SCYLLIUM 45 



separate out again to their various destinations. The ventral 

 roots are distributed to the muscles of the myotomes along 

 the trunk and in the fins, the dorsal roots to the sense-organs. 

 In addition, each spinal nerve sends a branch to the sympathetic 

 ganglia, which are joined to one another by nerve-fibres which 

 form two longitudinal chains, one on each side of the dorsal 

 aorta. 



The nerve-fibres are medullated, except those of the 

 sympathetic system. 



Cranial Nerves and Head-Segmentation. — The cranial nerves 

 are of importance in unravelling the segmentation of the 

 head. Some of them are dorsal roots, and some are ventral, 

 but they never join to form mixed nerves as in the region 

 of the trunk. 



The olfactory nerve (No. I) is not a true nerve like the 

 others, for it is formed of the fibres produced by the cells of 

 the nasal epithelium which grow in to the forebrain (in the 

 manner characteristic of all the nerves in Amphioxus). The 

 small nervus terminalis which accompanies it for some distance 

 is also devoid of segmental significance. 



Similarly the optic nerve (No. II) is not segmental, for the 

 whole optic cup and stalk are really parts of the brain itself. 



The dorsal root of the first (or premandibular) segment is 

 the profundus (No. V i), which unfortunately disappears in 

 Scyllium. In the closely related Squalus it is present, and 

 runs forwards through the socket for the eye (the orbit) under 

 the superior oblique muscle, and innervates the skin of the 

 snout. 



Fig. 24. — Scyllium : dorsal view of a dissection of the cranial nerves, 

 from a drawing by Mr. B. W. Tucker. 



II, optic ; III, oculomotor ; IV, trochlear ; V md, mandibular branch 

 of trigeminal ; V and VII op, superficial ophthalmic branches of trigeminal 

 and facial ; V mx and VII b, maxillary branch of trigeminal and buccal 

 branch of facial ; VII p, palatine branch of facial ; VII h, hyomandibular 

 branch of facial ; VIII, auditory ; IX, glossopharyngeal ; X, vagus ; X b 2, 

 second branchial branch of vagus ; X /, lateral-line branch of vagus ; 

 X v, visceral branch of vagus ; ac, auditory capsule ; c, cerebellum ; e y 

 eye ; er y external rectus eye-muscle ; gs 1 and 5, first and fifth gill-slits ; 

 h, hypoglossal nerve ; io, inferior oblique eye-muscle ; ir, inferior rectus 

 eye-muscle ; ol, olfactory lobe ; s, spiracle ; sc, spinal cord ; so, superior 

 oblique eye-muscle ; sr y superior rectus eye-muscle ; t, terminalis nerve. 



