ABSTRACT
Five brain divisions usually observed in fish are from cranial to caudal, telencephalon or forebrain (contain two olfactory lobes and cerebrum), diencephalon (contain epithalamus, thalamus, and hypothalamus), mesencephalon or midbrain (contain two optic lobes that are connected with torus longitudinalis internally and the torus semicircularis, and optic tegmentum medially), metencephalon or hindbrain [cerebellum (CR)], and myelenceph-alon (medulla oblongata). The optic tectum (OT) represents the principal anatomical termination of the retinal ganglion cell axons and is considered the most obvious part of the midbrain. The development of this region reflects the importance of the visual sense in different species. Grossly, the OT is divided into two optic lobes and contains the tectal ventricle. It displays a distinctive laminar histological architecture, which is made up of six layers. The CR in teleosts is well-developed and includes three main divisions: the valvula cerebelli, the corpus cerebelli, and the vestibulolateral CR. The cerebellar cortex consists of three distinguished layers: an outer molecular, intermediate ganglionic, and inner granular layer. Notably, the gray substance in the spinal cord of teleosts displays a characteristic difference in its organization from that of higher vertebrates, in which the dorsal horns lie quite close together giving the gray substance the shape of an inverted Y. The white matter surrounds the gray matter and harbors myelinated nerve fibers. The ventricular organization is composed of the olfactory, lateral, the third, the tectal, and the fourth ventricles. The peripheral nervous system consists of nerves and ganglia.
The nervous system is the structural and functional mechanism regulating the response of an animal to environmental changes. For the nervous system to perform these responses, it is widely arranged throughout the body. The nervous system includes two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is consisted of the brain and spinal cord (SC), while the PNS includes the spinal and cranial 354nerves with their associated roots and ganglia. The roots of the cranial and spinal nerves connect the PNS with CNS.
