LECTURE IV. 



Nature of Life. — Methodical Arrangement of Living Beings — 

 Species, Varieties, Genera, Orders, 8^c. — Progressive Simplijica- 

 tion of Organization, and of Functions. — Intellectual Functions 

 of the Brain, in the natural and disordered State, explained on 

 the same Principles as the offices of other Organs. 



The notion of life is too complicated, embraces too many 

 particulars, to admit of a short definition. It varies in the 

 different kinds of animals, as their structure and functions 

 vary ; so that a description drawn from one would not be 

 applicable to others differently situated in the animal series. 

 If we Include in the description tliose circumstances only, 

 wlilch are common to the whole animal kingdom, we must 

 direct our view to beings of the most simple structure, 

 where the phenomenon is reduced to its essential features, 

 and these are not obscured or confused by accessary cir- 

 cumstances. 



The distinguishing characters of living beings will be 

 found in their texture or organization ; in their component 

 elements ; In their form ; in their peculiar manifestations 

 or phenomena ; and In the limits, that is, in the origin and 

 termination of their vital existence. 



Their body is composed of solids and fluids ; the former 

 arranged in fibres and lamlnfe, so as to intercept spaces 

 which are occupied by tiie latter. The solids give the form 

 to the body, and are contractile. The fluids are generally 

 in motion. 



The component elements, of which nitrogen Is a princi- 

 pal one, united in numbers of three, four, or more, easily 

 pass into new combinations ; and are, for the most part, 

 readily convertible into fluid or gas. 



