184 FUNCTIONS OF THE NERVOUS SYSTEM. 



be affected by the combination of the Recti muscles, there is no reason why 

 the other diagonal movements should not also be due to them. 



246. The action of the Superior Oblique muscle has been a patter of dis- 

 pute. Unlike the other muscles which arise from the back of the orbit, its 

 tendon is not inserted into the front hemisphere of the eye, but into a point 

 behind its vertical axis ; and we should, therefore, be led to suppose that its 

 operation is to move the pupil in a direction contrary to that in which its ten- 

 don is inserted ; that is to say, as its tendon passes, from its insertion towards 

 the trochea, upwards and somewhat inwards, we should suppose that, in 

 shortening, it would draw the back of the eyeball in that direction, and turn 

 the pupil in the contrary one namely, downwards and a little outwards. 

 This theory of its action is borne out by experiments, both upon the muscle 

 and the nerve which supplies it ; for by laying bare the muscle, without dis- 

 turbing the eyeball or the neighbouring parts, and then exercising gentle 

 traction upon it, so as to draw the tendon in the same manner as ordinary 

 contraction of the muscle would have done, the eyeball is turned downwards 

 and somewhat outwards. The same effect is produced when the Fourth 

 pair of nerves is irritated, either mechanically or by galvanism, after it has 

 been separated from the brain. On the other hand, the Inferior Oblique mus- 

 cle may be shown, by experiments upon itself, to roll the eye upwards and 

 inwards ; the inward movement is much greater than the outward movement 

 caused by the Superior Oblique ; so-that these two muscles are not exactly 

 antagonists of each other. 



247. The distribution of nerves to these muscles is very peculiar. The 

 Superior Oblique has a nerve for itself alone, namely, the Fourth pair ; this 

 was formerly called the Patheticus nerve, from its being supposed to govern 

 that rotation of the ball upwards and inwards, which gives a pathetic expres- 

 sion to the countenance ; but, as just shown, its real action is the reverse. By 

 Sir C. Bell, this nerve was considered as belonging to his Respiratory system; 

 ahd he endeavoured to show, that the sudden movement of the pupil upwards 

 and imvards, which takes place in coughing and sneezing, and the fixation of 

 the ball in a similar position during sleep, are due to its operation. The 

 ascertained action of the muscle, however, constrained him to suppose, that 

 the operation of the nerve was not to cause contraction, but relaxation, of this ; 

 by which the antagonist muscles might be free to occasion the movement. 

 This idea affords a remarkable exemplification of the degree in which theory 

 may, in some minds, usurp the place of observation. There is, as we have 

 formerly seen, no ground for the assumption of a system of Respiratory nerves 

 distinct from those forming the general Excito-Motor system, from which a 

 part of every motor trunk in the body is derived ; and the supposition that the 

 action of a nerve is ever to cause relaxation in a muscle, is at variance with 

 all sound physiological induction. In this particular instance, it is at once 

 refuted, by such experiments on the trunk of the nerve as those just adverted 

 to. It may further be added, in regard to this nerve, that there is no decided 

 reason to. believe that it contains any sensory fibres. Its distribution is entirely 

 restricted to the Superior Oblique muscle ; but since in this, as in other mus- 

 cles of the. orbit, there is certainly a degree of sensibility, as is experienced by 

 the fatigue to which the long fixation or violent straining of them gives rise, it 

 may be questioned whether the Fourth pair of nerves is entirely motor. Its 

 course within the cranium renders it very unlikely that this point can be 

 satisfactorily determined by experiment. Muhler states that a connection 

 exists between this nerve and the ophthalmic branch of the Fifth pair ; so that 

 it is not improbable that, as in other instances, its sensory endowments are 

 derived from this source. 



248. Tine same may be said of the Sixth pair, which is termed the M- 



