857 (Ann. J. Sci. Nat., Tom. viii. p. 245), upon the brain of Dytiscus in relation to loco. motion, are of very considerable interest, showing, as they appear to do, that the power of coördinating the movements of the body is lodged in the infraesophageal ganglion. And such being the case, both the upper and lower pairs of ganglia ought to be regarded as forming parts of the insect's brain.” Quart. Jour. Micr. Sc., 1879, p. 342.

« SebelumnyaLanjutkan »