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Show 1 906.] NERVES OF CHLAMYEOSi LACHUS ANGUINEUS. 9 6 7 surface of the eyeball without, however, being attached to it. The Torpedo, for example, has an eye-stalk which differs considerably from that of Chlamydoselachus, inasmuch as it is attached to the cranium in a more anterior region of the orbit, is permanently fixed to the eyeball, and does not serve as a basis of attachment for any of the eye-muscles. The eye-stalk of Torpedo keeps the eye permanently protruded, while in Chlamydoselachus its function is two-fold-to prevent the eye sinking too far into the socket, and to supply a more lateral basis for the attachment of the muscles. The External Rectus muscle consists of two parts which have separate origins and insertions. The two parts of the muscle are, however, not independent, for they are united by strands of muscle-fibres passing from one to the other. Division A (Plate LXIX. figs. 4, 5, 6, R.Ext.A) of the muscle is the smaller. Its origin is on the outer part of the optic stalk, its insertion on the posterior surface of the eyeball, i. e, the normal insertion of an undivided externus rectus. Division B (R.Ext.B) is twice as large as A. Its origin is from the basis cranii, just anterior to the auditory capsule and beneath the foramen for the ganglia of the trigeminal and facial nerves, as well as along the proximal part of the optic stalk. The insertion is on the dorsal side of the eyeball, somewhat more external than that of the rectus superior, which it partly overlaps. From the position of its origin and insertion, division B (R.Ext.B) must be considered as a secondary or derivative portion of the rectus externus. The innervation supports this view. The main portion of the sixth nerve passes over division B to end in small branches in A, whilst several small, but side branches are given oft" to supply B. This secondary muscle was probably split oft' from a normal external rectus and acquired a more proximal origin, to aid the rectus superior and the superior oblique in tilting the eye upwards. It seems almost to form a pair with this latter muscle. Another modification in connection with this rotation is the shortening of the rectus superior and the lengthening of the rectus inferior, the latter being almost three times the length of the former. The Superior and Inferior Oblique muscles are both long and narrow. Their attachment is to the median anterior wall of the orbit immediately to the inner side of the orbito-nasal foramen. The superior oblique is attached a little more dorsally than its fellow, and is inserted nearly in the mid-dorsal line of the eyeball, close to the rectus superior and the B (R.Ext.B) division of the rectus externus. The inferior oblique is inserted nearly in the mid-ventral line of the eyeball near the rectus inferior. The Patheticus or fourth nerve has its origin in the usual position, on the dorsal side of the brain, between the optic lobe and the cerebellum ; it runs outwards and upwards to pass by a special foramen into the orbit, where it reaches the superior oblique muscle after a sinuous course. The Ocido-motor arises from the ventral surface of the brain P roc. Z ool. S oc.-1906, No. LXIV. 64 |
