184 COMPARATIVE ANATOMY 



also Fig. 121). The ganglia belonging to the cerebro-spinal system 

 are shown in both figures, those belonging to the sympathetic in 

 Fig. 149 only. 



Nerves of the Eye-muscles. The oculomotor (III) 

 trochlear or pathetic (IV) and abducent (VI) nerves (Figs. 

 148 and 149) supply the muscles which move the bulb of the eye 

 as shown in the table on p. 181. The oculomotor arises from 

 the base of the mid-brain, and comes into secondary connection with 

 an oculomotor or ciliary ganglion which primarily belongs to the 

 sympathetic system. 



The trochlear nerve, although actually arising in the interior 

 of the ventral part of the mid-brain, appears externally on the 

 dorsal side of the anterior margin of the hind-brain (valve of 

 Vieussens p. 156). Primitively it contains sensory as well as motor 

 fibres, and these in Fishes and Amphibians supply the connective- 

 tissue of the eye and the endocranium. 



The abducent nerve, which arises far back on the floor of 

 the medulla oblongata, also probably contains mixed fibres in the 

 Anamnia. In the Anura it becomes closely connected within the 

 skull with the Gasserian ganglion of the trigeminal. 



Trigeminal Nerve. This is one of the largest of the cerebral 

 nerves. It arises from the ventro-lateral region of the anterior 

 part of the medulla oblongata by a large lateral sensory and a 

 small ventral motor root, has a large intra- or extra-cranial 

 Gasserian ganglion at the origin of the former and then, in 

 Fishes (Fig. 148), divides into two main branches, an ophthalmic 

 (including a superficial and a deep or prqfundus portion), and a max- 

 illo-mandibular : in most terrestrial forms (Fig. 149) the maxillary 

 and mandibular nerves arise separately. From the presence of these 

 three characteristic branches, often known as the first, second, and 

 third divisions of the trigeminal, its name is derived. It passes 

 out from the skull sometimes through a single aperture, and some- 

 times by two or even three distinct ones. 



The superficial branch of the first division is usually distinct in 

 Fishes and Dipnoans and probably also in Urodeles, and passes 

 dorsally over the eye-ball, the deep branch passing below the supe- 

 rior and anterior recti and superior oblique muscles. In other 

 Fishes and in higher forms the two branches appear to be united. 

 It supplies the integument of the forehead and snout as well as 

 the eye-ball, eye-lids and conjunctiva, branches apparently going to 

 the lachrymal glands when present : it is entirely sensory. A con- 

 section of the profundus with the ciliary ganglion arises second- 

 arily. 



The second division of the trigeminal, which is also a sensory 

 nerve and with which a sphenopalatine ganglion derived from the 

 sympathetic is connected, extends first along the floor of the 



