Psychology - An elementary Text-Book

CHAPTER I

GENERAL PSYCHOLOGY

§ I. Brain and Mind

As we all know, the processes of our mental life stand in the closest relationship with the functions of the nervous system, especially with the functions of its highest organ, the brain. Local anemia, that is, a lack of blood in the brain, causes fainting, a cessation of consciousness; on the other hand, during mental work the blood pressure in the brain is higher than usual and metabolism is increased. Narcotic or poisonous drugs, as alcohol, caffein, and morphine, which influence mental activity, do this by means of their effect on the nervous system. Aside from such experiences, there are two special groups of facts upon which our knowledge of this relationship is based.

First the dependence of mental development on the development of the nervous system. This is most conspicuous when man and animals are compared. It is somewhat obscured, however, by the relation of the size of the brain to the size of the animal. The larger animal has as a rule the larger brain. Therefore the brain of man can be compared only with the brain of such animals as are of nearly the same size. When such a comparison is made, man is found to be no less superior in nervous organization than in intelligence. His brain is about three times as heavy, absolutely and relatively, as that of the animals most nearly approaching him, the anthropoid apes; eight to ten times as heavy as the brain of the most intelligent animals lower down in the scale, for instance large dogs. Similar relations between brain weight and intelligence are found in the human race itself. Of course, we cannot expect that this relation will always be found in a comparison of only two individuals. The conditions are too complex for such a regularity to exist; but it is easily demonstrated when averages of groups of intelligent and unintelligent men are compared. We do not expect, either, that in every individual case physical strength is exactly proportional to the weight of the muscles, although no one doubts that strength depends on the weight of the muscles.

The second of the facts upon which our knowledge of the relationship between mental life and nervous function is based, consists in the parallel effects of disturbances of their normal condition. Diseases or injuries of the brain are, as a rule, accompanied by disturbances of the mental life. On the other hand, mental disturbances can often be traced to lesions or structural modifications in the brain. This cannot be done in every case; but the actual connection is none the less certain. It is often very difficult to decide whether or not any mental abnormality exists. Expert psychiatrists have for weeks at a time observed men suspected of mental disease without being able to pronounce judgment. Equally difficult is the discovery of material changes in the brain and its elements. Much progress has been made in recent times in this respect; but it is still far from easy to recognize the more delicate changes in nervous structure resulting from disease. Certain abnormalities may never become directly visible although they involve disturbances of function, for instance, abnormalities in the nutrition of the nervous elements or changes in their normal sensitivity. No wonder, then, that for many mental diseases, as hysteria, corresponding material lesions are not yet known. But the correctness of our thesis is so strongly secured by the enormous number of cases in which it has been demonstrated, that no one doubts that it applies also to those cases in which, often for good reasons, its demonstration has thus far been impossible.

Of much importance is the particular form of this relationship between brain function and mental life. Popular thought attributes the chief classes of total mental activity to special parts of the brain. Judgment is thought to have its seat behind the thinker’s high forehead. The occipital part of the brain is, according to the medieval philosophers, the organ of memory. And so Gall’s phrenology met with ready acceptance from the public at large, which was delighted to learn that musical ability, mathematical talent, religious sentiment, egotism and altruism, and many other character traits had their special organs in the brain. But anatomists and physiologists have not been able to admit the plausibility of this doctrine.

Yet popular thought has, on the other hand, always emphasized the unity of mind. Those who regard its unity as the chief characteristic of mind have for centuries sought for the single point in the brain where the mind can be said to have its seat. If it were distributed all through the brain, would it not be possible to cut the mind into pieces by simply cutting the brain?

That both these views of the relation between brain and mind are inadmissible has become certain. Since about forty years ago the truth in this matter has been known. But to understand it clearly it is necessary first to familiarize ourselves with the construction of the nervous system.

QUESTIONS

23. What do we learn from a comparison of brain weight and intelligence?

24. What is the relation between nervous pathology and mental abnormality?

25. Is phrenology admissible?

26. What view concerning the relation of brain and mind is suggested by the unity of mind?

§ 2. The Nervous System

1. The Elements of the Nervous System

 

Fig. 1.—Multipolar Cell Body.

The number of elements making up the nervous system is estimated at about four thousand millions. It will help us to comprehend the significance of this number if we understand that a man’s life devoted to nothing but counting them would be too short to accomplish this task, for a hundred years contain little more than three thousand million seconds. These elements are stringlike bodies, so thin that they are invisible to the naked eye. They are generally called neurons. Within them different parts are to be distinguished. The part which is most important for the neuron’s life is a spherical, bobbin-shaped, pyramidal, or starlike body, called the ganglion cell or cell body, located usually near one of the ends of the long fiber of the neuron, but sometimes nearer the middle of the fiber. The length of the fiber varies from a fraction of an inch to several feet. The fiber may be compared with a telephone wire, inasmuch as its function consists in carrying a peculiar kind of excitatory process.

 

Fig. 2—Pyramidal Cell Body. a, Nerve fiber with collaterals.

 

Fig. 3.—Dendrites of a Nerve Cell of the Cerebellum.

At both ends of the neuron are usually found treelike branches. When the cell body is located near one of the ends of the fiber, many of these branches take their origin from the cell body and give it the pyramidal or starlike appearance illustrated by figures 1, 2, and 4. These branches are called dendrites, from the Greek word for tree, dendron. How wonderfully complicated the branching of a neuron may be is illustrated by figure 3. In addition to the dendrites a neuron possesses another kind of branches, resembling in character the tributaries of a large river, entering into it at any point of its course. These are called collaterals (lowest part of figure 2).

 

Fig. 4.—Various Types of Cell Bodies. 1 and 2, Giant pyramidal cell bodies; n, nerve fiber.

 

Fig. 5.—Longitudinal Section of a Nerve Fiber with Stained Fibrils. a, Medullated sheath.

The ganglion cells have a varying internal structure, which may be made visible to the eye when the cells have been stained by the use of different chemicals. They are found to contain small corpuscles with a network of minute fibrils between them, as shown in figures 1 and 4. The nerve fibers, too, in spite of being only 1/40 to 1/500 mm. thick, permit us to distinguish smaller parts (fig. 5). The core consists of a bundle of delicate, semi-fluid, parallel fibrils, the axis-cylinder. This is surrounded generally by a fatty, marrow-like sheath, and in the peripheral parts of the system this sheath is again inclosed in a membrane. Certain fibers attain a considerable length, for example, those which end in the fingers and toes, having their origin in the spinal region of the body.

The treelike branches of the main fiber and of the collaterals, if far away from the cell body, are sometimes called the terminal arborization, from the Latin word for tree, arbor (fig. 6). The treelike branching has most probably a functional significance of great importance. It enables the endings of different neurons to come into close enough contact to make it possible for the nervous processes to pass over from one neuron into another neuron, without destroying the individuality, the relative independence of each neuron.

 

Fig. 6.—Terminal Arborization of Optical Nerve Fibers.

Wherever large masses of neurons are accumulated, the location of the ganglion cells can be found directly by the naked eye. The fibers are colorless...