456 Grace B. Watkinson 
this muscle near its origin on the interorbital septum and breaks 
up into radiating branches among the musele fibres. 
IV. Nervus trochlearis arises from a single root on the dorsal 
surface of the mid-brain in its posterior region. From its point of 
origin it turns immediately lateral and, Iying close to the surface 
of the mid-brain, bends ventrally and anteriorally in a half eirele 
until it reaches a point on the ventral surface of the brain, somewhat 
posterior and lateral to the point of origin of nervus optieus. Slightly 
dorsal and posterior to the opening for the latter nerve, nervus 
trochlearis pierces the membranous wall of the brain cavity. Continu- 
ing its course directly anterior, it passes between the membrane 
wall of the brain cavity on its inner surface, and muse. rectus 
superior on its outer surface, and reaches musc. obliquus superior, 
where it divides into a large number of fine branches before enter- 
ing the posterior border of this muscle about equi-distant from its 
origin and insertion. Its course in the orbit is dorsal and parallel 
to ramus ophthalmicus of V., but whereas IV. passes external to 
musc. rectus superior, ramus ophthalmicus V. passes median to it. 
VI! Nervus abducens arises as a single nerve from the 
ventral surface of the medulla oblongata near the middle line and 
somewhat posterior to the point of origin of nervus trigeminus. The 
nerve remains for a short distance within the brain cavity, taking 
an anterior direction, and then emerges through a special foramen 
in the basi-sphenoid bone, dorsal to the basi-pterygoid process. Upon 
emerging from the foramen, the nerve pierces musc. retractor oculi, 
close to its origin, and breaks up into several branches which are 
distributed as follows: 
1. Ramus ad musculum retractorem oculi is a fine branch which 
breaks up among the fibres of muse. retraetor oculi. From the point 
where this branch arises, a larger, but also fine nerve, 
2. Ramus ad musculum bursalem is given off to muse. bursalis, 
which lies immediately ventral to muse. retractor oculi at this point. 
The remaining portion of nervus abducens, 
3. Ramus ad musculum reetum lateralem pierces muse. bursalis 
and splits into a number of fine fibres, one of which is considerably 
larger than the others. These all run parallel, close together, like 
a single nerve, over the ventral surface of musc. bursalis, to the 
ı VI. is described before V. to group the optie muscle nerves according 
to their interrelations. 
