1906. J NERVES OF CHLAilYEOSiiLACHUS AXGUINEUS. 967 



svirface of the eyeball without, however, being attached to it. 

 The Torpedo, for example, has an eye-stalk which clifTers con- 

 siderably from that of CJilamydoselachus, inasmuch as it is 

 attached to the ci'anium 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 ej^e-muscles. The eye-stalk of 

 Torpedo keeps the eye permanently protruded, while in Cldamy- 

 doselachus 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 stiunds 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 insei-tion on the posterioi* 

 sui'face of the eyeball, i. e. the noimal insertion of an undivided 

 externus rectus. Division B (R.Ext.B) is twice as large as A. Its 

 origin is from the basis cianii, 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 

 off to supply B. This secondary muscle was probably split off 

 fi-om a normal external rectus and acquired a more proximal 

 origin, to aid the rectvis 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 Siqjerior and Tnferior Ohliqae 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 eye- 

 ball, close to the rectus superior and the B (R.Ext.B) division of 

 the rectus externus. The inferior oblique is inserted neai-ly in 

 the mid-ventral line of the eyeball near the rectus inferior. 



The Fatheiicus 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 Oculo-motor arises from the ventral surface of the brain 

 Pkoc. Zool. Soc— 1906, Xo. LXIY. 64 



