Examine the top view photographs of the spiny dogfish shark with its cranial cavity exposed by clicking the blue lettered links in the column to the right. The specimen in the photographs was prepared by removing the skin from the dorsal surface of the head and shaving off thin horizontal chips of cartilagenous cranium until the brain and cranial nerves were exposed. The delicate vascular protective membrane called the primitive meninx was removed.

The nervous system functions in communication between the various parts of an organism and between the organism and its external environment. There are two major divisions of the shark's nervous system.

1.  Central Nervous System:  The central nervous system is composed of the brain and spinal cord.

2.  Peripheral Nervous System:  The peripheral nervous system which includes the sense organs, cranial nerves, and the spinal nerves (including their individual branches).

Shark Cranial Cavity

Labeled Cranial Cavity

Examine the dorsal view photographs of the shark's brain by clicking the blue lettered links in the column to the right.

Forebrain

• The telencephalon is the most anterior portion of the brain which includes the following parts:
• The olfactory bulbs are paired extensions of the anterior portion of the brain. They are rounded masses which make contact anteriorly with the spherical olfactory sacs, the organs of smell.
• The two cerebral hemispheres are the rounded lobes of the anterior brain. The anterior portions of thee cerebrum are known as the olfactory lobes.
• The diencphalon is the second portion of the forebrain, directly posterior to the telencephalon. It consists of the epithalamus, pineal body, thalamus, hypothalamus and pituitary body.

Midbrain

• The optic lobes are a pair of prominent bulged structure.

Hindbrain

• The cerebellum is an oval-shaped dorsal portion that partly overlaps the optic lobes.
• The medulla oblongata is the elongated posterior region of the brain that is continuous posteriorly with the spinal cord.

Shark Brain

Labeled Brain

Examine the bottom view photographs of the cranial nerves of the shark by clicking the blue lettered links in the column to the right.

CRANIAL NERVES

The cranial nerves originate in the brain and exit at the chondrocranium. These nerves may be sensory, carrying impulses to the brain; they may be motor, carrying impulses from the brain to muscles and glands; or they may be mixed nerves, carrying both sensory and motor fibers.

The cranial nerves of all vertebrates have similar names and similar functions. Fish are usually described as having ten pairs of cranial nerves including:

• (I) The olfactory is a sensory nerve originating in the olfactory epithelium of the olfactory sac and terminating in the olfactory bulb of the cerebral hemisphere.lt is concerned with the sense of smell.
• (II) The optic nerve is also a sensory nerve. It originates in the retina of the eye, exits the back of the orbit, passes medially and posteriorly to the optic chiasma and enters the optic lobes.
• (V) The trigeminal nerve arises from the anterior end of the medulla. It is a mixed motor and sensory nerve which has four branches that inervate the face, eyes, mouth and jaws. The superficial opthalmic nerve is one of the four branches. It has a general sensory function for the skin of the rostrum.
• (X) The vagus nerve is the longest of the cranial nerves. It is a mixed motor and sensory nerve that arises at the posterior end of the medulla. It innervates the gills, throat, esophagus, stomach, intestine and body wall.

Shark Cranial Nerves

Labeled Cranial Nerves

Examine the photographs of the shark's olfactory sac and eye by clicking the blue lettered links in the column to the right.

The olfactory sacs are spherical structures that contain a series of radial folds called olfactory lamellae. Their surfaces are covered with olfactory epithelium. Sea water taken into the nares is passed over these sensory areas. Here the odors stimulate the cilia-like endings of neuro-sensory cells.

The olfactory bulbs are a paired anterior extension of the brain leading into the posterior end of the olfactory sacs. Their fibers continue into the olfactory tract and the olfactory lobe of the cerebral hemisphere.

The sclera is the tough white fibrous outer coat of the eye. At places it is made even more firm by cartilage embedded in the sclera.

The iris is the pigmented anterior extension of the choroid layer. In its center is the pupil. The iris regulates the size of the pupil.

In the living shark the lens is a clear and flexible structure located behind the iris. It helps to focus the light upon the light sensitive retina.

The retina is the multi-layered sensory gray-white colored membrane. The rods and cones which receive light stimuli are located here. The optic nerve leaving the eye is a continuation of the light receptor cells in this membrane.

Shark Olfactory Sac

Labeled Olfactory Sac