A Guide for the Dissection of the Dogfish (Squalus Acanthias)

by Lawrence Edmonds Griffin

VASCULAR SYSTEM

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.

[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.