vii THE MEDULLA OBLONGATA 411 



We have already dealt fully with the trophic disturbances of 

 the eye after intracranial section of the 5th nerve (Chap. V. p. 330). 



The 6th cerebral nerve the Abducens or External Oculo- 

 motor is distributed solely to the external rectus muscle of the 

 eye. Its fibres arise from a small nucleus lying in the floor of 

 the fourth ventricle, immediately above the striae acusticae (Fig. 

 209). They issue in the form of a flattened bundle from the lower 

 edge of the pons, external to the pyramid. On paralysis of this 

 nerve the eyeball deviates inwards, owing to preponderance of the 

 antagonistic internal rectus muscle (convergent strabismus). 



The 4th or Trochlear nerve is the smallest of the cerebral 

 nerves. It arises in an elongated nucleus, a prolongation of the 

 nucleus of the 3rd nerve, which lies in the ventral grey matter 

 of the aqueduct of Sylvius at the level of the posterior quadri- 

 geminal bodies (Fig. 209). Its fibres bend around the aqueduct 

 and enter the superior medullary velum where they decussate 

 completely with those of the opposite side. After a long intra- 

 cranial course this nerve is distributed exclusively to the superior 

 oblique muscle. After section or paralysis of the trochlear the 

 outward and downward rotation of the eyeball is lost, so that 

 there is an upward and inward squint in the direction of the 

 nose owing to the unantagonised action of the inferior oblique 

 nerve. 



The 3rd or Oculomotor, the largest of the motor nerves to 

 the eyeball, arises in a nucleus in the grey matter of the aqueduct 

 of Sylvius, under the anterior quadrigeminal body (Fig. 209). 

 After passing through the tegmentum the nerve emerges at the 

 inner border of the cerebral peduncle at the upper margin of the 

 pons (Fig. 201). It innervates all the external muscles of the 

 eye except the external rectus and the superior oblique, which 

 are supplied by the 6th or 4th nerves, that is the superior, 

 inferior, and internal rectus muscles, the inferior oblique and the 

 levator palpebrae. The branch that innervates the inferior oblique 

 muscle sends fibres to the ciliary ganglion ; the short ciliary 

 nerves which spring from this penetrate the bulb in the form of 

 minute filaments, and innervate the sphincter iridis and ciliary 

 muscle. 



VIII. As a central organ the Medulla Oblongata has an 

 importance far greater than that of any other part of the nervous 

 system. When the bulb is severed by a transverse section from 

 the rest of the brain many important functions are preserved 

 which are immediately abolished when the section falls between 

 the bulb and the spinal cord. These are the functions of the 

 cerebral nerves which emerge from and have their centres in the 

 bulb, but in addition certain spinal functions become paralysed 

 because their dominating and co-ordinating centres lie in the 

 bulb. 



