Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/117



The nervous system controls most of the activities of the rest of the organism. It receives through the sense organs the stimuli from the organs of the body and the external world, transmits them as nervous impulses, coordinates them, and originates impulses which it carries to the muscles, glands, etc., stimulating these structures to action. It comprises the following parts: (1) central nervous system (brain and spinal cord), (2) peripheral nervous system (spinal and cranial nerves, and the sympathetic nervous system), and (3) the sense organs.

The early embryonic central nervous system of vertebrates is a comparatively simple tube (neural tube). The anterior portion develops into the brain, the remainder into the spinal cord. The embryonic brain at first comprises three divisions — in front the prosencephalon, then the mesencephalon, and posteriorly the rhombencephalon. The prosencephalon further differentiates into the telencephalon (which includes the cerebral hemispheres) and the diencephalon. The mesencephalon forms dorsally the optic lobes (corpora quadrigemina), while the rhombencephalon is transformed into the metencephalon (cerebellum and pons) anteriorly and the myelencephalon (medulla oblongata) posteriorly. The cavities of the brain are derivatives of the lumen of the neural tube.

The brain fills the cranial cavity. It has already been adequately preserved by chipping away a part of the roof