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journal of
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THE CONVOLUTIONS OF THE CEREBRUM OF THE
HORSE. By 0. CHARNOCK BRADLEY, Lecturer on Anatomy,
New Veterinary College, Edinburgh.
ANYONE who has had occasion to consult works on veterinary
anatomy must have been struck with the discrepancies in the
various descriptions of the fissures of the horse's brain. These
discrepancies cannot be attributed to any scarcity of literature
on the subject; for, though somewhat scattered, references are
fairly numerous.
Serres (1), in 1824, gave a drawing of the horse's cerebrum,
but unfortunately the attainment of artistic curves seems to
have been more sought after than a true representation of
the fissures as they actually exist. We do not, therefore,
much help from this source.
Leuret (2), in his classic work, places the horse in tA
of the fourteen groups into which he divides mammals accsi
to the degree of development of the cerebral convolutions.
atlas contains three figures of the horse's brain, which, on account
of their accuracy, must have been of inestimable value to subsequent workers. One fissure alone is named, this being the
fissure of Sylvius. The four chief arcuate convolutions are
named numerically, and supra-orbital convolutions are indicated.
P. Broca (3) names the fissures and convolutions, and illustrates his statement with two figures.
Krueg's (4) well-known investigations on the fissures of the
brain of the Ungulata include observations on six horses and
two donkeys. The fissures of the horse's brain are, consoVOL. XXXIII. (N.S. VOL. XIII.)
P
216
A2R 0. CHARN'OCK
BRADLEY.
quently, very fully discussed, three figures being employed by
way of illustration.
Dareste (5) and Owen (6) have both added considerably to the
literature on this subject.
M'Fadyean (7), in his Dissector's Guide to the Anatomy of
the Horse, in describingr the brain, employs a nomenclature
almost entirely his own.
Sir Win. Turner (8), in his address delivered before the
Anatomical Section of the Tenth Iiitertnational Medical Congress, briefly describes the horse's fissures and convolutions, and
rives two figures in illustration.
Chauveau and Arloing (9), in their exhaustive treatise on the
Anatomy of the Domestic Animals, have sought to establish a
parallel between the convolutions and fissures of the horse and
of man. They therefore employ names borrowed from human
anatomy.
Ellenberger (10) is, so far as I am aware, the latest writer who
has added to the literature of the subject. He shows a strong
leaning towards the nomenclature employed by Krueg, though
he does not slavishly follow his predecessor.
The above cannot be considered an exhaustive list, but is
sufficient to show that many writers in different languages have
added their quota to our knowledge of the subject.
For some time past I have sought to put the descriptions of
the various writers to the test by comparing them with the
actual brain of the horse. The more one makes such comparison, the more one sees room for addition, and, to a less degree,
for correction. I concluded, therefore, that some slight service
might possibly be rendered to comparative anatomy by giving
the results of my observations so far as they have gone. A considerable number of adult hemispheres have been examined, as
well as hemispheres from fcetuses of various ages. It is hoped
that when more embryological material has been examined,
something may be said concerning the time and sequence of
appearance of the various fissures. So far, however, the amount
of material at my disposal has not been sufficient to allow of
any definite and complete statements being made.
Seeing that the convolutions of the horse's cerebrum are arranged in four tiers, as in the dog, it seems not irrational to employ
TIlE CONVOLUTIONS OF THE CEREBRUM OF THE HORSE. 217'
the nomenclature suggested by Sir Wm. Turner (11) in his description of the brains of some Carnivora, and now very generally
adopted. The brain of the horse being more complicated in its
fissures than the same organ of the dog, a few additional
names will be necessary.
The rhinencephalon of the horse is marked off from the pallium by the customary fissures.
The rhiral and post-}rhinalfissures differ very materially in point
of depth: the former is shallow, whereas the latter is relatively
very deep. The post-rhinal fissure, when traced backwards, is
found to terminate on the tentorial surface of the hemisphere,
but does not join the calloso-marginal fissure.
The dentate or hippocampal fissure, as usual, forms the inner
boundary of the hippocampal lobe. The fissure is of no great
depth, and, moreover, becomes much shallower at its upper
extremity.
The surface of the hippocampal lobe is marked by one, sometimes two longitudinal fissures running parallel to the postrhinal fissure.
I. CRANIAL SURFACE OF THE PALLIUM.
The four-tier arrangement of the convolutions being most
clearly marked in the posterior part of the hemisphere, it will be
found most convenient for descriptive purposes to divide the
cranial surface of the pallium into anterior and posterior portions by a line joining the crucial fissure with the anterior limb
of the fissure of Sylvius. This is practically the line used by
Leuret for the purpose of dividing the convolutions into anterior
and posterior. Iln this division the convolutions below the
anterior limb of the fissure of Sylvius are counted as being
anterior.
(a) Posterior Part of the Cranial Sniface.
In describing the Sylvian fissure, it is perhaps best to consider
it as consisting of two limbs, thus following the example of
Krueg. The vertical limb is short and transverse. The anterior
limb is much more conspicuous and of much greater length.
It runs forwards and slightly upwards, and from its direction
218
MR 0. CHARNOCK BRADLEY.
appears to be the forward continuation of the post-rhinal
fissure. In some specimens this limb appears to be much
longer than usual, because of its joining a fissure described and
figured by Ellenberger under the name of diqyonal.
The fissure I would describe under the name of supra-sylvian
has received little attention at the hands of recent investigators,
though both Leuret and Broca figured it. Leuret, in particular,
FIG. 1.-Lateral view of left hemisphere of adult horse.
attached importance to it, inasmuch as he indicated it as
separating convolutions I. and II. Its early appearance in the
fetal brain adds to its claim to be considered as a fissure of
some moment (fig. 2, ss). This, in conjunction with its coinparative depth in the adult, will justify the attention here paid
to it.
In the fcetus this fissure consists, during the earlier stages of
its development, of two segments; a condition which may be
preserved for some time after birth, or even into adult life. In
the adult the segmented character is usually not apparent on
superficial examination, but is revealed by separating the
adjacent convolutions.
The lateral fissure is one of the deepest on the cranial surface
of the hemisphere. Its anterior extremity is on a level with
THE CONVOLUTIONS OF THE CEREBRUM OF THE HORSE. 219
a short and inconstant fissure, which the majority of authorities
seem to regard as corresponding to the crucial fissure of the Carnivora. From this point the lateral fissure extends backwards
and outwards, and finally terminates near the boundary of the
tentorial surface. Anteriorly it is continuous with the coronal
fissure, and partially on this account I have ventured to use the
A.
B.
i
C.
FiG. 2.-A, lateral view of right hemisphere of 129 days fetal mare. B, superior
view of the same. C, right hemisphere of 5 months fetal horse.
name of lateral to designate it, in spite of the fact that Krueg,
Owen, and Ellenberger have called it supra-sylvian. Chauveau
looks upon it as corresponding to the Rolandic fissure of Man.
The medio-lateral fissure runs very nearly parallel to the great
longitudinal fissure which separates the two cerebral hemispheres. It commences at a point slightly posterior to the socalled crucial fissure, and proceeding backwards with a slight
-220
MR
0. CHARNOCK BRADLEY.
inclination outwards, it may reach the tentorial surface, though
this is not usual. Its depth is considerable, sometimes equalling,
and indeed in some specimens exceeding, that of the lateral
fissure. Krueg and Ellenberger call this fissure lateral, while
Owen used the name lambdoidal.
The Sylvian and supra-sylvian convolutions are simple in their
arrangement, and seldom complicated by the presence of subsidiary fissures. Not so the medio-lateral and marginal convolutions.
The medio-lateral convolation is almost invariably complicated
by an antero-posterior fissure which runs parallel to the mediolateral, being in some instances as long as this latter fissure.
This minor fissure has been figured by Leuret, Krueg, Owen,
and Ellenberger. No name has been assigned to it by Leuret or
Krueg, but Owen has called it medi-lateral, and Ellenberger
ecto-lateral. In order to bring it in line with the names
suggested above, it might be called the ecto-medio-lateral fissure.
The two secondary convolutions into which the medio-lateral
gyrus is thus divided are usually, in their turn, indented by
inconstant furrows, whose general direction is longitudinal.
The marginal convolution follows the lead of the medio-lateral
in being divided into two parallel parts by a longitudinal fissure,
which might be named endo-medio-lateral. This fissure commences behind the crucial fissure, and is continued on to the
tentorial surface; the whole of its length is marked by extreme
simplicity.
(b) Anterior Part of the Cranial Surlface.
The anterior part of the. cranial surface consists of an orbital
surface and an area surmounting this.
The orbital surface contains olfactory and intra-orbital fissures,
with straight, internal supra-orbital and external supra-orbital
convolutions.
The olfactory fissure is always shallow, and not very constant
in its constitution, being sometimes double.
The intra-orbital fissure exhibits some variations in the mode
of termination of its posterior extremity. Three variations have
been met with. First, it may join the rhinal fissure; secondly,
THE CONVOLUTIONS OF TIHE CEREBRUM OF THE HORSE.
221
it may join the pre-sylvian; and thirdly, it may join neither.
It is difficult to say which is most common.
In somne instances the intra-orbital fissure appears to be continuous with the rostral fissure of the iliesial surface; in these
cases, however, a hidden convolution will always be found.
The gyrus rectus and internal supra-orbital convolution are
FIG. 3.-Upper surface of left hemisphere of horse's brain.
always continuous with each other around the anterior extremity
of the olfactory fissure.
The prw-sylvian fissure forms a connecting link between the
rhinal fissure below and the coronal fissure above. It is quite
impossible to say where the proe-sylvian ends and the coronal
begins.
22999
MR 0. CHARNOCK BRADLEY.
The coronalfissure joins the anterior end of the lateral fissure.
The pre-sylvian, coronal, and lateral fissures are always continuous, and always of great depth, so that they forin a long
curved indentation, which at once strikes the eye of an observer.
The diagonal fissure runs parallel to and at no great distance
from the imaginary line which has been used to divide the
cranial surface of the hemisphere into anterior arid posterior
areas.
What exactly in the horse corresponds to the insula of man
seems to be somewhat doubtful. So far as I am aware, Clark
S.
tot
FIG. 4.-Orbital surface of left hemisphere of horse's brain.
(12) has made the latest statement. He has examined five
equine brains, and has concluded that the insula consists of a
variable number of ' gyres' included between the rhinal fissure
and the posterior part of the interior limb of the fissure of
Sylvius. This area is indicated in fig. 1 by deep shading.
Usually (as shown in. the figure) two small eminences can be
seen in this position without opening up the fissures. Clark
quotes Lussana and Lenmoigne (13) as stating that the insula of
the horse is of considerable size. Further observation appears
to be necessary before arriving at any definite conclusion.
II. MESIAL SURFACE OF THE PALLIUM.
The two chief convolutions-marginal and callosal-on this
surface are separated by a deep and continuous calloso-marginal
THE CONVOLUTIONS OF THE CEREBRUM OF TILE HORSE. 223
fissure, which begins anteriorly on a level with and below the
anterior extremity of the corpus callosum, and ends on the
tentorial surface. The depth of this fissure is not uniform, the
anterior portion (corresponding to the genial fissure of the dog)
being much deeper than the posterior portion (splenial fissure
of the dcog).
The marginal convolution is indented by a so-called crucial
Assure, which may appear to joiln both calloso-marginal and
lateral fissures; on careful examination, however, hidden convolutions will always be found. The hidden convolution on the
cranial surface may be considered as representing the sigmoid
gyrus of Carnivora.
A rost ral fissure extends fromt the margin of the anterior and
N. ~
~
~ ~ m
FIG. 5.-Mesial and tentorial surfaces of right hemisphere of horse.
inferior part of the mesial surface to a level with the genu of
the corpus callosum.
The callosal convolution is never perfectly simple, but always
possesses a longitudinal fissure, which may be continuous, but is
more commonly composed of two or more segments. The
anterior part of this fissure is the deepest and most constant,
and has been called the genial fissure by Krueg, whose example
has been followed by Ellenberger. If it be admitted, however,
224
MR 0. CHARNOCK BRADLEY.
that the calloso-marginal fissure corresponds to splenial and
genual fissures, obviously the view of Krueg is incorrect.
Although the fissure in the callosal convolution has been
figured by Leuret, Broca, Turner, and others, a name has not
been bestowed upon it; by them. Perhaps the term iftracallosal might not be entirely inappropriate.
A distinct suprea-callosal conwVolution may always be found in
the callosal sulcus. It bends round the genii of the corpus callosum in front and the spleniumr behind, being continuous with
the fascia dentata. Its presence seems to have been overlooked,
probably on account of its variable size, though in some instances
it is sufficiently conspicuous.
The callosal sulcus is continuous with the hippocampal fissure.
This becomes obvious if the hippocampal fissure be. traced
upwards; its depth has decreased considerably on reaching the
splenium, and the furrow connecting the hippocampal and callosal
fissures may in some cases be little more than a suggestion. This
probably explains why previous writers have overlooked the
connection.
III. TENTORIAL SURFACE OF THE PALLIUM.
On this surface the posterior extremities of the post-rhinal
and callosal-marginal fissures form the most conspicuous depressions. These fissures are never continuous in the horse, a
definite retro-limbic gyrus intervening. An oblique fissure,
which might be called retro-limbic, always indents this bridging
convolution.
RELATIVE DEPTH OF THE FISSURES.
In estimating the relative depth of the various fissures, it was
found that, in order to obtain reliable results, transverse sections
were necessary. In the following list the figures are to be taken
as of relative value only, as in all the specimens examined hardeniing had previously been resorted to.
As was anticipated, the depth was found to vary relatively in
different brains, and even in the two hemispheres of the same
brain. This made the task somewhat difficult, and no doubt
more extended observation would to some extent chance the
following list:-
TILE CONVOLUTIONS OF
THE CEREBRUM OF THlE HORSE.
225
Depth.
.
.
Coronal fissure, .
.
.
.
Lateral fissure,
Calloso-marainal fissure,
.
.
Post-rhinial fissure,
Sylvian (anterior limb) fissure,
.
Medio-lateral fissure, .
.
.
Prae-sylvian fissure,
.
Supra-sylvian fissure, .
.
.
Diagonal fissure, .
Ecto-nledio-lateral fissure, .
Ento-medio-lateral fissure, .
.
Intra-orbital fissure, .
.
.
.
Rostral fissure,
.
.
.
Rhinal fissure,
.
Intra-callosal fissure, .
.
.
Olfactory fissure-, .
.
.
*
.
16 mm.
12
)
10
.
.
.
)
9
9
.
8
.
.
.
.
.
.
.
7
6
5
)
j
3
2
The figures speak for themselves, and therefore no comment is
necessary.
It is interesting to notice that the crucial fissure was so variable in its depth that it was thought advisable to exclude it
from the above list. In one instance its depth was only 5 mm.,
whereas in another it reached 13 mm.
The subjoined list gives the chief synonyms employed by
various writers to denote the different fissures, and will probably be found of use in comparing their descriptions.
226
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227
REFERENCES.
(1) SERRES, Anatomie comparee du cerveaux dans les quatre classes
des animaux vertebres, with Atlas, 1824-27, pl. xv. figs. 274 and
275.
(2) LEURET, Anatonie compare'e du Systeme Nerveux, 1839-57.
(3) P. BROCA, "Anatomie comparee des circonvolutions cerebrales," Revue d'Anthropologie, 1878, figs. 14 and 22.
(4) KRUEG, "Ueber die Furchung der Grosshirnrinde der
Ungulaten," Zeitschr. .f. Wissensch. Zoologie, xxxi., 1878.
(5) DARESTE, "Troisieme me'moire sur les circonvolutions du
cerveau chez les mammifdres," Annales d. sci. naturelles, 4me serie,
Zoologie, t. iii.
(6) OWEN, Anatomy of the Vertebrates, vol. iii., 1868.
(7) M'FADYEAN, Anatomy of the Horse, 1884.
(8) TURNER, "The Convolutions of the Brain," Jour. of Anat.
and Pliys., vol. xxv., 1890.
(9) CHAUVEAU and ARLOING, Tr-aite' d'Anatomie comnparee des
Animaux domestiques, 4me edition, 1890.
(10) ELLENBERGER and BAUM, Topogrcphische Anatomie des
Pferdes, 1894.
(1 1) TURNER, Jour. of Anat. and Phys., vol. xxii. p. 580.
(12) CLARK, " Comparative Anatomy of the Insula," Journal- of
Comparative Neurology, vi., 1896, p. 59, figs. 24 and 25.
(13) LUSSANA and LEMOIGNE, Fisiologia dei centri nervosi Encefalici, 1871.
EXPLANATION OF REFERENCE LETTERS IN FIGURES.
Fissures:-s', vertical limb of Sylvian fissure; s", anterior limb of
same; ss, supra-sylvian; 1, lateral; em-i, ecto-medio-lateral; em-i,
ento-medio-lateral; d, diagonal; co, coronal; p s, prae-sylvian; cr,
crucial; o, olfactory; io, intra-orbital; r, rhinal; pr, post-rhinal;
c m, calloso-marginal; ca, callosal; ro, rostral; rl, retrolimbic; h,
hippocampal; g, intra-callosal.
Convolutions:-S, sylvian; SS, supra-sylvian; M-L, medio-lateral;
M, marginal; C, callosal; SC, supra-callosal; R, gyrus rectus; SO,
internal supra-orbital; SO', external supra-orbital; RL, retrolimbic;
H, hippocampal lobe.

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