THE BRAIN. 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 alk 
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, 
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- 
eatures, tortuous cavities for the reception of blood, which 
are called simuses. 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- 
