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

the posterior positions, sensory functions.

Somewhat similar conclusions were reached also by Dr.

Hughlings-Jackson, in England, from his studies of epilepsy. But

no positive evidence was forthcoming until 1861, when Dr. Paul

Broca brought before the Academy of Medicine in Paris a case of

brain lesion which he regarded as having most important bearings

on the question of cerebral localization.

 

The case was that of a patient at the Bicetre, who for twenty

years had been deprived of the power of speech, seemingly through

loss of memory of words. In 1861 this patient died, and an

autopsy revealed that a certain convolution of the left frontal

lobe of his cerebrum had been totally destroyed by disease, the

remainder of his brain being intact. Broca felt that this

observation pointed strongly to a localization of the memory of

words in a definite area of the brain. Moreover, it transpired

that the case was not without precedent. As long ago as 1825 Dr.

Boillard had been led, through pathological studies, to locate

definitely a centre for the articulation of words in the frontal

lobe, and here and there other observers had made tentatives in

the same direction. Boillard had even followed the matter up with

pertinacity, but the world was not ready to listen to him. Now,

however, in the half-decade that followed Broca’s announcements,

interest rose to fever-beat, and through the efforts of Broca,

Boillard, and numerous others it was proved that a veritable

centre having a strange domination over the memory of articulate

words has its seat in the third convolution of the frontal lobe

of the cerebrum, usually in the left hemisphere. That part of the

brain has since been known to the English-speaking world as the

convolution of Broca, a name which, strangely enough, the

discoverer’s compatriots have been slow to accept.

 

This discovery very naturally reopened the entire subject of

brain localization. It was but a short step to the inference

that there must be other definite centres worth the seeking, and

various observers set about searching for them. In 1867 a clew

was gained by Eckhard, who, repeating a forgotten experiment by

Haller and Zinn of the previous century, removed portions of the

brain cortex of animals, with the result of producing

convulsions. But the really vital departure was made in 1870 by

the German investigators Fritsch and Hitzig, who, by stimulating

definite areas of the cortex of animals with a galvanic current,

produced contraction of definite sets of muscles of the opposite

side of the body. These most important experiments, received at

first with incredulity, were repeated and extended in 1873 by Dr.

David Ferrier, of London, and soon afterwards by a small army of

independent workers everywhere, prominent among whom were Franck

and Pitres in France, Munck and Goltz in Germany, and Horsley and

Schafer in England. The detailed results, naturally enough, were

not at first all in harmony. Some observers, as Goltz, even

denied the validity of the conclusions in toto. But a consensus

of opinion, based on multitudes of experiments, soon placed the

broad general facts for which Fritsch and Hitzig contended beyond

controversy. It was found, indeed, that the cerebral centres of

motor activities have not quite the finality at first ascribed to

them by some observers, since it may often happen that after the

destruction of a centre, with attending loss of function, there

may be a gradual restoration of the lost function, proving that

other centres have acquired the capacity to take the place of the

one destroyed. There are limits to this capacity for

substitution, however, and with this qualification the

definiteness of the localization of motor functions in the

cerebral cortex has become an accepted part of brain physiology.

 

Nor is such localization confined to motor centres. Later

experiments, particularly of Ferrier and of Munck, proved that

the centres of vision are equally restricted in their location,

this time in the posterior lobes of the brain, and that hearing

has likewise its local habitation. Indeed, there is every reason

to believe that each form of primary sensation is based on

impressions which mainly come to a definitely localized goal in

the brain. But all this, be it understood, has no reference to

the higher forms of intellection. All experiment has proved

futile to localize these functions, except indeed to the extent

of corroborating the familiar fact of their dependence upon the

brain, and, somewhat problematically, upon the anterior lobes of

the cerebrum in particular. But this is precisely what should be

expected, for the clearer insight into the nature of mental

processes makes it plain that in the main these alleged

“faculties” are not in themselves localized. Thus, for example,

the “faculty” of language is associated irrevocably with centres

of vision, of hearing, and of muscular activity, to go no

further, and only becomes possible through the association of

these widely separated centres. The destruction of Broca’s

centre, as was early discovered, does not altogether deprive a

patient of his knowledge of language. He may be totally unable to

speak (though as to this there are all degrees of variation), and

yet may comprehend what is said to him, and be able to read,

think, and even write correctly. Thus it appears that Broca’s

centre is peculiarly bound up with the capacity for articulate

speech, but is far enough from being the seat of the faculty of

language in its entirety.

 

In a similar way, most of the supposed isolated “faculties” of

higher intellection appear, upon clearer analysis, as complex

aggregations of primary sensations, and hence necessarily

dependent upon numerous and scattered centres. Some “faculties,”

as memory and volition, may be said in a sense to be primordial

endowments of every nerve cell—even of every body cell. Indeed,

an ultimate analysis relegates all intellection, in its

primordial adumbrations, to every particle of living matter. But

such refinements of analysis, after all, cannot hide the fact

that certain forms of higher intellection involve a pretty

definite collocation and elaboration of special sensations. Such

specialization, indeed, seems a necessary accompaniment of mental

evolution. That every such specialized function has its

localized centres of co-ordination, of some such significance as

the demonstrated centres of articulate speech, can hardly be in

doubt—though this, be it understood, is an induction, not as yet

a demonstration. In other words, there is every reason to

believe that numerous “centres,” in this restricted sense, exist

in the brain that have as yet eluded the investigator. Indeed,

the current conception regards the entire cerebral cortex as

chiefly composed of centres of ultimate co-ordination of

impressions, which in their cruder form are received by more

primitive nervous tissues—the basal ganglia, the cerebellum and

medulla, and the spinal cord.

 

This, of course, is equivalent to postulating the cerebral cortex

as the exclusive seat of higher intellection. This proposition,

however, to which a safe induction seems to lead, is far afield

from the substantiation of the old conception of brain

localization, which was based on faulty psychology and equally

faulty inductions from few premises. The details of Gall’s

system, as propounded by generations of his mostly unworthy

followers, lie quite beyond the pale of scientific discussion.

Yet, as I have said, a germ of truth was there—the idea of

specialization of cerebral functions—and modern investigators

have rescued that central conception from the phrenological

rubbish heap in which its discoverer unfortunately left it

buried.

 

THE MINUTE STRUCTURE OF THE BRAIN

 

The common ground of all these various lines of investigations of

pathologist, anatomist, physiologist, physicist, and psychologist

is, clearly, the central nervous system—the spinal cord and the

brain. The importance of these structures as the foci of nervous

and mental activities has been recognized more and more with each

new accretion of knowledge, and the efforts to fathom the secrets

of their intimate structure has been unceasing. For the earlier

students, only the crude methods of gross dissections and

microscopical inspection were available. These could reveal

something, but of course the inner secrets were for the keener

insight of the microscopist alone. And even for him the task of

investigation was far from facile, for the central nervous

tissues are the most delicate and fragile, and on many accounts

the most difficult of manipulation of any in the body.

 

Special methods, therefore, were needed for this essay, and brain

histology has progressed by fitful impulses, each forward jet

marking the introduction of some ingenious improvement of

mechanical technique, which placed a new weapon in the hands of

the investigators.

 

The very beginning was made in 1824 by Rolando, who first thought

of cutting chemically hardened pieces of brain tissues into thin

sections for microscopical examination—the basal structure upon

which almost all the later advances have been conducted. Muller

presently discovered that bichromate of potassium in solution

makes the best of fluids for the preliminary preservation and

hardening of the tissues. Stilling, in 1842, perfected the

method by introducing the custom of cutting a series of

consecutive sections of the same tissue, in order to trace nerve

tracts and establish spacial relations. Then from time to time

mechanical ingenuity added fresh details of improvement. It was

found that pieces of hardened tissue of extreme delicacy can be

made better subject to manipulation by being impregnated with

collodion or celloidine and embedded in paraffine. Latterly it

has become usual to cut sections also from fresh tissues,

unchanged by chemicals, by freezing them suddenly with vaporized

ether or, better, carbonic acid. By these methods, and with the

aid of perfected microtomes, the worker of recent periods avails

himself of sections of brain tissues of a tenuousness which the

early investigators could not approach.

 

But more important even than the cutting of thin sections is the

process of making the different parts of the section visible, one

tissue differentiated from another. The thin section, as the

early workers examined it, was practically colorless, and even

the crudest details of its structure were made out with extreme

difficulty. Remak did, indeed, manage to discover that the brain

tissue is cellular, as early as 1833, and Ehrenberg in the same

year saw that it is also fibrillar, but beyond this no great

advance was made until 1858, when a sudden impulse was received

from a new process introduced by Gerlach. The process itself was

most simple, consisting essentially of nothing more than the

treatment of a microscopical section with a solution of carmine.

But the result was wonderful, for when such a section was placed

under the lens it no longer appeared homogeneous. Sprinkled

through its substance were seen irregular bodies that had taken

on a beautiful color, while the matrix in which they were

embedded remained unstained. In a word, the central nerve cell

had sprung suddenly into clear view.

 

A most interesting body it proved, this nerve cell, or ganglion

cell, as it came to be called. It was seen to be exceedingly

minute in size, requiring high powers of the microscope to make

it visible. It exists in almost infinite numbers, not, however,

scattered at random through the brain and spinal cord. On the

contrary, it is confined to those portions of the central nervous

masses which to the naked eye appear gray in color, being

altogether wanting in the white substance which makes up the

chief mass of the brain. Even in the gray matter, though

sometimes thickly distributed, the ganglion cells are never in

actual contact one with another; they always lie embedded in

intercellular tissues, which came to be known, following Virchow,

as the neuroglia.

 

Each ganglion cell was seen to be irregular in contour, and to

have jutting out from it two sets of minute fibres, one set

relatively short, indefinitely numerous, and branching in every

direction; the other set limited in number, sometimes even

single, and starting out directly from the cell as if bent on a

longer journey. The numerous filaments came to be known