As the nervous system contains the organs of sensation and volition, and distinguishes animal from vegetable structures, it has long occupied the attention of anatomists, and is now employed by naturalists as the basis of systematical arrangement. The discoveries with which these investigations have been rewarded, are indeed numerous; but much yet remains to be done, in order to ascertain the structure and actions of various parts of this system, and to reconcile the contradictory statements of different authors. In taking our view of the subject, it will be necessary to avoid all minuteness of detail, and to attend chiefly to those circumstances which characterise the different races of animals, instead of investigating the peculiarities of particular species.

1. Structure of the Nervous System.–The nervous system, as it appears in its most perfect form in the vertebral animals, consists of the Brain, the Spinal Marrow, and the Nerves.

The Brain occurs in the anterior part of the body, surrounded by the bony covering of the skull. Between the brain and the skull, there are three membranes which are considered as the integuments of this part of the nervous system. The exterior of these is termed the Dura mater, and may be considered as the inner periosteum of the skull. It forms various processes for dividing and supporting different portions of the brain, and contains, within its dupKcatures, tortuous cavities for the reception of blood, which are called sinuses.