THE BRAIIST. 141 



ment. 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 circum- 

 stances which characterise the different races of animals, 

 instead of investigating the peculiarities of particular spe- 

 cies, 



1. Structure of the Nervous System. The nervous sys- 

 tem, 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, sur- 

 rounded 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 y 

 and may be considered as the inner periosteum of the skull. 

 It forms various processes for' dividing and supporting dif- 

 ferent portions of the brain, and contains, within its dupli- 

 catures, tortuous cavities for the reception of blood, which 

 are called sinuses. Underneath the dura mater, is the 

 Arachnoid coat ; so named from its resemblance to a spi- 

 der's web in thinness. Like the former, it is extended 

 merely over the surface of the brain, without entering its 

 various convolutions. The third membrane of the brain is 

 termed Pia mater. It adheres every where to its surface, 

 following the course of all its irregularities, and lining its 

 different cavities. It is thin and vascular, containing nu* 



