A History of Science, vol 4 - Henry Smith Williams (epub e ink reader .txt) 📗

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protoplasmic processes; the other fibre was named, after its

discoverer, the axis cylinder of Deiters. It was a natural

inference, though not clearly demonstrable in the sections, that

these filamentous processes are the connecting links between the

different nerve cells and also the channels of communication

between nerve cells and the periphery of the body. The white

substance of brain and cord, apparently, is made up of such

connecting fibres, thus bringing the different ganglion cells

everywhere into communication one with another.

 

In the attempt to trace the connecting nerve tracts through this

white substance by either macroscopical or microscopical methods,

most important aid is given by a method originated by Waller in

1852. Earlier than that, in 1839, Nasse had discovered that a

severed nerve cord degenerates in its peripheral portions. Waller

discovered that every nerve fibre, sensory or motor, has a nerve

cell to or from which it leads, which dominates its nutrition, so

that it can only retain its vitality while its connection with

that cell is intact. Such cells he named trophic centres.

Certain cells of the anterior part of the spinal cord, for

example, are the trophic centres of the spinal motor nerves.

Other trophic centres, governing nerve tracts in the spinal cord

itself, are in the various regions of the brain. It occurred to

Waller that by destroying such centres, or by severing the

connection at various regions between a nervous tract and its

trophic centre, sharply defined tracts could be made to

degenerate, and their location could subsequently be accurately

defined, as the degenerated tissues take on a changed aspect,

both to macroscopical and microscopical observation. Recognition

of this principle thus gave the experimenter a new weapon of

great efficiency in tracing nervous connections. Moreover, the

same principle has wide application in case of the human subject

in disease, such as the lesion of nerve tracts or the destruction

of centres by localized tumors, by embolisms, or by traumatisms.

 

All these various methods of anatomical examination combine to

make the conclusion almost unavoidable that the central ganglion

cells are the veritable “centres” of nervous activity to which so

many other lines of research have pointed. The conclusion was

strengthened by experiments of the students of motor

localization, which showed that the veritable centres of their

discovery lie, demonstrably, in the gray cortex of the brain, not

in the white matter. But the full proof came from pathology. At

the hands of a multitude of observers it was shown that in

certain well-known diseases of the spinal cord, with resulting

paralysis, it is the ganglion cells themselves that are found to

be destroyed. Similarly, in the case of sufferers from chronic

insanities, with marked dementia, the ganglion cells of the

cortex of the brain are found to have undergone degeneration. The

brains of paretics in particular show such degeneration, in

striking correspondence with their mental decadence. The position

of the ganglion cell as the ultimate centre of nervous activities

was thus placed beyond dispute.

 

Meantime, general acceptance being given the histological scheme

of Gerlach, according to which the mass of the white substance of

the brain is a mesh-work of intercellular fibrils, a proximal

idea seemed attainable of the way in which the ganglionic

activities are correlated, and, through association, built up, so

to speak, into the higher mental processes. Such a conception

accorded beautifully with the ideas of the associationists, who

had now become dominant in psychology. But one standing puzzle

attended this otherwise satisfactory correlation of anatomical

observations and psychic analyses. It was this: Since, according

to the histologist, the intercellular fibres, along which

impulses are conveyed, connect each brain cell, directly or

indirectly, with every other brain cell in an endless mesh-work,

how is it possible that various sets of cells may at times be

shut off from one another? Such isolation must take place, for

all normal ideation depends for its integrity quite as much upon

the shutting-out of the great mass of associations as upon the

inclusion of certain other associations. For example, a student

in solving a mathematical problem must for the moment become

quite oblivious to the special associations that have to do with

geography, natural history, and the like. But does histology give

any clew to the way in which such isolation may be effected?

 

Attempts were made to find an answer through consideration of the

very peculiar character of the blood-supply in the brain. Here,

as nowhere else, the terminal twigs of the arteries are arranged

in closed systems, not anastomosing freely with neighboring

systems. Clearly, then, a restricted area of the brain may,

through the controlling influence of the vasomotor nerves, be

flushed with arterial blood while neighboring parts remain

relatively anaemic. And since vital activities unquestionably

depend in part upon the supply of arterial blood, this peculiar

arrangement of the vascular mechanism may very properly be

supposed to aid in the localized activities of the central

nervous ganglia. But this explanation left much to be desired—in

particular when it is recalled that all higher intellection must

in all probability involve multitudes of widely scattered

centres.

 

No better explanation was forthcoming, however, until the year

1889, when of a sudden the mystery was cleared away by a fresh

discovery. Not long before this the Italian histologist Dr.

Camille Golgi had discovered a method of impregnating hardened

brain tissues with a solution of nitrate of silver, with the

result of staining the nerve cells and their processes almost

infinitely better than was possible by the methods of Gerlach, or

by any of the multiform methods that other workers had

introduced. Now for the first time it became possible to trace

the cellular prolongations definitely to their termini, for the

finer fibrils had not been rendered visible by any previous

method of treatment. Golgi himself proved that the set of fibrils

known as protoplasmic prolongations terminate by free

extremities, and have no direct connection with any cell save the

one from which they spring. He showed also that the axis

cylinders give off multitudes of lateral branches not hitherto

suspected. But here he paused, missing the real import of the

discovery of which he was hard on the track. It remained for the

Spanish histologist Dr. S. Ramon y Cajal to follow up the

investigation by means of an improved application of Golgi’s

method of staining, and to demonstrate that the axis cylinders,

together with all their collateral branches, though sometimes

extending to a great distance, yet finally terminate, like the

other cell prolongations, in arborescent fibrils having free

extremities. In a word, it was shown that each central nerve

cell, with its fibrillar offshoots, is an isolated entity.

Instead of being in physical connection with a multitude of other

nerve cells, it has no direct physical connection with any other

nerve cell whatever.

 

When Dr. Cajal announced his discovery, in 1889, his

revolutionary claims not unnaturally amazed the mass of

histologists. There were some few of them, however, who were not

quite unprepared for the revelation; in particular His, who had

half suspected the independence of the cells, because they seemed

to develop from dissociated centres; and Forel, who based a

similar suspicion on the fact that he had never been able

actually to trace a fibre from one cell to another. These

observers then came readily to repeat Cajal’s experiments. So

also did the veteran histologist Kolliker, and soon afterwards

all the leaders everywhere. The result was a practically

unanimous confirmation of the Spanish histologist’s claims, and

within a few months after his announcements the old theory of

union of nerve cells into an endless mesh-work was completely

discarded, and the theory of isolated nerve elements—the theory

of neurons, as it came to be called—was fully established in its

place.

 

As to how these isolated nerve cells functionate, Dr. Cajal gave

the clew from the very first, and his explanation has met with

universal approval.

 

In the modified view, the nerve cell retains its old position as

the storehouse of nervous energy. Each of the filaments jutting

out from the cell is held, as before, to be indeed a transmitter

of impulses, but a transmitter that operates intermittently, like

a telephone wire that is not always “connected,” and, like that

wire, the nerve fibril operates by contact and not by continuity.

Under proper stimulation the ends of the fibrils reach out, come

in contact with other end fibrils of other cells, and conduct

their destined impulse. Again they retract, and communication

ceases for the time between those particular cells. Meantime, by

a different arrangement of the various conductors, different sets

of cells are placed in communication, different associations of

nervous impulses induced, different trains of thought engendered.

Each fibril when retracted becomes a non-conductor, but when

extended and in contact with another fibril, or with the body of

another cell, it conducts its message as readily as a continuous

filament could do—precisely as in the case of an electric wire.

 

This conception, founded on a most tangible anatomical basis,

enables us to answer the question as to how ideas are isolated,

and also, as Dr. Cajal points out, throws new light on many other

mental processes. One can imagine, for example, by keeping in

mind the flexible nerve prolongations, how new trains of thought

may be engendered through novel associations of cells; how

facility of thought or of action in certain directions is

acquired through the habitual making of certain nerve-cell

connections; how certain bits of knowledge may escape our memory

and refuse to be found for a time because of a temporary

incapacity of the nerve cells to make the proper connections, and

so on indefinitely.

 

If one likens each nerve cell to a central telephone office, each

of its filamentous prolongations to a telephone wire, one can

imagine a striking analogy between the modus operandi of nervous

processes and of the telephone system. The utility of new

connections at the central office, the uselessness of the

mechanism when the connections cannot be made, the “wires in use”

that retard your message, perhaps even the crossing of wires,

bringing you a jangle of sounds far different from what you

desire—all these and a multiplicity of other things that will

suggest themselves to every user of the telephone may be imagined

as being almost ludicrously paralleled in the operations of the

nervous mechanism. And that parallel, startling as it may seem,

is not a mere futile imagining. It is sustained and rendered

plausible by a sound substratum of knowledge of the anatomical

conditions under which the central nervous mechanism exists, and

in default of which, as pathology demonstrates with no less

certitude, its functionings are futile to produce the normal

manifestations of higher intellection.

 

X. THE NEW SCIENCE OF ORIENTAL ARCHAEOLOGY

 

HOW THE “RIDDLE OF THE SPHINX” WAS READ

 

Conspicuously placed in the great hall of Egyptian antiquities in

the British Museum is a wonderful piece of sculpture known as the

Rosetta Stone. I doubt if any other piece in the entire exhibit

attracts so much attention from the casual visitor as this slab

of black basalt on its telescope-like pedestal. The hall itself,

despite its profusion of strangely sculptured treasures, is never

crowded, but before this stone you may almost always find some

one standing, gazing with more or less of discernment at the

strange characters that are graven neatly across its upturned,

glass-protected face. A glance at this graven surface suffices to

show that three sets of inscriptions are recorded there. The

upper one, occupying about one-fourth of the surface, is a

pictured scroll, made up of chains of those strange outlines of

serpents, hawks, lions, and so on, which are recognized, even by

the least initiated, as hieroglyphics. The middle inscription,

made up of lines, angles, and half-pictures, one might surmise to

be a sort of abbreviated or short-hand hieroglyphic. The third or

lower inscription is Greek—obviously