226 SIR CHARLES BELL ON THE THIRD PAIR OF NERVES. 



and of the trunk in consequence. The whole active machinery of the frame is in 

 most intimate union. 



If we consider the office of the eyes — the necessity for their free and uncon- 

 trolled motions — their sole dependence on the sensation in the retina — we shall 

 be ready to acknowledge that they must be relieved from the train of concate- 

 nated actions which occurs in every movement of the frame besides. Thus unem- 

 barrassed, the recti muscles of the eye are suited to that s>Tnpathy, and that ex- 

 traordinary minuteness of accordance with the state of sensation in the retina, 

 which are necessary to vision. 



Resuming the consideration of the columns of motion and sensation : — they 

 may be traced up into the cerebrum ; the great cms cerebri is formed of these 

 combined columns, as each diverges, and is lost in the hemisphere. The column 

 of motion is still anterior, so that the anterior part of the crus belongs to mus- 

 cular action, and the posterior part to sensation. 



Now, considering that the essential difference in these coliunns is this — that 

 in the anterior, the course of impulse is outward from the sensorium commune, 

 and inwards, or towards the sensorium in the posterior, the origins of aU the 

 nerves must conform, or the system is overthrown. 



We look with increasing interest on the roots of nerves, as conclusive on this 

 subject. 



The first nerve, the olfactory, being traced backwards, divides into three 

 roots, and disperses in the inferior part of the anterior lobe of the cerebrum, 

 without the intervention of the columns, and without interference with them. 



The second or optic nerve, though in du-ect contact with the column of mo- 

 tion, takes no origin from it ; but in a long and circuitous course, under the name 

 of Tractus opticus, turns round the crus cerebri, to fall into the rear or back part 

 of the column of sensation ; and so, is combined with those nerves, through which 

 the impulse is towards the sensorium, or inwards. 



Even on proceeding so far, it is fair to infer that a nerve of sense gives off 

 no branch — that it can communicate no endo\\Tnent — that it is unequal to con- 

 fer either motion or sensation, or any property but that which is its limited of- 

 fice. 



Further, if we take the pen, and trace the nerves of sense — the olfactory, 

 optic, auditory, and gustatory — we shall find them all avoiding the anterior co- 

 lumn, and fallhig into the back part of the sensitive column ; so that already in 

 part we perceive the cause of iri-egulaiity in the base of the brain, in the neces- 

 sity of the nerves of sense avoiding the anterior column, to gain the posterior or 

 sensitive column. 



We come next to the Third Nerve. This nei-ve is distinguished from all 

 others ; its origin is peculiar, and its distribution limited. By universal consent, 

 it has got the name of motor oculi, being distributed to the voluntai-y muscles of 



