3ournat of Anatomn aub Xpoitlogp.

24 Pages·2005·2.69 MB·English

3ournat of Anatomn aub Xpoitlogp.
THE RETE MIRABILE OF THE NARWHAL. By HENRY
SEASONWILSON, M.D.,Superintendent ofPracticalAnatomy,
London Hospital. (PLATES XXII., XXIII.)
THE specimen on which the following investigations were made
was a fetal Narwhal, 641 inches long, and 481 inches in its
greatest circumference. Its mother was captured in 1861 on
the north side of Pond's Bay, Davis Strait, by Captain George
Deuchars of the screw-steamer "Narwhal" of Dundee. The
foetus was removed at the time of capture by Dr Robert Brown,
who kindly permitted me to dissect the soft parts. Owing,
however, to the decomposed state of the animal, I could only
investigate the vascular system, and even that but imperfectly.
My remarks will apply only to the thoracic rete mirabile, and
are selected from the notes and drawings taken during my dis-
sections when Demonstrator of Anatomy in the University of
Edinburgh in 1862.
It seems unfortunate that the term "rete mirabile" should
have been applied so indiscriminately to all vascular plexiform
arrangements, for, both anatomically and teleologically, when
the various plexuses are compared with each other, a good deal
of confusion and a greatwant of harmonyareatonce observable.
I shall not enter here into that question. It will be best under-
stood and more easily handled when we come to examine into
the probable function of retia in general, and especially of the
thoracic plexus in Cetacea. Suffice it to say that of all the
retia mirabilia throughout the vertebrate, that in the thorax of
the Cetacea resembles no other so-named plexus. John Hunter
is the only author who has likened it to any other network, and
VOL. XIV; D
378 DR HENRY SEASON WILSON.
even that similitude is but trivial, for the spermatic arteries of
the Bull resemble it only in the tortuosities of their branches.
I will first state the facts evinced by dissection, deferring deduc-
tions and other observations for subsequent attention.
As the thoracic rete mirabile in the Narwhal is arterial, and
its position is bythe side of the thoracic aorta, in order to under-
stand the sources whence the vascular supply is derived, a brief
enumeration of the modifications exhibited by the primary
branches of the aortic arch and the thoracic aorta are necessary.
Aortic arch.-The arch lay at first entirely concealed by the
pulmonary artery. Beyond this only a narrow portion could be
seen, owing to the close manner in which it was embraced
by the lobes of the thymus (Plate XXII. A). The right lobe
separated it from the vena cava anterior, whilst both lobes were
interposed between the arch and the pneumo-gastric nerves,
more especially on the left. By dissecting away the thymus,
the aorta was seen to be continued forwards and to the right for
a short distance, and thence to arch obliquely upwards towards
the left of the spine. Close to its exit from the left ventricle
the aorta supplied its coronary branches, each having its usual
origin and distribution. From the convexity of the arch arose
the innonlinate, the left common carotid, and the left posterior
thoracic arteries.
Theinnominate (Plate XXII.A,1) hadits originfrom thecom-
mencement of the transverse portion of the arch, on a level with
the head of the first rib, and greatly exceeded the others in size.
It extended forwards towards the neck for about half an inch,
and then divided into two large branches-the right posterior
thoracic,andtherightcommoncarotid (PlateXXII.A,3, 2, 11,8).
The right posterior thoracic in its origin and in the relations
ofits first part resembled the right subclavian, but differed from
it in its ultimate distribution. From its originitarchedhorizon-
tally forwards and outwards towards the first rib. Reaching
the latter at the distance of about three inches from the spine,
it suddenly curved backwards, and continued along the inner
aspect of the thoracic wall. In this course it crossed the first
eight ribs and ended in the eighthintercostal space. Itwas thus
divisible into two portions-the arch, situated in the neck, and
the terminal portion related to the ribs, and intrathoracic in
.379
THE RETE MIRABILE OF THE NARWHAL.
position (PlateXXII.A). Forthe sakeof descriptionthese por-
tions may be called the arch and the costal portion. The arch
was much the shorter, and was cervical in position. It had the
usual relation to the innominate vein, the pneumogastric, the
branches of the sympathetic, and the phrenic nerves, as well as
to the scalenus muscle. At first it was covered by the cervical
part of the right thymic lobe. Only one branch, cervical (Plate
XXII. A, 4) passed from this arch. It arose from the parent
trunk, where that was crossed by the right innominate vein.
This branch corresponded in its distribution with the"cervicalis
ascendens" in man. In addition to its muscular branches it
supplied offsets in great numbers to the cervical portion of the
rete mirabile, and also formed large anastomoses with the
anterior (superior), thyroid, and pharyngeal branches of the
external carotid (Plate XXII. A). On the left side this branch
came from the left internal mammary. The costal portion of
the posterior thoracic ended, as above stated, in the eighth inter-
costal space. In its course it lay, to a certain extent, embedded
in the substance of the thoracic portion of the rete, running
along the external margin of the rete, and keeping parallel to
the spine. Immediately before its termination, however, it
diverged fromthemesial line. Itsbrancheswereverynumerous,
and were chieflydirected inwards towards the spine,or outwards
from it. Towards the vertebral column it gave hundreds of
long, delicate offsets,nearly equal in their calibre. These passed
entirely to the rete, and a fuller account of their distribution
will be more appropriate when considering that network. The
branches passing outwards constituted groups which, with the
exceptionofthefirst,occupiedtheintercostalspaces fromthethird
to the eighth (Plate XXII. A, 16, 7). The first group was given
off immediately before the artery entered the thorax, and was
muchlarger than the others. It consisted of numerousbranches,
directed forwards to the cervical muscles, and backwards to the
first and second intercostal spaces. The other groups resembled
intercostal vessels, and need not occupy time in minute descrip-
tion. As the right common carotid artery has very little to do
with the rete, supplying comparatively very few branches, and
those simply to its cervical portion, I shall pass it over.
The left common carotid (Plate XXII. A, 2') sprung from the
380
DR HENRY SEASON WILSON.
aorticarch,ata distanceofaboutthreequartersofaninchfromthe
innominate trunk. In size it was somewhat more than half the
calibre of the latter. Its course was forwards, and slightly out-
wards. Within the thorax it was related externally to the
narrow anterior prolongation of the left thymic lobe, by which
it was separated from the left pueumogastric, and internally it
was in contact with the thyroid body. Its cervical portion need
not be described.
The left posterior thoracic (Plate XXII. A, 3'), in its place of
origrin resembled the left subclavian in man; that is, it arose
from the left aspect of the last portion of the aortic arch,directly
opposite, but immediately preceding the ductus arteriosus. It
was an inch and a half beyond the left common carotid, and
about half its size. Its course was forwards to a level with the
first rib and then backwards to end in the ninth intercostal
space. The arch was intrathoracic and very short. It had the
same relations to the sympathetic and pneumogastric as the
corresponding subclavian in man. Its only branch was given
off as it neared the first rib. This branch soon broke up into a
number of offsets, and represented the first group of the costal
portion of the right posterior thoracic (Plate XXII. A, 6'). The
costal part of the left posterior thoracic extended one intercostal
space beyond that of the same branch on the left side. It
supplied the same innumerable branches inwards to the rete,
and similar intercostalgroups outwards. The first group, already
mentioned, resembled that on the right side in supplying the
first and second intercostal spaces. It may here be added that
these intercostal groups anastomosed freely with corresponding
intercostal branches of the internal mammary arteries, and the
last or terminal group also inosculated with those aortic inter-
costals supplying the neighbouring intercostal spaces.
Between the origins of the left common carotid and the left
posterior thoracic, but closer to the latter, a very small fourth
branch arose from the aortic arch. It was directed forwards to
supply the apex of the left thymic lobe, and then ended in
the left mass of the rete mirabile (Plate XXII. A).
fThoracic aorta.-The thoracic aorta occupied the usual posi-
tion along the spine asfar as the diaphragm. Its relations were
similar to those in man, with this exception, that the rete
THE RETE MIRABILE OF THE NARWHAL. 381
separated it from the spinal column. Its branches, seven in
number, corresponded with the intercostal arteries, and arose
from the vertebral aspect of the trunk, not in pairs, but as single
offsets. These branches varied considerably in size. From the
first to the fourth there was a gradual decrease, whilst the last
three became greatly enlarged. The direction of the individual
branches also differed. The first three passed slightly forwards
to reach their destination, but the last two inclined backwards.
Each terminated in two lateral branches at variable distances
from its origin. These lateral divisions coursed along the
transverse vertebral grooves and ended in branches which were
destined to supply, some the rete mirabile alone, others partly
the latter and partly the intercostal muscles. Their ramifica-
tions in the intercostal spaces were peculiar in that thebranches
of distribution passed off at acute angles from the trunks. As
the same description does not answer for all, each must be
examined separately. The first intercostal artery came off from
the aorta at a level somewhat posterior to the fifth intercostal
space, and, turning forwards, soon broke up into two divisions.
One of these divisions continued onwards and subdivided into
lateral branches to supply that portion of the rete corresponding
with the two fourth intercostal spaces. The other division also
became bifid, the resulting branches being spent in the mesh-
work of the rete occupying the fifth interspaces. The second,
third, and fourth intercostals branched almost directly into
lateral divisions, and were lost in the rete under the sixth,
seventh, and eighth interspaces respectively of the right and
left sides. The fifth intercostal artery was a very large branch
comparatively. Its lateral divisions were very unequal in size.
That to the left far exceeded the other, and was in calibre the
continuation of the parent trunk. It coursed along the trans-
verse vertebral groove into the ninth intercostal space, lying
above the constituent vessels of the rete. Having reached the
space it subdivided into two unequal offsets. ,The smaller of
these ran along the middle of the interspace and anastomosed
with the ultimate twigs of the posterior thoracic and musculo-
phrenic arteries, whilst the larger offset turned obliquely back-
wards and followed the last rib. In its course along the rib it
first supplied a considerable branch to the tenth interspace, and
382 DR HENRY SEASON WILSON.
then terminated in offsets to the abdominal muscles on the one
side, and the intercostals on the other. These terminal offsets
of the left lateral division, as each passed along its intercostal
space, supplied dorsal vessels which pierced the intercostal
muscles and ended in the dorsal vertebral muscles, and while
beneath the rete each, as well as the parent trunk, distributed
numerous delicate twigs to that network. The right lateral
division of the fifth intercostal was similar in size and distribu-
tion to those of the preceding. It was lost in that part of the
rete opposite the ninth interspace. The sixth intercostal
branch was the largest of the last three. Its lateral divisions
were proportionately as unequal in size. That to the left,.the
smaller, continued entirely imbedded in the rete, as far as the
intertransversales, through which it passed to ehd in the dorsal
muscles of the spine. Whilst surrounded by the rete it gave to
the latter a large supply of delicate, slender branches. The
right lateral division subdivided into two branches opposite the
head of the tenth rib. The smaller of these, after sending a
dorsal muscular offset, coursed along the ninth rib and was lost,
partly in distribution to the ninth and eighth interspaces, and
partlyin anastomoses with the last ramifications of the posterior
thoracic and musculo-phrenic arteries. The larger branch gave
origin also to a dorsal muscular offset, and, continuing outwards
for a short distance, bifurcated for the supply of the last two
intercostal spaces. The whole of the right lateral division as
well as part of its subdivision were covered by the rete to which
they distributed innumerablevessels. The seventh of the inter-
costal series of branches corresponded to a "last dorsal." Its
course, being close to the posterior margin of the last rib, was in
the abdominal wall. It was the smallest of the last three, and
its lateral divisions were, like those of the others, unequal in
size. That to the left, the larger, was directed above the rete
mirabile as far as the posterior borderof thehead of the eleventh
rib, along which it ramified, supplying the muscles in that
region, and communicating with offsets from the fifth intercostal
artery. When lying above the rete the latter received a rich
supply of vessels, and when lying on the intertransversales it
gave a dorsal muscular branch. The right lateral division was
exceedingly small, and soon broke up into numerous twigs for
THE RETE 'MIRABILE OF THE NARWHAL. 383
that portion of the rete corresponding with the tenth inter-
costal space.
Nowthat the main branches, whence the rete mirabile derives
its vascular supply, have been described, the position of the
principal part of the rete, that is, its thoracic portion, as well
as the relations it bears to the thoracic vessels, will be easily
understood.
By the thoracic rete mirabile is understood that large mass of
anastomosing vessels situated along the centre of the dorsalwall
of the thorax. For convenience of description it may be said to
consist of two elongated lateral masses, each lying athwart the
ribs, and close to the side of the spine, the lateral masses being
connected to each other by an intermediate central band, simi-
larly constituted, and arching over the vertebral bodies. Each
lateral mass measured 3 inches in breadth. They both pre-
served their characteristic arrangement and structure from the
base to the apex of the thorax, but beyond these limits they
became altered and comparatively insignificant. Their exten-
sion into the abdomen was scarcely perceptible, though their
prolongation into the neck was more distinct and more readily
traced. The intermediate band had the same extent in an
antero-posterior direction, but was only 1A inch in breadth.
The whole mass had thus a transverse measurement of about
7j inches. As the rete closely invested the structures over
which it lay, it necessarily varied considerably in thickness.
This was greatest nearest to the vertebral bodies where the
anastomosing mass sunk deeply between the transverse pro-
cesses and the heads of the ribs, and into the hollows of the
vertebral grooves. It was least in the intermediate central
band, which but thinly covered the bodies of thevertebrae. The
parts of the thoracic wall concealed by the entire rete mirabile
were the bodies of the dorsal vertebrae, the intervertebral fora-
mina, and part of the ribs with the corresponding portion of
their intercostal muscles. Though the meshwork closely in-
vested these structures, fitting accurately into all theinequalities
of their surfaces, yettheconnective tissue between them and the
rete was of such a loose nature as to permit the entire mass to
be reflected from them without much dissection. It was most
firmly adherent over the transverse processes and heads of the
384 DR HENRY SEASON WILSON.
ribs. The opposite aspect of the rete was almost entirely in
contact with the pleura. The thoracic aorta, however, crossed
lengthwise along the left of the central band, but the lateral
masses were covered only by the pleurae. The free margins of
the rete, well defined on each side, kept parallel to the spinal
column, and were bounded along nearly the whole of their
extent by the two posterior thoracic arteries.
The three principal sources whence the rete mirabile derived
the vessels entering into its composition were the posterior
thoracic arteries on each side, and the thoracic aorta in the
middle. The anastomotic vessels of the posterior thoracics
branched in vast numbers from those portions of their trunks
and of their intercostal offsets which were above the rete. As
they emerged from these sources these branches presented one
of the most beautiful of anatomical objects. The delicacy of
their coats, the vast immensity of their numbers, their gently
sinuous courses, their almost equal calibres, and their almost
equally distanced origins at once marked them as peculiarly
characteristic and uncommon. They soon became lost in intri-
cate meanderings which, with care and patience, were gradually
unravelled and their ramifications traced into the substance of
the labyrinthine maze. These ramifications will be best under-
stood by following as far as practicable any given branch.
Quitting its parent trunk as a minute vessel it entered the rete
with a sinuous course, retaining the same calibre throughout
this extent. Once within the mass it lost its wavy undulations
and wandered very tortuously. At the same time it became
sometimes rapidly, sometimes gradually, widened into twice,
thrice, and even four times its former size, and distributed here
and there, at irregular intervals, secondary branches, having its
own calibre, or occasionally rather smaller. Passing onwards in
its intricate course, it branched on all sides until at last its
individuality was lost in the complex network of the anastu-
motic maze. Its secondary branches each presented a like
distribution, and supplied tertiary offsets, which in their turn
but repeated the same vascular intricacy, adding more and more
to the general labyrinth. The interval between any two
branches was occasionally very considerable, and in such cases
the vessels wended about in a very tortuous manner, as if
THE RETE MIRABILE OF THE NARWHNL. 385
profiting by their freedom from lateral ties. Nowhere did a
secondary or a tertiary offset anastomose with like branches
from a primary vessel. The other vascular source of the rete
mirabile, the thoracic aorta, added its supply through the seven
intercostal branches which have already been described. They
entered the mass along its middle, and thus divided it into
symmetrical halves. Though not presenting the same beautiful
regularity in their origins, the vascular supply from these aortic
branches was in every other respect similar to that-from each of
the posterior thoracics. They entered the substance of the rete,
and were there lost in the intricacies of the plexus.
On unravelling the rete throughout its depth and to the
extent of about the breadth of two or three intercostal spaces,
the characters and arrangement of its constituent vessels were
readily examined. In character and size they were divisible
into three sets, easily distinguished from each other. For easier
reference I have called them vasa maxima, vasa media, and
vasa minima. The first set, vasa maxima, contained vessels of
a large calibre, but possessing very thin walls. They were
situated chiefly along the middle of the inferior aspect of the
rete towards its posterior extent, and more particularly opposite
the ninth and tenth intercostal spaces. Their walls were pecu-
liarly puckered so as to give them an appearance somewhat like
that of the large intestines. It was due to this puckering that
these vessels became so curiously contorted when inflated,
causing the whole superficial aspect of the rete to resemble a
mass of cerebral convolutions in miniature. After loosening
the vasa maxima from their connective tissue, the puckerings
still remained, and seemed to be produced by a band or bands
running more or less longitudinally in the coats of the vessels.
The second set of vessels, vasa media, were smaller than the
preceding, though their walls possessed a greater proportionate
thickness. They were the most generally distributed through-
out the mass, meandered in a very tortuous manner, and pre-
sented puckered walls, though less distinctly marked than the
preceding. The third set, vasa minima, consistedofvessels very
much smaller than the preceding, but having proportionally the
thickest walls. Their coats were entirely devoid of the pucker-
ing so characteristic of the others, and they were generally
386 DR HENRY SEASON WITLSON.
found deeply imbedded in the rete. In their course they were
only slightly tortuous. They formed here and there distinctly
isolated plexiform areas, easily distinguished from the rest by
the straight, narrow, thread-like appearance of their constituent
vessels. The anastomoses of each set were continuous with
those of the others. The walls of the vasa maxima and media
were lax, and the vessels appeared as if empty. This was
doubtful in the case of the vasa minima. Occasionally vessels
of a size intermediate to those specified were met with, but they
were only exceptions.
When compared with its great extent, the branches supplied
by the rete mirabile to the structures beneath it were very few.
These were chiefly muscular and osseous, and were most dis-
tinctly seen on reflection of the central band from the bodies of
the vertebrae, or of the lateral masses from the intercostal
spaces. From their small and insignificant size, they probably
but connected the vascular mass of the rete with nutrient
vessels of the surrounding structures. They presented no
puckering of their coats. In addition to the preceding, larger
branches passed through the vertebral foramina into the spinal
canal. They had puckered walls, and were evidently communi-
cations between the rete and the vessels within the vertebral
canal, or they extended a meshwork of the same character as
the rete into that canal.
In unravelling the rete mirabile many structures were found
imbedded within its substance, or surrounded by its anasto-
motic vascular plexus. These were-several veins, the first
portion of the intercostal nerves, and the sympathetic ganglia
with their branches. Some of the veins were large, all of them
had very delicate walls. They seemed to pass towards the
spine, and, while in the substance of the rete, they received
many branches apparently derived from a very fine network
situated within the interspaces and over the coats of the anasto-
motic arteries. But they were much too small and delicate,
and the parts much too decomposed for any satisfactory dissec-
tion to be made with the scalpel. The intercostal nerves lay
very deeply in the substance of the rete, and while thus covered
each received its sympathetic root. The sympathetic ganglia
were situated in the vertebral grooves close to the roots of the