The nervous system of Scoliodon consists of three parts:
(1) Central nervous system:
It includes brain and spinal cord.
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(2) Peripheral nervous system:
It includes cranial and spinal nerves.
(3) Automatic nervous system:
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(a) Brain:
In Scoliodon the brain lies enclosed within the chordrocranium and is made of the same three basic parts of the vertebrate brain-forebrain, midbrain and hindbrain.
(1) Forebrain:
It consists of two parts, anteriorly cerebrum and posteriorly diencephalon. Cerebrum is large, undivided, bears a stout olfactory stalk or peduncle on each antero-lateral side.
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Each peduncle terminating into a large bilobed olfactory lobe lies in olfactory sac. The sac opens to outside through the nostrils or naris.
The mid-ventral surface of cerebrum has a small opening, the neuropore. Tow delicate terminal nerves, each bearing a ganglion, come out of neuropore to innervate the mucous membrane of the olfactory sac.
Diencephalon is rather small and narrow part of the brain and its thin, membranous and vascular roof is called the anterior choroid plexus.
Mid-dorsally a slender pineal stalk terminating into pineal body or epiphysis of unknown function extends upwards.
Ventrally its anterior margin bears optic chiasma of two optic nerves. Just behind the chiasma a hollow projection called infundibulum is situated on the floor.
Attached posteriorly to infundibulum is a sac like hypophysis and both make up the pituitary body.
(a) Mid brain:
It is not a prominent part and remains closely concealed dorsally by cerebellum and ventrally by the infundibullar outgrowths.
It consists mainly of a pair of large, rounded dorsal swellings the optic lobes or carpora bigemina, which are the centres of sight and hearing. Ill and IV cranial nerves arise from midbrain.
(b) Hind brain:
It consists of two parts cerebellum and medulla oblongata. Cerebellum is large, elongated and rhomboidal structure overhanging the optic lobes in front and part of medulla behind.
Its dorsal surface is irregularly folded and divided into three lobes by two transverse furrows.
Medula oblongata, the last part of the brain is a triangular structure and continues posteriorly into the spinal cord.
Its anterodorsal end bears on either side a hollow outgrowth, the restiform body. Both the bodies are connected together by a transverse band of nervous tissues.
Medulla is roofed over by a thin, non-glandular and vascular membrane, the posterior choroid plexus.
Cavities of brain:
In H L S of Brain, it is seen hollow and its cavities are called ventricles, filled with ceratrospinal fluid. These cavities continue with the central canal of spinal cord.
(a) Rhinocoel:
Cavities of olfactory sacs.
(b) Lateral Ventricles:
Cavities of cerebrum also called paracoels.
(c) Foramen of Monro:
Opening between I and II paracoels (ventricles) of cerebrum and III ventricle.
(d) Diacoel:
Cavity of diencephalon also called III ventricle.
(e) Iter:
Cavity of midbrain. Also called Aqueduct of Sylvius.
(f) Optocoels:
Cavities of optic lobes.
(g) Epicoel:
Cavity of cerebellum.
(h) Metacoel:
Cavity of medulla also called (IV) ventricle.
(2) Spinal Cord:
It is the extension of medulla oblongata upto the end of tail, within the neural canals of vertebral column.
It encloses a central canal and inner grey matter covered by outer white matter. Its transverse section shows that it is made up of two halves connected together by a bridge.
(3) Cranial nerves:
There are 10 pairs of cranial nerves recognized by their serial Roman numbers as well as the names. The serial numbers, names, origin and distribution of cranial nerves are as follows:-
Nerve “0” Terminal or Pre-olfactory:
These are sensory arising through neuropore of cerebrum and innervating the olfactory region.
Nerve I Olfactory:
These are sensory, serves for smelling. Arise from olfactory lobe supply to olfactory sac.
Nerve II Optic:
These are sensory in relation to light. Thick and short nerves arise from ventral side of diencephalon form optic chiasma and innervate retina of eye.
Nerve III Oculomotor:
These are motor, controlling movements of eyeball, iris and lens. They are slender, arise from ventral surface of midbrain.
They divide into four branches to innervate the inferior, superior and anterior recti muscles and the inferior oblique muscles of eye.
Nerve IV Trochlear or Pathetic:
These are motor, help in rotation of eye ball. They arise dorso-ventrally from midbrain and supply the superior oblique muscle of eye.
Nerve V Trigeminal:
They are mixed nerves arise from antero-lateral sides of medulla. They bear a Gasserian ganglion while still in the cranium and divide into 4 main branches as follows:-
(a) Ophthalmicus superficialis:
It is sensory, supplies to the skin of snout. It passes forward above the orbit intermingled with fibers of similarly named branches of VII.
(b) Ophthalmicus profundus:
It is sensory, supplies to the dorsal skin is snout. It runs forward along the inner surface of eyeball.
(c) Maxillaris:
It divides into two branches. Maxillaris superior, a flat ribbon-like sensory nerve and supplies to the ventral skin of the snout. While Maxillaris inferior supplies the skin of posterior part of upper jaw.
(d) Mandibulars:
It is a mixed nerve and supplies to the muscles of lower jaw, tongue and gill region.
Nerve VI Abducens:
It is a small slender and motor nerve arising mid ventrally from medulla to innervate the posterior rectus muscle of eye.
Nerve VII Facial:
It is a mixed large nerve arising from the side of the medulla and dividing into four main branches:
(a) Ophthalmicus superficialis:
It is a large nerve, runs along the similar named branches of Nerve V and supplies to the ampullae of lorenzini on the dorsal side of snout.
(b) Buccalis:
It is sensory and supplies to the infraorbital canal and associated ampullae of lorenzine.
(c) Palatinus:
It runs across the floor of the orbit and supplies to the roof of buccal and pharyngeal cavities.
(c) Hyomandibular:
It runs down along the posterior wall of orbit giving off 3 main branches:
(i) Hyoidean (or hyomandibular) to muscles of throat (hyoid arch)
(ii) Mandibularis internus to mucous membrane of buccal floor.
(iii) Mandibularis externus to mandibular canal.
Nerve VIII Auditory:
It is sensory and helps in audition. It passes into auditory capsule to innervate the membranous labyrinth.
Nerve IX Glossopharyngeal:
It is a mixed nerve and divides into three branches.
(i) Pretrematic running along the anterior border of first gill pouch.
(ii) The post trematic along its posterior border and
(iii) Pharyngeal to the mucous membrane of pharynx.
Nerve X Vagus or pneumogastric:
It is the largest and mixed nerve gives off 3 main branches as follows:
(i) Lateralis:
It runs along the whole length of fish under the lateral line canal innervating its neuromast organs.
(ii) Viscerelis:
It enters the body cavity to supply viceral organs and heart.
(iii) Branchialis:
These are 4 branchialis nerves, one supplying each of the remaining gill pouches from second to fifth.
Each nerve divides into three branches, post-trematic and pretrematic for the gill pouches and a middle pharyngeal to the mucous lining of pharynx.
4. Spinal nerves:
Several pairs of spinal nerves arise from spinal cord at regular intervals along its entire length. Each nerve originates by two roots, a sensory dorsal root and a motor ventral root. Dorsal root bears a ganglion.
After piercing through the neural arch both the roots unite outside to form a mixed spinal nerve. Each spinal nerve gives off three branches:
(i) Ramus dorsalis and (2) ramus ventralis to the dorsal and ventral body wall muscles and skin and (3) ramus communicans joining the autonomic system.
A branchial plexus is formed at the level of pectoral girdle, but no plexus is formed in the pelvic region.
5. Autonomic nervous system :
In dogfish it includes a series of paired and irregularly arranged ganglia in the dorsal wall of posterior cardinal sinuses and in dorsal parts of kidneys.