The Nervus trochlearis (fourth pair), if present with an independent origin, is always thin, taking its origin on the upper surface of the brain from the groove between lobus opticus and cerebellum; it goes to the Musculus obliquus superior of the eye.
C. Nerves taking their origin from the Medulla oblongata (Figs. [41–45]).
The Nervus abducens (sixth pair) issues on the lower surface of the brain, taking its origin from the anterior pyramids of the Medulla oblongata, and supplies the Musculus rectus externus of the eye, and the muscle of the nictitating membrane of Sharks.
The Nervus trigeminus (fifth pair) and the Nervus facialis (seventh pair) have their origins close together, and enter into intimate connection with each other. In the Chondropterygians and most Teleostei the number of their roots is four, in the Sturgeons five, and in a few Teleostei three. When there are four, the first issues immediately below the cerebellum from the side of the Medulla oblongata; it contains motory and sensory elements for the maxillary and suspensorial muscles, and belongs exclusively to the trigeminal nerve. The second root, which generally becomes free a little above the first, supplies especially the elements for the Ramus palatinus, which sometimes unites with parts of the Trigeminal, sometimes with the Facial nerve. The third root, if present, is very small, and issues immediately in front of the acustic nerve, and supplies part of the motor elements of the facial nerve. The fourth root is much stronger, sometimes double, and its elements pass again partly into the Trigeminal, partly into the Facial nerve. On the passage of these stems through the skull (through a foramen or foramina in the alisphenoid) they form a ganglionic plexus, in which the palatine ramus and the first stem of the Trigeminus generally possess discrete ganglia. The branches which issue from the plexus and belong exclusively to the Trigeminus, supply the organs and integuments of the frontal, ophthalmic, and nasal regions, and the upper and lower jaws with their soft parts. The Facial nerve supplies the muscles of the gill-cover and suspensorium, and emits a strong branch accompanying the Meckelian cartilage to the symphysis, and another for the hyoid apparatus.
The Nervus acusticus (eighth pair) is strong, and takes its origin immediately behind, and in contact with, the last root of the seventh pair.
The Nervus glossopharyngeus (ninth pair)[12] takes its origin between the roots of the eighth and tenth nerves, and issues in Teleostei from the cranial cavity by a foramen of the exoccipital. In the Cyclostomes and Lepidosiren it is part of the Nervus vagus. It is distributed in the pharyngeal and lingual regions, one branch supplying the first branchial arch. After having left the cranial cavity it swells into a ganglion, which in Teleostei is always in communication with the sympathic nerve.
The Nervus vagus or pneumogastricus (tenth pair) rises in all Teleostei and Palæichthyes with two discrete strong roots: the first constantly from the swellings of the corpora restiformia, be they thinner or thicker and overlying the sinus rhomboidalis, or be they developed into lateral plaited pads, as in Acipenser and Chondropterygians. The second much thicker root rises from the lower tracts of the medulla oblongata. Both stems leave the cranial cavity by a common foramen, situated in Teleosteous fishes in the exoccipital; and form ganglionic swellings, of which those of the lower stem are the more conspicuous. The lower stem has mixed elements, motory as well as sensory, and is distributed to the muscles of the branchial arches and pharynx, the œsophagus and stomach; it sends filaments to the heart and to the air-bladder where it exists. The first (upper) stem forms the Nervus lateralis. This nerve, which accompanies the lateral mucous system of the trunk and tail, is either a single longitudinal stem, gradually becoming thinner behind, running superficially below the skin (Salmonidæ, Cyclopterus), or deeply between the muscles (Sharks, Chimæra), or divided into two parallel branches (most Teleostei): thus in the Perch there are two branches on each side, the superficial of which supplies the lateral line, whilst the deep-seated branch communicates with the spinal nerves and supplies the septa between the myocommas and the skin. In fishes which lack the lateral muciferous system and possess hard integuments, as the Ostracions, the lateral nerve is more or less rudimentary. It is entirely absent in Myxinoids, but the gastric branches of the Vagus are continued, united as a single nerve, along the intestine to the anus.
No fish possesses a Nervus accessorius. Also a separate Nervus hypoglossus (twelfth pair)[13] is absent, but elements from the first spinal nerve are distributed in the area normally supplied by this nerve in higher vertebrates.
The number of Spinal nerves corresponds to that of the vertebræ, through or between which they pass out. Each nerve has two roots, an anterior and posterior, the former of which has no ganglion, and exclusively contains motor elements. The posterior or dorsal has a ganglionic enlargement, and contains sensory elements only. After leaving the vertebral canal each spinal nerve usually divides into a dorsal and ventral branch. The Gadoids show that peculiarity that each of the posterior roots of some or many of the spinal nerves possesses two separate threads, each of which has a ganglion of its own; the one of these threads joins the dorsal and the other the ventral branch. In fishes in which the spinal chord is very short, as in Plectognaths, Lophius, the roots of the nerves are extremely long, forming a thick Cauda equina. The additional function which the (five) anterior spinal nerves of Trigla have to perform in supplying the sensitive pectoral appendages and their muscles has caused the development of a paired series of globular swellings of the corresponding portion of the spinal chord. A similar structure is found in Polynemus.