NERVOUS SYSTEM.

129

CONVERSATION VI.

Nervous system-its progressive developement in the inferior animals-the spinal marrow-nerves-brainfunctions of the nervous system-sensation-volitionMr. Bell's discoveries--theories of the nervous power— Dr. Wilson Philips' experiments—phrenology-comparison of the brain of man and those of the inferior animals-facial angle-instinct-reason.

Dr. B.-The functions of organic life-those which we have examined are all possessed to a certain degree by vegetables, as well as animals. But the latter class of beings, and particularly, he who stands at the head of it-man, enjoys a higher and nobler state of existence. They are conscious of their own existence and of the presence of things around them; and they are affected with pleasure or pain, according to the manner in which these objects affect their constitution. For the enjoyment of this function, they are indebted to a complex series of organs called the Nervous System. With this alone however, they would have remained imperfect beings, enjoying an existence less enviable than that of the humblest vegetable. Capable of receiving impressions from the external world, the power of acting in consequence of these impressions, of making their condition change as the objects around them change, was obviously necessary to complete the perfection of their organization, and the harmony of relations existing between them and the

130 DEVELOPEMENT OF THE NERVOUS SYSTEM.

general system of nature.

To accomplish this latter purpose, we are provided with a system of organs, called organs of locomotion.

Emily.—Am I tɔ understand that the latter are entirely subservient to the former organs-acting only when stimulated by them?

Dr. B. Certainly; no voluntary motion takes place without the action of the nerves, but this view of the subject must be considered at another time, for at present we must look at the structure of the nervous system. In order that you may obtain a clearer notion of this system, we will first examine it in the inferior animals, and thence trace it up into the nervous system of man. In this manner, you will see in a very interesting light, its gradual developement, and the relative importance of its different parts.

Emily. This system, like the others, I suppose, is simple in the lower animals, increases in complexity as we ascend the scale of being, and finally exists in its utmost perfection in man.

Dr. B.-Yes-but its gradual developement differs from that of the other organs, in one very remarkable point. It is that in the earliest stages of existence, it is the same in all; take it in the highest state of perfection in which it is observed, and we find that before it could attain that state, it had to pass through all the forms which it possessed in the different classes of inferior animals in their perfect state.

Emily. If I understand your meaning correctly, the nervous system of man, before it attains the form which we behold, has first presented that of the zoophytes, then that of the worms, then of the shell-fish, then of the fishes, and so on through the whole series.

Dr. B.-Precisely so; and now we will look at the proofs of this doctrine. In the lowest orders of the Zoophytes, the only trace of this system is nothing more than nervous molecules unconnected with one another, disseminated through the substance of their simple, pulpy

DEVELOPEMENT OF THE NERVOUS SYSTÈM. 131

structure. In Zoophytes of a little higher organization, the nervous matter appears in the form of minute and delicate threads arranged around the principal organs of nutrition, with other threads proceeding from them in a radiated manner, to go to the different parts of the body. In the next orders we find a series of little knots of nervous matter, called ganglia, connected by two nervous threads, still placed around the organs of nutrition. In the earth worm belonging to a still higher order, the nervous thread is single, instead of double as before, and the ganglia instead of appearing independent bodies, are merely swellings of this thread, from which proceed single and double pairs of threads alternately. In the first orders which possess senses, we find the anterior ganglia enlarged and sending out filaments to supply the organs of sense. Thus, as the organs of the animals increase, the nervous threads, or nerves we may as well call them, increase in number and perfection, the principal mass still being placed around the oesophagus. In the fishes we meet for the first time, the most remarkable modification which the system undergoes. The main nervous cord becomes enclosed in a long case or canal, and take the name of spinal marrow, from the long canal which is called the spine. But this is not all: at the summit of the spinal marrow, are observed several large ganglia connected together but quite distinct from one another.

This constitutes the brain, and here you may see it in this cut. In the brain of the reptiles, we find an addition of two more ganglia. In the birds, these last ganglia are much developed, and in the mammiferous tribes, the brain, instead of being composed of separate ganglia, has the appearance of a single organ, whose parts are no longer divisible into distinct bodies.

Emily. This cut, then, according to your theory, will also represent the brain and spinal marrow of man, at one period of his existence, will it not?

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THE NERVES.

Dr. B.-Yes; and to be perfectly convinced of this fact, you have only to compare the figure of the fishes' brain with this of man's, representing the form which it possesses when any traces of organization can be first discerned.

Emily. Well this is strange indeed! they seem to be perfectly alike; at least, I can discern no essential difference. The difference between them however, must be manifested afterwards; this being the permanent form of the fishes' brain, while in man it is only temporary. Dr. B.-It is succeeded by the form of the brain, as it appears in reptiles; then as it exists in birds; and finally by that which is destined to be its permanent form.

Emily. But you also said that in the lowest classes, the nerves were formed before the spinal marrow and brain showed any signs of existence,-now is this the case too, in man?

Dr. B. Most certainly; the nerves may be seen quite distinctly, while the spinal marrow is yet in a semifluid state. When this is fully organized, the nerves extend themselves to its sides, and join it by two divisions, called the anterior and posterior roots. This figure

CHEMICAL COMPOSITION OF THE BRAIN. 133

two pairs of nerves proceeding towards the spinal marrow, one on each side, and inserted into it by two roots.

Emily. But if these two divisions of the nerves, have their termination, not their origin in the spinal marrow, is it not improper to give them the name of roots?

Dr. B.—This name was given them when the nerves were supposed to originate from the spinal marrow, and is now retained merely for the sake of convenience.

Emily.-Pray, tell me how many nerves go to the spinal marrow in this manner?

Dr. B.-The whole number of spinal nerves is thirty-one on each side, and are perfectly regular and symmetrical.-Tracing the spinal marrow upwards into the head, we find it terminated by the oblong marrow, as a column is surmounted by its capital. Nearly all the rest of the brain is distinguished into two portions, the cerebellum, occupying the inferior and posterior parts of the head; and the cerebrum, which fills up the remaining space. Inclosed in its bony case, and enveloped by thick membranes, this delicate organ has all the security and protection, which its pre-eminent importance demands. The quantity of blood which it receives is remarkably large, for though it does not weigh more than one-fortieth of the whole body, it is estimated that onetenth of all the blood sent from the heart, goes to this organ. Its external portions are convoluted in structure, and the whole has a soft, pulpy consistence.

Emily. Has its chemical composition ever been ascertained? I feel not a little curious to know "what stuff our brains are made of."

Dr. B.-It has been found that about three-fourths of its weight is water, and in the solid parts are considerable fat, a peculiar animal principle, called ozmazome, a quantity of albumen, a minute portion of phosphorus, and some salts, chiefly phosphates of lime, soda and am

monia.

Emily-Fat, phosphorus, lime and soda! Of all the strange facts which you have related, this seems to be