STRUCTURE AND FUNCTION OF HEART AND BRAIN IN SCOLIODON
B.Sujatha,
Lecturer in Zoology
STRUCTURE AND FUNCTION OF HEART IN SCOLIODON
The circulatory system of Scoliodon consists of the circulatory fluid called blood, the heart, the arteries and the veins.
(i) Blood:
The blood consists of a colorless plasma and corpuscles suspended in the plasma. Two kinds of corpuscles are encountered the RBC (or erythrocytes) and the WBC (or leucocytes). The erythrocytes are oval bodies containing a nucleus. Hemoglobin is present in the erythrocytes. The leucocytes are amoeboid in structure.
(ii) Heart:
Heart is a bent muscular tube and consists of the receiving parts, comprising of a sinus venosus and a dorsally placed auricle and the forwarding parts, consisting of a ventricle and a conus arteriosus . The heart is situated on the ventral side of the body between two series of gill pouches.
The sinus venosus is a thin-walled tubular chamber. It is highly contractile and the beating of the heart originates from this part. Two great veins, the ductus Cuveiri, open into the sinus venosus, one on each lateral side. Two hepatic sinuses enter the sinus venosus posteriorly. The sinus venosus opens into the auricle by sinuauricular aperture which is guarded by a pair of valves.
The auricle is a large, triangular and thin walled chamber situated dorsal to the ventricle but in front of the sinus venosus. The auricle communicates with the ventricle through a slit-like auriculo-ventricular aperture guarded by two lipped valves. The receiving chambers (sinus venosus and auricle) receive the venous blood from all parts of the body.
The ventricle has a very thick muscular wall. Its inner surface gives many muscular strands, which gives it a spongy texture. It is an oval chamber and constitutes the most prominent part of the heart. The conus arteriosus is a stout median muscular tube arising from the ventricle.
The lumen of the conus arteriosus is provided with two transverse rows of semilunar valves. To keep the valves in position the free ends of the valves are attached to the ventricular wall by fine tendinous threads called chordae tendinae. The conus arteriosus is continued forward as the ventral aorta.
The function of the heart is to receive the deoxygenated blood from all parts of the body and to pump it for aeration to the gills. Such a type of heart is designated as the venous or branchial heart, because only the deoxygenated blood circulates through it.
STRUCTURE AND FUNCTION OF BRAIN OF SCOLIODON
The forebrain consists of a massive undivided cerebral hemisphere. The cerebral hemisphere is relatively larger than that of other fishes. From the anterior end of cerebral hemisphere arise two stout olfactory peduncles; each terminates into a large bilobed olfactory lobe.
The olfactory lobes lie close to the olfactory capsules. Each olfactory nerve is composed of many bundles of nerve fibres. The surface of the cerebrum is smooth and the walls are thick. A small opening called the neuropore is present on the mid-ventral surface of the cerebrum. The posterior part of forebrain (diencephalon) is very short. The roof of the diencephalon is thin, non-nervous and contains the anterior choroid plexus.
The lateral walls of the diencephalon form two thickened bodies called thalami. A long and slender tube, the pineal organ or epiphysis cerebri projects from the roof of the diencephalon. The floor of the diencephalon (or hypothalamus) is well-formed. A hollow infundibulum is given off from the floor of the diencephalon.
The infundibulum is dilated to form two oval thick-walled bodies called lobi inferiores whose distal ends are produced into two thin-walled glandular sacs called sacci vasculosi. The lobi inferiores are the centres for gustation and smell.
The hypophysis is attached to the infundibulum. The optic chiasma lies in front of the infundibulum. The optic chiasma is formed by the decussation of the nerve fibres of two optic nerves (Fig. 1.38B).
The midbrain is large and consists of two round optic lobes. The optic lobes are situated behind the diencephalon. The floor and the side walls are relatively thicker. The midbrain is considered as the centre of coordination.
The hindbrain consists of a highly developed cerebellum and a medulla oblongata. The dorsal surface of the cerebellum produces many irregular convolutions. The cerebellum contains a small cavity. The cerebellum is also a centre of co-ordination. The cerebellum is divided into three lobes by two well-marked transverse furrows.
The medulla oblongata is triangular and the anterior end gives a pair of hollow corpora restiformia with trace of convolutions in adults. The medulla controls respiration. Two corpora restiformia are connected by the transverse nerve band. The roof of the medulla oblongata is non-nervous and bears the posterior choroid plexus. The hind- brain controls swimming movements.
The ventricles of the brain are moderately developed cerebral hemispheres contain narrow lateral ventricle. The third ventricle is extended forward about half the length of the cerebral hemispheres. The floor of the fourth ventricle is very much thickened.
The fourth ventricle is large and extends dorsally into the cerebellum and is continuous behind with the cavity of the spinal cord. The iter (i.e., the communicating duct between the third and the fourth ventricles) is wider. Although the cerebrum is undivided, there are two lateral ventricles which are continued to the rhinocoels (cavity of the olfactory lobes).