HEMIOPIA; 
Mechanism of its Causation on the Theory 
of TOTAL Decussation of the Optic 
Nerve Fibres in the Optic Tract 
at the Chiasma (Optic 
Commissure.) 
By Wm. Dickinson. M. D., 
ST. LOUIS. 
Reprint from 
The Alienist and Neurologist, 
St Loui s, Jan nary, 1881. 
ConppliipervtA of the Author, 
ISlo. 1131 Washington A.ve. Art. ViII.—Hemiopia.—Mechanism of its 
Causation on the Theory of TOTAL 
Decussation of the Optic Nerve Fibres 
in the Optic Tracts at the Chiasma, 
(Optic Commissure). 
By William Dickinson, M. D., St. Louis, 
TN the discussion of this subject, which, during th&;-4|Lst 
score of years especially, has engaged the taler it :3f a 
large number of investigators in this country and m 
Europe, I beg to premise that I assert no claim to orig- 
inal research, but have simply availed myself of the 
achievement of others; and, in as brief manner as is 
consistent with the importance which I attach to the 
subject, I shall present the results to which I have 
arrived, from a faithful perusal of the literature which 
they have furnished, and which has been accessible to me. 
For the better elucidation of my subject, I briefly 
refer to the anatomy of the optic apparatus. The term 
“ optic nerves ” I shall employ exclusively to designate 
that portion of this apparatus extending from the optic 
commissure anteriorly to the globe; while that of the 
“optic tract” embraces all that portion extending from 
the commissure, posteriorly, to its multiple sources of 
origin in different parts of the brain. Systematic works 
on “ Anatomy ” teach that each optic tract derives its 
origin from the Corpora Geniculata, from the posterior 
and inferior aspect of the Optic Thalamus, and from the 
optic thalamus itself, and from the Corpora Quadrigemina. 
From these points of origin it proceeds forward and joins 
with its fellow in front of the Tuber Cinereum, to form 
the optic commissure; that, within the commissure, the 
central fibres of each tract cross each other to pass to 2 
William Dickinson. 
the opposite eye ; while the outer fibres of each continue 
their course uninterruptedly to the eye of the correspond- 
ing side. This mode of crossing, of a portion of the optic 
nerve fibres, is termed partial or semi-decussation. 
According to this doctrine, the nasal half of each retina 
is supplied by the optic tract of the opposite side ; while 
the temporal half receives its contingent from the tract 
of the same side. Fibres continued across the anterior 
border of the chiasma are described, connecting the optic 
nerves of the two sides, having no relation with the optic 
tracts. In respect to these, Gudden, of acknowledged 
high authority, states: “they cannot be demonstrated.’’ 
Fibres corresponding to these are also described upon 
the inner margin of each optic tract, continued across from 
one to the other side of the brain, having no connection 
with the optic nerves. The same anatomist states : “these 
do exist, but they have no physiological relation to the 
nerves; on the contrary, they are wholly independent of 
them.” 
Thus divested of all collateral fibres, we have remain- 
ing, as the constituents of the optic commissure, fibres 
which cross each other, and fibres which do not cross 
each other ; both together subsequently in their course, 
forming the optic nerve of each eye. Now, is this 
description true, or is it not ? Does a part only of the 
fibres of each optic tract cross each other in the chiasma, 
or do all the fibres thus cross each other? If all the 
fibres cross each other, then the optic nerve proceeding 
to each eye is the continuation through the chiasma 
solely of the optic nerve fibres of the optic tract of the 
other side ; if they do not, then the former description is 
correct, viz : that each optic nerve is composed of both 
the crossed fibres and the lateral fibres. Our inquiry 
is, in other words : Is decussation total or is it partial ? 
I shall endeavor to recite arguments in proof that all 
the optic nerve fibres in the optic tract do decussate in 
the chiasma, and thus disprove the doctrine of semi-de- 
cussation taught in all our text-books. Total Decussation in Optic Chiasm. 
3 
We shall assume as granted the existence of the law 
of isolated conduction in reference to the fibres of the 
optic tracts and optic nerves, i. e., that the nerve fibrils 
are multiple, and constitute an essential element in the 
mechanism of vision ; that they have a real, continuous 
course from localities far behind those usually assigned, 
even from the cortex cerebri to their termination in the 
retina, or to be more exact, in the ganglion cell-layer of 
the retina. Morphologically, therefore, the optic nerve 
may be regarded as a part of the cortex cerebri, and the 
retina itself as an outpost of the brain, since a portion of 
its fibres attach themselves to a fasciculus of the corona 
radiata, which has its origin in the cortex. 
The chiasma or optic commissure consists of the 
aggregate plexiform interlacement of the optic nerve fibres 
placed side by side, destitute of a fibrous sheath, the 
whole being covered in by the pia-mater. It is situated 
beneath the corpus callosum, with which it is in connec- 
tion through the Lamina Cinerea, and through the medium 
of the former, it is in connection with the anterior, middle 
and part of the posterior lobes of the brain. 
The texture of the several parts of which it is com- 
posed is soft, succulent and, therefore, readily compressible. 
It rests upon the olivary process of the sphenoid bone, a 
hard, unyielding body presenting, conspicuously, an eleva- 
tion in reference to its surroundings. The chiasma thus 
situated, is to some degree ?iormally pressed upon by the 
corpus callosum and also by the parts which it in turn 
supports above. It is bounded in front by the Lamina 
Cinerea, which forms the anterior part of the inferior 
boundary of the third ventricle ; at either lateral angle, by 
the gray matter of the substantia perforata anterior, cor- 
responding to the under-surface of the corpus striatum; 
behind by the Tubera Cinerea, which forms a part of the 
floor of the third ventricle; here are also situated the 
Pituitary Bodies, the posterior lobes of which contain a 
cavity; which, through the medium of the Infundibulum 
communicates with the third ventricle; also, the Corpora William Dickinson. 
4 
Albicantia and the Substantia Perforata posterior, which 
forms the posterior part of the floor of the third ventricle, 
and corresponds to the under surface of the Optic 
Thalami. 
The anterior perforated space is perforated by numer- 
ous orifices for the transmission of small vessels to the 
Corpus Striatum. These orifices, through which they pass, 
are in size three times as large as the vessels themselves, 
and therefore afford but slight support to them. These 
vessels are derived from the anterior and middle cerebral 
arteries of the “Circle of Willis.” The posterior perforated 
space is perforated in like manner for the passage of 
blood vessels to the Optic Thalami; these are derived from 
the posterior cerebral arteries. 
The relatively large size of the Chiasma, its peculiar 
and exceptional position render it eminently obnoxious to 
variable degrees of pressure in consequence of changed 
conditions of the numerous and diverse cerebral factors 
by which it is surrounded. In addition, “ Michel has 
devoted much attention to the upper surface of the com- 
missure, and its relation to a layer of gray matter over it 
containing a recessus or cavity communicating with the 
third ventricle in the middle line and the lateral ventricle 
at the sides, in such a way that fluid injected into one 
lateral ventricle would distend this cavity, and so press 
on the commissure. The communication of this recessus 
with the third ventricle is of great importance patholo- 
gically, as in this way the front, lateral or posterior parts 
of the commissure may be pressed on.” 
I am the more desirous of giving special emphasis to 
the form and situation of this body and of its relations to 
the several parts immediately adjacent, since, on the theory 
of total decussation, upon its four angles and their vicinity 
must be impressed the proximate causes of all hemiopic 
symptoms. The region within which these causes are to be 
found, is, therefore, very limited; while on the theory of 
semi-decussation, the causes, being the same, may be 
sought at any point in the course of the optic tract, between Total Decussation in Optic Chiasm. 
5 
the Optic Chiasm and its origin in the cerebral ganglia, and 
in its more remote points of origin in the gray substance of 
the hemispheres. 
Pressure, either directly or in its results, inducing 
nutritional changes, as will be shown, at length, in the 
sequel is the occasion, probably, of all forms of hemiopia 
from intra-cranial causes, whatever may be the nature of 
the cause by which the pressure is exerted. If this be 
applied at the anterior angle of the Chiasma, temporal 
hemiopia will result; the central portions only of the 
entire ordinary visual field being illuminated; for the optic 
nerve fibres supplying the nasal halves of the two retinae are 
compressed, and their sensibility destroyed, or conducti- 
bility interfered with. 
If it be applied at the right lateral angle, the right 
halves of each retina will be incapacitated, and, conse- 
quently, vision in the right visual fields alone is possible; 
if, on the other hand, it be applied at the left lateral 
angle, vision in the left visual fields alone remains. Again, 
if the pressure be applied at the posterior angle, the optic 
nerve fibres destined to the outer halves of both retinae 
being compressed, their faculty power of conduction 
destroyed, and these portions of the retinae incapacitated, 
vision in the outer fields alone remain, constituting nasal 
hemiopia ; a form of hemiopia inexplicable on anatomical 
grounds on the theory of semi-decussation. 
Now, this is a simple and very intelligible presentation 
of the entire subject of the mechanism of causation, and 
an unequivocal guide to the solution of all the forms of 
hemiopia. 
The array of authority in support of semi-decussation, 
the theory now generally prevalent is truly formidable, 
viz.: Newton, Vater, Wollaston, Mueller, Graefe, Jackson, 
Hirschberg, Knapp, Wilson, Gudden, Reich, Schoen, Monk 
and others ; but names, however distinguished, avail but 
little, if their doctrines, subjected to the crucial test of 
later discoveries, are found to have been predicated upon 
the sandy foundation of mere hypothesis. But yet, to 6 
William Dickinson. 
illustrate the doctrine of total decussation, we are able to 
record a galaxy of worthy names, viz : Biesiadecki, Man- 
delstamm, Michel, Brown-Seqnard, Pawlozvsky, Cohn, Illing, 
Bastian and others, whose names, though of less repute 
in the republic of medical science, will ever receive the 
merited honor of bringing to naught the hypothesis and 
teachings of their predecessors; for their doctrines are 
founded upon the everlasting rock of anatomical demonstra- 
tion. 
I shall now offer reasons why the theory of semi- 
decussation cannot be sustained; and 
First. It is founded upon an hypothesis.—I have not 
been able to satisfy myself as to the opinions and teach- 
ings of the earlier anatomist respecting the subject of 
decussation, but upon consulting the writings of Hieronymus 
Fabricius, a distinguished anatomist of the seventeenth cen- 
tury, I find this statement, viz: “ The optic tracts approach 
each other and appear to decussate at the chiasma, but 
they do not thus decussate; the right optic tract after 
this approximation leaves its fellow, and becomes the 
optic nerve and proceeds to the right eye, and left optic 
tract in like manner to the left eye.” It was, therefore, 
his opinion that no decussation whatever took place, each 
eye being supplied exclusively by optic nerve fibres from 
its own side. This declaration, we know, is widely at 
variance with the observations of later anatomists. Vesalius, 
however, who flourished a century earlier, records an 
anomalous case, in which “ there was 110 junction of the 
optic nerves, and the sight was never double,” nor were 
other symptoms mentioned by him; and an instance of con- 
genital absence of the optic commissure is now preserved 
in the museum of Westminister Hospital, London. These 
are certainly very exceptional cases. 
Whatever may have been the accepted doctrine prior 
to his time, Sir Isaac Newton was the first, who, in the 
year 1704, from physiological reasons, as he conceived, 
advanced the possibility of a partial decussation of the 
optic nerve fibres to account for the phenomena of Total Decussation in Optic Chiasm. 
7 
Hemiopia. It was with him but an hypothesis, a mere con- 
jecture. He himself may not have been convinced of its 
reality, but on this assumption, certain observed phe- 
nomena obtained a more ready solution than on any 
other hitherto suggested. Perchance he only designed 
this as a mere hypothesis, after the manner of astrono- 
mers, who in their effort to account for perturbations 
observed in the revolutions of certain planets, assume the 
existence of an intermediate planet hitherto undiscovered, 
and then pursuing a course of reasoning and calculation 
upon this assumption, determine whether the hypothetic 
planet, if it actually did exist, is capable of producing the 
phenomena observed. It was thus “ Bruno conjectured 
the fundamental fact of the nebular origin of the heavenly 
spheres; Kant reasoned out its foundation idea, and 
LaPlace developed it.” 
Twenty years after the promulgation of this hypothesis, 
three cases of Hemiopia by Vater, in a dissertation, were 
explained on the theory of semi-decussation, thus lending 
it additional confirmation. A century later the same theory 
was adopted by Dr. Wollaston, of London, to account for 
hemiopic symptoms, of which he thrice was the subject. 
Second. The theory of semi-decussation is disproved 
by physiological experiment.—It has been ascertained 
that section of one optic tract in living animals gives rise, 
not to bilateral hemiopia, as would result if decussation 
was partial, but to amaurosis of one eye only, and that 
of the opposite side; also that section longitudinally 
through the chiasma in the median line, causes complete 
amaurosis; which is in harmony with the doctrine of 
total decussation, since all the fibres of both tracts 
would thus be divided : while on the theory of semi-de- 
cussation temporal hemiopia would result. Flourens ascer- 
tained that destruction of either Corpus Quadrigeminum 
was followed by loss of vision in the eye of the opposite 
side. If both these are destroyed, blindness, double and 
complete, is the result. In like manner it has been ascer- 
tained that destruction of one angular gyrus, where Ferrier William Dickinson. 
8 
located the origin of the optic nerve, produces temporary 
blindness in the opposite eye; and when the angular gyri 
of both hemispheres are destroyed, blindness of both eyes 
is produced and that permanent. 
Third. The theory of semi-decussation is disproved by 
clinical experience.—In cases of uniocular neuritis, from a 
cerebral tumor, recorded by Dr. H, Jackson, the neuritis 
was on the side opposite to that of the tumor. Wundach 
reports a case in which the left eye had, for a long time, 
been blind from detachment of the retina ; he found, on 
examination after death, atrophy of the left optic nerve, and 
also of the right optic tract. Beer relates a case of disease 
of the substance of the left hemisphere which had resulted 
in blindness of the right eye only, from intra-cranial disease 
of the left side. It was found to be occasioned from the 
effects of a vascular cyst containing two ounces of fluid in 
the substance of the posterior part of the left hemisphere. 
Magendie also records a case in which the right eye having 
been for a long time lost, the optic nerve was atrophied 
throughout its entire length to the optic commissure, and 
also the left optic tract to its visible source of origin, 
that condition usually found consequent upon loss of 
function. Irrefragible evidence of our position is furnished 
by the case of Professor De Morgan, a distinguished 
mathematician, examined by Dr. Bastian, and by him 
reported: “ The patient was blind in the right eye 
almost from birth; the corresponding optic nerve, upon 
section, was found to be atrophied, and, likewise, the 
left optic tract; while the optic tract of the same side 
presented a healthy appearance.” And, akin to this case, 
and rendering accumulative proof, is that of a dog exam- 
ined by Michel. This animal had been affected with congen- 
ital malformation of the eye of the right side ; the right 
optic nerve and the left optic tract were found, on section, 
to be atrophied. In all these cases, on the theory of 
semi-decussation, one optic tract alone having suffered 
lesion, there should have resulted bilateral hemiopia, since 
each tract supplies corresponding portions of both retinae. Total Decussation in Optic Chiasm. 
9 
Fourth. The theory of semi-decussation is disproved 
by the teachings of comparative anatomy.—It is admit- 
ted even by the advocates of partial decussation, that 
in birds, the bat, in the osseous fishes—the cod, 
halibut and sturgeon—in rabbits and dogs, the decus- 
sation is total. The same condition has been found 
in all vertebrates in which decussation has been demon- 
strated ; and we are authorized to assert, on the authority of 
Biesiadecki, that the same obtains in all vertebrates, 
including man, one scheme of conformation everywhere 
prevailing. And why should there be found an exception 
in man? Is it not possible? Some one may inquire. 
Yes, it is possible, and nature is competent to inaugurate 
this anomaly, but she is not fickle or capricious; nor does 
she delight in departures from certain well-defined models 
in creation, nozv adopting one scheme and now another. 
She works according to simple, general and inflexible 
laws, and, when uninfluenced by extraneous causes, 
produces uniformity of results. The autocracy of gen- 
eral law everywhere prevails, and wherever anomalies 
or monstrosities occur, they are due to some mechanical 
impediments or interference in the process of development, 
which were insuperable. 
Fifth. The theory of semi-decussation of the optic 
nerve fibres in man is disproved, finally, by the anatomy 
of the optic commissure itself.—This affords the coronal 
and ne plus ultra testimony, and must forever disarm 
argument, dispel doubt and silence conjecture. Hypoth- 
esis on this subject, has dominated the medical world 
for a century and a-half. But hypothesis and theories 
thereon constructed, that will not triumphantly endure 
the ordeal of demonstration must fall, and speculative 
physiology must ever yield to the revelations of ana- 
tomy. Biesiadecki appears! And he alone enjoys the 
honor of first grasping this fallacy and given to it a suc- 
cessful refutation, though so long maintained and embalmed 
in all works on descriptive anatomy. He was so for- 
tunate as to be able to pursue every single fibre of one 10 
William Dickmson. 
optic tract through the chiasma to the optic nerve of the 
opposite side, by the separation of the fibres of the chiasma, 
and thus succeeded in demonstrating total decussation. 
Michel and Mandelstamm, simultaneously and independently 
of each other, arrived at the same result. They never 
found the fibres of one tract bending ro7md to the nerve 
of its own side, but they all pass to the optic nerve of 
the opposite side. Michel states: “ the optic commissure 
in man is made up of the fibres of both nerves arranged in 
a kind of basket-work, whose meshes form more or less 
irregular squares.” He thus renders unequivocal corrobor- 
ation to the anatomical demonstration of Biesiadecki. It 
was venial in the dim twilight of medical science of 
Newton’s time, that this hypothesis should have been 
proposed and taught, awaiting the advent of better and 
truer things; but now, in the full blaze of its meridian 
splendor, hypothesis gracefully yields to demonstration; 
anatomy, confirming the substitution, imparts fitting 
correction to the printed page, and time, the great arbiter 
of all things, transmits the record of the present to the 
judgment, revision and perfection of the future. 
[Other aspects of this subject will occupy our attention 
in the next number.] 
The forms of Hemiopia, with few exceptions, may be 
embraced under one of the following, viz.: 1st, Bilateral— 
(a) right-sided, (b) left-sided; 2d, Temporal; 3d, Nasal. 
All hemiopic phenomena are readily explained on the 
theory of total decussation. It cannot be denied that most 
of them can be accounted for on the theory of partial 
decussation; but those must always be excepted which are 
due to nasal hemiopia. In respect to these, when occa- 
sioned by cerebral causes, V. Graefe, though favoring the 
hypothesis promulgated by Newton and re-affirmed by 
Muller, frankly confessed himself incapable of giving an 
anatomical explanation. Total Decussation in Optic Chiasm. 
11 
That which I have attempted to prove may be form- 
ulated as follows, viz: 
ist. Diseases involving the optic tract of one side will 
produce total blindness of the eye on the opposite side; 
and not bilateral hemiopia. 
2d. Diseases affecting either lateral angle of the 
chiasma, involving both the optic nerve fibres entering 
and also those leaving it, will produce bilateral hemiopia; 
and not nasal hemiopia in one eye. 
3d. Diseases affecting the posterior angle, in the 
median line, involving the optic nerve fibres of both tracts 
that enter the chiasma, will affect the outer half of each 
retina and produce nasal hemiopia; a form utterly inex- 
plicable on the theory of partial decussation, and 
4th. Disease in front of the commissure will affect 
the inner halves of each retina, and produce temporal 
hemiopia; the phenomena of which are alike explicable 
on either theory. 
Mandelstamm epitomizes the entire subject in the 
brief, terse, oracular declaration “ no facts yet contradict 
the doctrine of total decussation; but anatomical in- 
vestigations, physiological experiments and clinical ex- 
perience harmonize to confirm it.”