HEART AND VENTRAL AORTA. Continue the longitudinal incision through the
skin as far as the mandible.[1] Dissect away the sheet of muscles
between the gill pouches and the mandible, exposing a slender muscle
which extends from the pectoral girdle to the middle of the mandible.
The _thyroid gland_ lies dorsal to the anterior end of this muscle,
close against the mandible. (The thyroid of Eugaleus is a broad,
flattened structure covering the anterior ends of the coracohyoideus
muscles.) Carefully dissect out the muscles lying between the branchial
pouches of the right and left sides.
Footnote 1:
See footnote, p. 31.
In front of the pectoral girdle lies a thin-walled sac, the _pericardial
sac_. Open it by a median ventral incision. Remove about one-half inch
of the middle of the pectoral girdle, being careful not to cut the
thin-walled part of the heart lying dorsal to it. The _pericardial
cavity_ is a pear-shaped chamber containing the heart, and lined by the
smooth _pericardium_ which is morphologically equivalent to the
peritoneum. At the anterior extremity of the chamber the pericardium is
seen to be reflected backward over the surface of the heart, thus
forming its smooth outer coat.
The _heart_ may be considered as a bent tube, enlarged in certain
regions to form the chambers. Anteriorly and ventrally is a short,
thick-walled tube, the _conus arteriosus_; this leads out of the
pericardial sac anteriorly, while posteriorly it opens into a large
muscular chamber, the _ventricle_. Dorsal to the ventricle, and
projecting on either side of it is the thin-walled _auricle_. Dorsal to
both ventricle and auricle is the extremely thin-walled _sinus venosus_.
This is triangular in shape, the apex opening into the posterior side of
the auricle, the base attached to the posterior wall of the pericardial
cavity; the lateral angles are drawn out into the _ducti cuvierii_,
which receive veins from the anterior and posterior parts of the body.
From the conus arteriosus springs a smaller vessel, the _ventral aorta_,
which passes forward between the gill pouches. Take note of the small
arteries passing over the surface of the conus and along the inner ends
of the gill pouches, and take care not to cut them or their branches in
the subsequent dissection.
Two pairs of arteries leave the ventral aorta as it emerges from the
pericardial sac. The aorta then passes forward some distance and finally
divides into two branches which pass to either side. Follow the branches
of the aorta outward on the left side and demonstrate their courses. The
anterior branch quickly divides into two, the anterior of these passing
along the base of the first demibranch. The posterior enters the septum
between the first and second pouches, and supplies the second and third
demibranchs. The middle branch of the aorta passes directly to the
fourth and fifth demibranchs. The posterior branch divides almost as it
leaves the aorta, its branches supplying the remaining demibranchs.
There is considerable variation in this branch of the aorta. It usually
divides as stated, but it frequently passes some distance toward the
gills before dividing, and in a considerable number of cases two vessels
arise directly from the aorta instead of one.
The arteries carrying blood from the ventral aorta to the gills are
named the _afferent branchial arteries_. Observe the relation of these
vessels to the gills.
VENOUS SYSTEM. All the blood of the body is conveyed to the sinus
venosus. The sides of the sinus venosus are extended as large vessels,
already referred to as the ducti cuvierii. Open the sinus and ducti by a
transverse ventral incision. The ducti pass directly into the lateral
veins. Near the middle of the posterior wall of the sinus is an aperture
of varying size, the opening of the _hepatic sinus_; there are rarely
two openings in Squalus, always two in Eugaleus. A large opening on the
posterior wall of each ductus leads into the _posterior cardinal vein_.
On the anterior wall of the ductus, near the sinus venosus, is a small
aperture, that of the _inferior jugular vein_. Lateral to this is
frequently a somewhat larger opening of the _anterior cardinal vein_.
This is absent, however, in the majority of specimens; the anterior
cardinals opening into the anterior ends of the posterior cardinals in
about six out of ten cases.
A large cavity, the _hepatic sinus_, exists in the anterior end of the
liver just posterior to the suspensory ligament. Cut into the liver at
this point until the sinus is found, open it, and observe the large
_hepatic veins_ bringing blood into it from the liver, as well as its
communication with the sinus venosus.
Trace all veins by passing a flexible probe or guarded bristle along
them and then opening the vein with the probe as a guide. All smaller
vessels emptying into those described should be noted.
The _lateral veins_ pass forward to the posterior edge of the pectoral
girdle, bend sharply dorsad, and enter the lateral extremities of the
ducti cuvierii. Open a lateral vein near the anterior end and trace it
toward the heart. The right and left lateral veins are joined by a vein
passing along the ventral bar of the pectoral girdle. Open the lateral
veins at a point about two inches in front of the pelvic girdle and
trace the veins backward as far as they can be followed. The blood from
the pelvic fins enters the lateral vein through the _femoral vein_. The
lateral veins finally unite back of the cloaca.
Just before the lateral vein enters the ductus cuvierius it is joined by
a large _coracoid vein_ which runs dorsad and posteriorly along the
posterior edge of the pectoral arch. Follow its course. It receives a
good-sized _pectoral vein_ from the pectoral fin, and sometimes several
smaller veins from the same region. Traced dorsad it is found to open
into a large blood sinus above the liver and oesophagus, the _cardinal
sinus_.
In Eugaleus this connecting vein between the lateral vein and the
cardinal sinus is wanting, the pectoral vein opening directly into the
lateral.
The _ventral cutaneous vein_, which runs along the ventral midline of
the body wall, should be followed; anteriorly it joins the vessel
uniting the two laterals; posteriorly it divides at the pelvic arch and
anastomoses with the laterals.
Pass a bristle from the sinus venosus into one of the _posterior
cardinal veins_ and trace the vein backward between the kidneys as far
as possible. Open both posterior cardinals in this way, washing them out
and observing that they receive blood from the kidneys by a series of
_renal veins_, and that they are separate in their posterior parts, but
communicate with each other anteriorly, where they are greatly expanded;
the communicating portions and coincident enlargement forming the
_cardinal sinus_. The anterior portion of the cardinal vein receives
_ovarian_ or _spermatic veins_ from the female or male gonad, _anterior
oviducal veins_ from the anterior part of the oviduct, and segmental
veins from the corresponding region of the body wall. There sometimes is
more than a single opening from the posterior cardinal vein into the
cuvierian duct.
Cut across the tail an inch behind the cloaca. Two vessels lie in the
cartilaginous arch below the centra of the vertebrae; the dorsal of the
two is the _caudal artery_, the ventral one is the _caudal vein_. Follow
the vein forward. Dorsal to the cloaca it divides into two, which should
be followed along the dorsal surfaces of the kidneys. These are the
_renal portal veins_, conveying blood to the kidneys. Besides collecting
the blood of the tail the renal portals also receive the _posterior
oviducal_ and _segmental veins_. They pass into the capillaries of the
kidneys.
The _inferior jugular vein_ opens into the medial end of the cuvierian
duct. Trace it forward along the ventral ends of the gill pouches; it
receives vessels from the arches and finally joins the _hyoidean_
_veins_ which follow the hyoid arch. At the outer end of the cuvierian
duct there is often a small opening on the anterior wall opposite the
mouth of the posterior cardinal vein. This leads into the _anterior
cardinal vein_. As mentioned before, in a slight majority of the cases
examined, the anterior cardinal vein opens into the posterior cardinal
vein, not directly into the cuvierian duct. If possible, pass a bristle
into the anterior cardinal. To follow the vein, and usually this is the
best way to find it, make a vertical longitudinal incision upon the
dorsal side of the neck, between the gill pouches and the mass of muscle
lying beside the vertebral column. This will open the anterior cardinal,
which is considerably expanded in this region, and it may be traced from
this point toward the heart and the head. The anterior cardinal narrows
suddenly in front of the anterior gill pouch, and leads downward to the
orbit, where it expands into the _orbital sinus_ surrounding the eyeball
and its muscles. Trace the anterior cardinal only as far as the opening
into the orbital sinus at this time. Veins from the anterior portion of
the head and from the brain can be followed when the dissection of the
eye is undertaken.
Just back of the spiracle the anterior cardinal receives the _hyoidean
vein_, which passes ventrad along the base of the first demibranch and
unites with the hyoidean of the opposite side. Ventrally, it also
communicates with the inferior jugular vein.
The principal veins of the body have now been dissected with the
exception of the _hepatic portal vein_, which it is better to trace
after the arteries of the digestive tract have been studied.
THE EFFERENT BRANCHIAL ARTERIES AND DORSAL AORTA. Commencing at the
mouth, cut through the floor of the pharynx close to the left side of
the ventral aorta and the heart. The cut should leave the gill arches
uninjured, and may be continued into the oesophagus.
Examine the interior of the mouth and pharynx, observing particularly
the form and arrangement of the teeth, the spiracular and branchial
clefts, the gill rakers, and the character of the mucous coat of the
pharynx.
Remove the skin from the roof of the pharynx. This exposes four pairs of
_efferent branchial arteries_ bringing blood from the gills and uniting
in pairs to form the _dorsal aorta_. Follow each vessel of the left side
out to its gill-cleft. At the dorsal end of the gill-cleft it divides
into a large posterior and small anterior branch. These respectively
pass along the posterior and anterior demibranchs of the gill pouch,
receiving fine branches from the gill lamellae, and finally unite again
at the ventral end of the gill pouch. Thus a complete loop is formed
around the branchial cleft. The posterior branch of each efferent artery
and the anterior branch of the succeeding one are united by several
short vessels. The efferent artery of the last demibranch possesses only
these connections with the branch next anterior to it, and none with the
aorta directly. From the ventral ends of the efferent loops small
vessels pass toward the midline to unite with a longitudinal artery, the
_hypobranchial artery_, which will be traced farther a little later in
the dissection.
In Eugaleus the dorsal aorta extends forward beyond the union of the
first pair of efferent branchials and then divides into small right and
left branches which pass forward and outward to unite with the common
carotid arteries.
A _common carotid artery_ leaves the dorsal end of each anterior
efferent branchial loop, passing forward and inward. At the level of the
spiracles it divides into _external_ and _internal carotids_; the
internal carotid unites with its fellow of the opposite side and enters
the skull. The external carotid arteries run outward and forward around
the eyes and are distributed to the regions of the mandible and snout.
Do not, at present, trace them beyond the posterior edge of the eye.
Another vessel arises from the middle of the anterior side of the first
efferent branchial loop and runs forward to the spiracle, where it ends
in the capillaries of the pseudobranch. This is the _afferent hyoidean
artery_. The term pseudobranch is used for the branchial lamellae of the
spiracle rather than demibranch because of the arterial blood supply of
this organ.
Immediately after uniting the internal carotids divide and diverge,
forming an X-shaped figure. Each anterior limb of the X again divides
into two branches. The lateral branch passes to the ventral surface of
the skull; it presently gives off an anterior twig (_ophthalmic artery_)
which enters the eye. It then passes on as the _efferent hyoidean
artery_ to the pseudobranch. The inner of the two branchs mentioned
above passes on as the _internal carotid, sensu strictu_, and is
distributed to the brain.
If the dissection is made with care, the branches of the internal
carotid can all be found without cutting any important nerves. The
branches passing to the eye and brain are best traced to their
terminations in connection with the dissection of the nervous system.
Near the union of the first pair of efferent branchial arteries a small
_posterior vertebral artery_ arises from each, and runs anteriorly along
the vertebral column.
Near the divisions of the common carotids two _anterior vertebral
arteries_ arise from these vessels and pass posteriorly, often
anastomosing with the posterior vertebral arteries. These vertebral
arteries are vestiges of the former anterior part of the dorsal aorta
(compare with Eugaleus, in which the dorsal aorta sends forward two
vessels which join the common carotids).
An _oesophageal artery_ springs from the second efferent branchial, and
passes back until it enters the wall of the oesophagus. It also gives
off nutrient branches to the second, third, and fourth gill pouches. The
nutrient artery of the first gill pouch arises directly from the first
efferent branchial.
Near the point at which the fourth pair of efferent branchials join the
aorta, two small _subclavian arteries_ leave the aorta and pass into the
pectoral fins. There is some variation in regard to the point of origin
of these vessels; it may be either in front of or behind the junction of
the fourth efferent branchials with the aorta.
The _hypobranchial artery_ passes along the ventral ends of the gill
pouches. It is either connected with the efferent branchial loops by
short branches, or is formed, in part at least, by short vessels
connecting these loops. The hypobranchials are important nutrient
vessels, supplying the gill pouches and the muscles of the throat and
the oesophagus by means of numerous small arteries; from the
hypobranchials also arise small _posterior coronary arteries_ which pass
to the ventral and posterior walls of the pericardium and the sinus
venosus, and larger _anterior coronary arteries_ supplying the ventricle
and conus arteriosus. The hypobranchials can frequently he followed
along the dorsal side of the pericardium and then outward to junctions
with the subclavian arteries.
The _coeliac artery_ (_coeliac axis_) arises from the aorta just back of
the subclavians. Passing posteriorly and ventrad close to the right side
of the stomach and reaching the gastro-hepatic omentum, it divides into
two branches, the _gastro-hepatic_ and _anterior intestinal arteries_.
The first gives off a small _hepatic artery_ to the liver and a large
_gastric artery_ to the cardiac limb of the stomach. The _anterior
intestinal artery_ supplies the pyloric limb of the stomach, the
pancreas, duodenum, and right side of the large intestine.
Small _genital arteries_, supplying the reproductive glands, arise from
the coeliac near its origin. (In Eugaleus the genital arteries arise
from the anterior and posterior mesenteric arteries.)
At about the middle of the abdominal cavity two arteries arise close
together from the aorta. The anterior of the two is the _anterior
mesenteric artery_; it passes to the left side of the large intestine
and its branches anastomose more or less with those of the anterior
intestinal artery. The posterior vessel is the _lienogastric_; it goes
to the spleen, pancreas, and loop of the stomach.
The _posterior mesenteric artery_ leaves the aorta a little distance
back of the lienogastric and passes to the rectal gland, rectum, and
cloaca.
Free the kidney from the body wall along its outer edge and turn it up
so as to expose its dorsal surface. Observe the numerous _parietal
arteries_ (going to the body wall) and _renal arteries_ (to the kidney),
which spring from the dorsal aorta. Branches of the parietals also pass
into the kidney.
A pair of small _iliac arteries_ pass into the pelvic fins.
_Oviducal arteries_, one or several on each side, arise from the aorta
behind the coeliac artery and pass to the oviduct. Their size varies
largely with the development and physiological condition of the oviduct.
The aorta is continued in the tail as the _caudal artery_.
DISSECTION OF THE HEART. Remove the heart together with the ventral
aorta from the body and fasten it, dorsal side up, under water. Open the
sinus venosus with scissors, wash it out, and observe the vertical
slit-like opening into the auricle and the two membraneous valves which
guard it.
Continue the cut through the _sinu-auricular aperture_ along the median
dorsal line of the auricle; observe the thin walls of the auricle and
their strengthening by an irregular mesh of muscles, the _musculi
pectinati_; the shape and position of the _auriculo-ventricular
aperture_; the flaps of the _auriculo-ventricular valve_. Press upon the
sides of the ventricle and, if possible, observe the mode of action of
the valve.
Cut across the ventricle from the auriculo-ventricular aperture. Carry
another incision from this along the dorsal side of the conus
arteriosus. Observe the small size of the cavity of the ventricle, the
thickness of its walls, and the projecting network of muscles, the
_columnae carneae_, some of which are attached to the edges of the
auriculo-ventricular valves.
In the conus arteriosus observe the rows of three pocket-like valves
each around the proximal end (_semilunar valves_), and a single row of
three similar but larger valves at the junction of the conus and ventral
aorta. There is some variability in the number of rows of valves in the
conus of Squalus; there are always three rows of three valves each in
that of Eugaleus.
In the aorta notice the apertures without valves which lead into the
afferent branchial vessels.
HEPATIC PORTAL SYSTEM. The _hepatic portal vein_ is the large vein
entering the liver alongside the hepatic artery and bile duct. It
receives branches from the stomach, pancreas, spleen, intestine, and
rectal gland.
At the surface of the liver it divides into two branches, which enter
the two lobes of this organ. Within the liver the hepatic portal veins
branch until a capillary system is formed from which the blood is
collected by the hepatic veins and carried into the sinus venosus.
In general, the branches of origin of the hepatic portal vein follow
closely the arteries of the digestive organs. Trace the following parts
of the system: A _posterior intestinal vein_, from the rectal gland and
rectum, the large intestine and spiral valve, across to the end of the
pancreas, along the pancreas to the hepatic portal vein; an _anterior
intestinal vein_, from the large intestine and spiral valve, along the
duodenal lobe of the pancreas; _gastric_, _duodenal_, and _pyloric
veins_ joining the veins already traced; a _splenic vein_ joining the
posterior intestinal vein.
The liver, with the bile duct, may now be removed from the body if it is
desired to trace the bile duct into the bladder or to trace the hepatic
ducts. This can be done best by gently scraping away the soft liver
tissue until the bladder and ducts are exposed.