TRANSACTIONS OF THE SECTIONS. 451 



from these facts, to suppose that the whole nervous system, if suf- 

 ficiently expanded, and divested of all coverings, would be found 

 too tender to give any resistance to the touch, too transparent 

 to be seen, and probably would entirely escape the cognizance 

 of all our senses. Consistently with this view of the matter, the 

 author thinks that we can hardly take upon us to say that the 

 simplest animals, and even plants, may not have some modifi- 

 cation of sentient substances incorporated in their structure, in- 

 stead of being collected, as in the higher classes of animals, into 

 palpable membranous cords and filaments. 



The term plexus has been generally employed to signify an 

 interweaving or crossing of filaments ; but Dr. Macartney is 

 satisfied that there is an actual union or intermixture of sub- 

 stance in both the plexuses of the brain and of the other parts 

 of the nervous system. He has discovered that the roots of 

 the spinal nerves, instead of being connected with the medulla 

 by mere contact or insertion, as hitherto supposed, actually 

 enter into the composition of the filaments of the spinal marrow, 

 and that these roots of nerves (as they are called) form com- 

 munications with each other within the substance of the me- 

 dulla. With regard to the cerebral nerves also, it can be 

 shown that they are continuous with the cerebral plexuses in 

 their immediate neighbourhood. 



Many of the communications formed between the right and 

 left sides of the nervous system are well known, such as the 

 commissures of the brain, the crossing white filaments of the 

 spinal marrow, the decussation of the pyramids, and the intei'- 

 change of the two optic nerves in fishes. The author has 

 found so many communications to exist between the origins of 

 the nerves on the right and left sides of the body, that he is 

 disposed to believe it to be a general fact. The optic nerves 

 in the human subject do not decussate, as some have supposed, 

 but form a very intricate plexus where they come into contact. 

 This mode of conjunction accounts for atrophy of the tractus 

 opticus being in some instances found on the same side, and at 

 others on the opposite side to that of the eye affected with 

 blindness. 



The facts already observed would justify the opinion that 

 the sentient substance is in no place distant or isolated ; that 

 it is essentially one and indivisible; and consequently the ner- 

 vous system differs from all the other systematic arrangements 

 in nature. 



It appears to the author that this view of the sentient system 

 will alone serve to explain the numerous sympathies which 

 exist in animal bodies, the occurrence of disease in the higher 



<-:> n. 9. 



