640 STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLIV. 



Ascending Tracts. The most important of these are (a) the 

 tract of thefillet, and (b), the central tract of the cranial sensory nerves. 

 We must also remember the fibres that connect the cord to the 

 cerebellum ( (c) dorsal and (d) ventral cerebellar tracts), and (e) the 

 fibres that pass into the restiform body (inferior cerebellar peduncle) 

 from the olivary body. 



The superior cerebellar peduncles originate from the dentate nucleus of the 

 cerebellum ; we have already seen them converging to the middle line as they pass 

 upwards, and decussating in the mid-brain at the level of the posterior C. quadri- 

 gemina ; they terminate in the red nucleus of the tegmentum. Before they cross 

 they give off branches which Cajal describes as forming a descending cerebellar 

 bundle, which gives off collaterals to the motor nucleus of the fifth, the nucleus 

 of the seventh, and perhaps to the nucleus ambiguus. One bundle of fibres in the 

 superior cerebellar peduncle starts from cells in the optic thalamus, and conveys 

 impulses downwards to the cerebellum. 



Origins and Functions of the Cranial Nerves. 



Having now studied the internal construction of these parts, we 

 can take up more fully the origins and functions of the cranial nerves 

 which originate there. The olfactory nerve is connected to the 

 cerebrum, and will be considered with the sense of smell. The 

 optic nerve will be studied with vision, though it is, as we have seen, 

 immediately connected with the mid-brain. 



The third, fourth, and sixth nerves are wholly motor, and supply 

 the muscles of the eye. Gaskell discovered among the rootlets of 

 the third and fourth nerves the vestiges of a degenerated and function- 

 less ganglion, which indicates the previous existence of a sensory por- 

 tion of these nerves. 



The third nerve (motor oculi) arises in a group of nerve-cells in 

 the grey matter on the side of the Sylvian aqueduct underneath the 

 superior corpus quadrigeminum, and close to the middle line. The 

 anterior part of this nucleus is composed of small cells from which 

 small nerve-fibres originate for the oiliary muscle and sphincter of 

 the iris (intrinsic muscles of the eyeball). These fibres correspond 

 to the visceral fibres of a spinal nerve, and, like them, have a cell 

 station, namely, in the ciliary ganglion. The posterior part of the 

 nucleus is composed of larger cells, and these give rise to larger fibres 

 which supply the following extrinsic eye-muscles : superior rectus, 

 inferior rectus, internal rectus, inferior oblique and levator palpebrae. 



The fourth nerve (trochlear) takes origin from the grey matter 

 immediately below the centre of the third, but slightly more lateral 

 in position. It is situated underneath the inferior corpus quadri- 

 geminum. It supplies the superior oblique muscle of the opposite 

 eyeball. 



The sixth nerve (abducens) arises from a centre beneath the 

 eminentia teres in the upper part of the floor of the fourth ventricle 

 near the middle line. It supplies the external rectus. 



