234 MUTUAL DEPENDENCE OF THE FUNCTIONS. 



Dr. B. It is an imaginary line passing through the 

 middle of the body in a longitudinal direction, dividing 

 it into two similar halves. 



Emily. And on this line are placed the nose, mouth, 

 tongue, &ic. and on each side of it are ranged the organs 

 of vision, hearing, &;c. 



Dr. B. The organs of organic life show none of 

 this symmetry. They are irregular in their form, and 

 situated without any regard to the median line. The 

 stomach, liver, heart and spleen are placed in this man- 

 ner, and their halves bear no resemblance to each other. 



We have now gone through with the functions of ani- 

 mal existence ; we have examined them in the various 

 classes of animals, and observed their points of resem- 

 blance and difference. As yet, however, we have not 

 spoken particularly of their mutual relations, influence, 

 and dependence, and the laws which regulate their com- 

 binations. As we have traced them separately and in 

 detail from one class to another, we have seen them 

 constantly undergoing some modification or other, and to 

 one therefore who is but imperfectly acquainted with the 

 subject, it would seem that by combining together all 

 these modifications in an arbitrary manner, we could 

 vary to infinity the structure of organic beings. 



Emily. Certainly ; we should only have to alter the 

 condition of a single function, no matter how little, leav- 

 ing all the rest unchanged, to have a specifically differ- 

 ent being. For instance, if we change the structure of 

 the digestive organs, we have a different animal, whether 

 we alter those of respiration, circulation, sensibility, &LC. 

 or not. 



Dr. B. But such combinations as these which at 

 first sight appear possible, do not exist in nature. For 

 it must be recollected that each organ has not only its 

 peculiar function to perform in the animal economy, but 

 that it contributes with all the rest to the production of a 

 common object. Now, this is effected not by joining 

 their results together and making a sum total, but by 



