INTRODUCTION. jcxi 



The old wom-out material, together with other matters conveyed 

 into the circulation, having now become useless and excrementi- 

 tious, we find a channel for their expulsion in the urinary 

 SYSTEM. This includes the kidneys and the bladder; the former 

 being organs of secretion — by which is meant, organs having the 

 power, by an action peculiar to themselves, of separating a sub- 

 stance or fluid from the blood altogether different from the blood 

 itself, and which either answers some important purpose in the 

 animal economy, or becomes the medium for the ejection of 

 useless or excrementitious matters ; the latter, simply a reservoir 

 for the urine, containing it until convenience may suit for its 

 discharge. 



In order for the animal machine, thus constituted, to be set in 

 action, the nervous system becomes necessary ; that system 

 which is the source of all sensation, as well as of all motion, and 

 by which the animal is signally elevated beyond the vegetable 

 creation. Sir Charles Bell, who has thrown such splendid new 

 light upon this department of anatomy, arranges the nervous 

 system in four classes: — First, comprising nerves of sensation; 

 second, nerves of voluntary motion; third, nerves of respiratory 

 motion ; fourth, nerves of the sympathetic system, by which are 

 regulated the functions of nutrition, growth, and decay, and 

 whatever is directly necessary to animal existence. The simplest 

 form of nervous system is found to consist in a central ganglion, 

 with a single nerve attached to it ; and is exhibited in animals 

 whose organs of motion are found to be imperfect. But, should 

 the creature possess organs of progression, and be endowed with 

 organs of sense in addition, then nerves and ganglia are super- 

 added, and a sensorium or brain is formed, which becomes at 

 once the seat and source of all motion and sensation, the throne 

 of reason, and grand focus of every intellectual faculty. In this 

 manner, 



" Nature, in her productions slow, aspires 



By just degrees to reach perfection's height." 



Intimately in structure connected with, and in function entirely 



dependent upon, the nervous system, is the sensitive system ; 



comprehending the nose, the ei/e, and the ear. Were it not for the 



nerves interpassing between these organs and the brain, we could 



