ANATOMICAL STUDIES ON THE CRANIAL NERVES AND CRANIAL GANGLIA OF ANGUILLA ANGUILLA (LINNAEUS, 1758) EYE MUSCLE NERVES

The aim of this study is to analyze the ocular muscle nerves and the ciliary ganglion of the anguillid fish Anguilla anguilla . The ocular muscle nerves comprise the nervi oculomotorius, trochlearis and abducens. The oculomotor nerve leaves the cranial cavity together with the nervus abducens through a common foramen. It innervates four eye muscles; rectus superior, rectus inferior, rectus medialis and the obliquus inferior muscles. It carries pure somatic motor fibers and visceromotor (parasympathetic) ones. There is no ciliary ganglion but, there is one ciliary nerve arising from the nervus oculomotorius. The trochlear nerve has its own foramen and carries pure somatic motor

Behaviorally,European eels are essentially a solitary species, there is no evidence that any form of schooling is present (Suzuki et al, 2003). They migrate to various regions during different stages of their life (Deelder, 1970) and they are active mainly during the day (Deelder, 1970;Tsukamoto et al., 2003). European eels sense the environment using their sense of taste (Sola and Tongiorgi, 1998). There is little if any documentation of social communication between eels, although they have strong sense of olfaction, that is used most probably for homing purposes (Deelder, 1970).
European eels have completely different diets during different life stages, they are carnivore, insectivore, eats noninsect arthropods molluscivore, eats other marine invertebrates and scavenger (Sinha and Jones, 1975). European eels are reported to leap out of the water during the winter and feed on terrestrial invertebrates (Deedler, 1970).
Traditionally anguilliforms, the largest order of elopomorphs, comprise three suborders (Robins, 1989;Nelson, 2006): the Anguilloidei (freshwater eels); the Congroidei (short tail eels) and the Muraenoidei (false morays). Anguilliforms have traditionally been thought to be closely related to the Saccopharyngiformes (gulper eels and allies), a group formed by four families of deep-sea fishes (Nelson, 2006). Taxonomically, both morphological (Forey et al., 1996) and molecular studies of elopomorph interrelationships based on mitochondrial sequences (Wang et al., 2003;Inoue et al., 2004& 2010 were done. The recently described Protanguillidae (Johnson et al., 2012) brings the total diversity of Anguilliformes to 937 species spread across 20 families (Wiley and Johnson, 2010;Froese and Pauly, 2012). Previous phylogenetic studies of anguilliform relationships based on morphological data alone (Forey, 1973;Nelson, 1973;Greenwood, 1977;Patterson and Rosen, 1977;Robins, 1989;Forey et al., 1996) have been unable to resolve the relationships among the three anguilliform suborders. These relationships are illustrated successfully through mitochondrial analyses (Wang et  More recently, Johnson et al. (2012) erected the new family Protanguillidae on the basis of the species Protanguilla palau, a recently discovered, enigmatic anguilliform. This species possesses a number of morphological traits that are absent in most living eels including collar-like gill openings, a pseudobranch, a premaxilla, unfused symplectic, and metapterygoid (Johnson et al., 2012).
Phylogenetically, it has been known that the Indo-Pacific region was the origin of the speciation of the freshwater eels of the genus Anguilla (Aoyama & Tsukamoto, 1997;Lin et al., 2001). The ancestors of both temperate and tropical eels originated from the Indo-Pacific region, particularly in the archipelagic area of Indonesia, Malaysia and the Philippines.
Functionally, the extrinsic eye muscles are the effector organs for voluntary and reflexive movements of the eyes (Dakrory et al., 2018). Spencer and Porter (2006) stated that, the coordinating activity of the six extraocular muscles, must be accomplished with high precision as the fovea, subtends a very small angle of visual space. Extraocular muscles are innervated by motoneurons in the oculomotor, trochlear, and abducens nuclei (Spencer and Porter, 2006).

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The disposition and innervation of the extraocular (extrinsic) muscles is a highly conserved and presumably ancient system among the vertebrates (Branson, 1966;Isomura, 1981;Young, 2008). They appear in the lower vertebrates in essentially the same form, as in man (Neal, 1918). Indeed their number and their nerve relations are the same in man as in the dogfish (Young, 2008).
There is a consistency among living and extant vertebrates, regarding to the innervation pattern by the three eye muscle nerves (Fritzsch et al. 1990). Although, of the homology in the innervation pattern of the ocular muscles among vertebrates; yet, the details of the eye muscle nerves (the nerve"s origin, roots, intracranial pathway, location of exit from the cranium, extracranial course, anastomosis, relevant branches and to its innervations, ganglion (Ganglia) requer further investigation (Dakrory et al., 2018).
Due to the absence of lens muscles, accommodation is not possible. Furthermore, the eel does not have a corpus chorioidae nor a musculus ciliaris (Stramke, 1972). Again, Biometric studies on the nucleus of the oculomotor nerve have shown that the yellow eel probably makes relatively little use of its eyes (Kirsche, 1966). All other species of fish studied, including the burbot ( Lota Iota ) which is well known as a 'non-visual' animal-have a larger nucleus than the eel. Just before its spawning migration the eel has, in comparison with other fish, relatively small eyes (Wunder, 1936). This suggests a similarly reduced visual capacity just before the eel's marine period. With metamorphosis into a silver eel the diameter of the eye increases in size (Matschenis, 1965) ; this growth should be accompanied by an increase in the efficiency of the eye.
The cranial nerves are an important collection of nerves, where, they connect the brain with all the important centers of perception of the outer surface of the head, as well as the inner surface of the buccopharyngeal and other visceral regions. They travel directly to the brain rather than through the spinal cord. The cranial nerves have several functions vital for day-to-day life (Shaheen, 1987;Dakrory, 2000).
Of the first investigations done on the cranial nerves of Osteichthyes were those of Stannius (1849) and Goronowitsch (1888) on Acipenser ruthenus; these classical studies are still useful to investigators. Dakrory (2000) on Ctenopharyngodon idellus, Dakrory (2003)  It is quite evident from the above historical reviews that there are numerous works on the cranial nerves of bony fishes, but few ones on Anguillid fishes which is an interesting group among teleosts. Thus, it was recommended that a detailed microscopic investigation on the eye muscle nerves in Anguilla anguilla belonging to family Anguillidae will be very fruitful. This investigation will highlight the nerves" origin, roots, intracranial pathway, exit from the cranium, the extracranial course and their innervation. In addition, the current study will give a concise and comprehensive explanation of the important characters of the nerves including their components, anastomosis, correlated ganglion (Ganglia), and relevant branches.

Material and Methods:-
The species chosen for this study is the European eel, Anguilla anguilla which is a fresh water bony fish belongs to the FamilyAnguillidae. This family is characterized by the elongate body, numerous vertebrae, small elliptical scales which are difficult to see casually, a small and elliptical gill opening just in front of the pectoral fin base, very long dorsal and anal fins confluent with a reduced caudal fin, a terminal mouth with the lower jaw projecting a little, small teeth in several rows on the jaws and palate, the dorsal fin origin well behind the pectoral fin level but in front of the anus level, no pelvic fins, and by a suite of osteological characters (Deelder, 1970;Nelson, 1994;Coad, 2016).
The eel shape is characteristic along with the long and spineless dorsal and anal fins and the absence of pelvic fins, the scales are small, elliptical in shape and embedded in the skin, the lateral line is distinct (Nelson, 1994;Coad, 2016). Fish approaching sexual maturity develop very large eyes, the olfactory organs atrophy, the lateral line becomes more conspicuous, a tougher and thicker skin develops, and the colour changes (Coad, 2016). Colour is variable but the back is usually grey-brown, olive-brown, brownish-green, yellowish, or black and the belly is whitish to yellowish. The dorsal fin is dark, other fins are yellowish. The iris is yellow. This yellow or green eel 40 stage changes to the silver or bronze eel at maturity (Coad, 2016). The mature fish is turn green, yellow or brownish in color darker on the back, has silvery or bronze to coppery flanks and belly, a black pectoral fin and a clear contrasting black lateral line, as well as enlarged eyes and lose their ability to feed, (Van Ginniken and Van Den Thillhart, 2000; Coad, 2016).
Female eels are generally larger than males (Dekker, van Os and van Willigen, 1998). The maximum published length of a European eel was 133 cm (Dekker et al., 1998).
The geographic range of adult European eels includes the English Channel and coasts of the Mediterranean Sea and northern Atlantic Ocean from Iceland to Mauritania (Ringuet et al., 2002). Their range also encompasses the Baltic and North Seas, as well as all accessible continental or coastal hydrosystems (Ringuet et al., 2002). In the early spring months, European eels migrate to the Sargasso sea for breeding. Larvae are hatched from the Sargasso Sea and can also be found along the coast of Europe. Silver (juvenile) stage eels of Anguilla anguilla live in tributaries along the European coast (Ringuet et al., 2002;Ringuet et al., 2002;Tsukamoto et al., 1998). Depending on the life stage of the individual eel, European eels can be found in marine, freshwater, and brackish aquatic environments. Typically, the European eel is found in depths of 0-700 m, most often on the floor of the ocean or river in which it is living (Tsukamoto et al., 1998).
The 5 youngs of the chosen species will be collected from fish farm at Ras El-Bar City, Damietta Governorate, Egypt at July 2010. In the lab, the heads together with the branchial region of youngs were cut and immediately fixed in aqueous Bouin`s solution for 24 hours.
The specimens were washed several times to remove the excess Bouin`s solution. Decalcification is necessary before sectioning and staining for the specimens, this will be carried out by placing the heads in EDTA solution for about 40 days with changing the solution every 4 days.
After that, the heads will be prepared for blocking and sectioning. Two specimens were sectioned transversely at 10 micron by microtome. One of the two serial sections will be mounted on slides and stained with hematoxylin and eosin. The serial sections will be drawn by the aid of a projector microscope. From these drawings an accurate graphic reconstruction for the brain, eye and the eye muscle nerves will be made in a lateral view. Also, parts of certain sections will be photomicrographed to demonstrate the relation of these nerves with the other cranial structures.

Nervus Oculomotorius
In the current study, the oculomotor nerve originates from the mid-lateral side of the mesencephalon by a single root (Figs. 1 &2, RO. III). It runs forwards, for a somewhat long distance, in a depression on the lateral side of the brain, at the level of Gasserian ganglion. After this forward course, the oculomotor nerve leaves the depression and runs within the cranial cavity, passing lateral to the brain, medial to the anterior end of Gasserian ganglion of the nervus trigeminus and ventromedial to the trochlear nerve.
Thereafter, it passes medial and dorsal to the cranial wall and lateral to the brain. It then shifts anteromedially extending ventral to the brain (cerebral hemisphere), dorsolateral to the nervus abducens and dorsal to the internal jugular vein.
More forwards, the nervus oculomotorius continues penetrating the meninx primitiva passing lateral to the nervus abducens and medial to the internal jugular vein. After this course, it leaves the cranial cavity through a common foramen together with the abducent nerve. This foramen (Fig. 3, COM. F) is located between the lateral edge of the parasphenoid bone medially and the basisphenoid bone laterally.
Extracranially, the nervus oculomotorius runs forwards passing ventrolateral to the cranial wall, dorsolateral to the nervus abducens, ventomedial to the internal jugular vein and medial to the maxillo-mandibular trunk of the nervus trigeminus. Shortly anterior, the nervus oculomotorius divides into a dorsal ramus superior (

Ramus Superior
After its separation from the nervus oculomotorius, the ramus superior (Figs. 1 &4, R.SP.III) extends anteriorly in the dorsolateral direction passing dorsal to the ramus inferior of the nervus oculomotorius and ventral to the rectus superior muscle. Shortly anterior, the ramus superior enters the latter muscle from its ventrolateral side, where it distributes and ends between its fibres (Figs. 1&5, R. SP. III).

Ramus Inferior
Immediately, after its separation from the nervus oculomotorius, the ramus inferior (

Nervus Trochlearis
In anguillid species studied, the nervus trochlearis arises from the lateral side of the mid-brain just anterior to the origin of the nervus trigeminus by a single small root (Figs. 1 &8, RO.IV). After its origin, this nerve extends anteriorly within the cerebral cavity passing lateral to the brain and dorsomedial to Gasserian ganglion. After a long forward course, it becomes dorsomedial to the ganglion of the nervus trigeminus, ventromedial to the anterodorsal lateral line nerve and lateral to the brain (Fig. 2, N Forwards passing lateral to the brain and medial to the cranial wall. After a considerable course in the orbital region, it leaves the cranial cavity by penetrating the meninx primitiva through its own foramen (Fig. 9, F. IV). Extracranially, the 45 nervus tochlearis runs forwards passing lateral to the cranial wall and ventomedial to the ramus ophthalmicus lateralis and trigeminus and dorsomedial to the eyeball (Fig. 10, N. IV).
Reaching the mid-way of orbital region, the nervus trochlearis continues ventral to the supraorbital lateral line canal and the ramus ophthalmicus superficialis trigeminus and lateralis. Finally, the nervus trochlearis enters and ends between the fibres of the obliquus superior muscle (Figs. 1 &7, N. IV).  abducens becomes ventromedial and then medial to the nervus oculomotorius, lateral to the ramus palatinus of the nervus facialis and dorsal to the cranial wall. More and more cephaled, the nervus abducens leaves the cranial cavity by piercing the meninx primitiva together with the nervus oculomotorius through a common foramen (Fig. 3, COM. F). Extracranially, the nervus abducens extends anterolaterally for a short distance passing ventral to the nervus oculomotorius and dorsal and medial to the origin of the rectus lateralis muscle.
In the present investigation, the oculomotor nerve gets its exit from the cranial cavity together with the abducens nerve throught one and the same foramen, the common foramen. This foramen was found to be located between the parasphenoid bone and the basisphenoid bone. This foramen is locatedin the pleurosphenoid bone as reported by Dakrory (2000) in Ctenopharyngodon idellus, Taha (2010) in Hypophthalmichthys molitrix and by Ali (2012) in Liza ramada. Different localities for the oculomotor foramen were described in other fishes by some authors. It was found in the lateral ethmoid bone in Amphipnous cuchia (Saxena, 1967), in the basisphenoid bone in Trichiurus lepturus (Harrison, 1981), in the orbitosphenoid bone in Polypterus senegalus (Piotrowski and Northcutt, 1996) or surrouned by the pleurosphenoid bone in Ctenopharyngodon idellus (Dakrory, 2000) and in Hypophthalmichthys molitrix (Taha, 2010). However, Ray (1950) described a special oculomotor foramen in the membranous cranial wall of the orbitotemporal region in Lampanyctus leucopsarus, while Srinivasachar (1956) described this foramen in the preoptic root of the orbital cartilage in Ailia.This foramen is found between the prootic and the pleurosphenoid bones by Ali (2005) in Tilapia zillii and that ofMattar(2012) and Dakrory et al. (2012) in Gambusia affinis affinis.
In the jawless fishes, Johnels (1948) described an optic fenestra through which emerge the optic and the three eye muscle nerves from the cranial cavity in Petromyzon. However, Jollie (1968) described a separate oculomotor foramen in lampreys. The author added that this may confluent with a large optic foramen located anterior to it. On the other hand, the three eye muscle nerves along with their muscles are lacking in the hagfishes (Jollie, 1968;Northcutt, 1985;Wicht, 1996). Fernholm and Holmberg (1975) stated that the hagfishes have relatively small eyes and there was tendency toward eye reduction. Parallel with these results, Wicht (1996) recorded that the external eye muscles as well as the accompanying nerves are entirely lacking in all species of hagfishes even in that retained relatively large and differentiated eyes as in Eptatretidae.
It is clear from the detailed anatomical study of the head serial sections of Anguilla anguilla that the nervus oculomotorius carries special somatic motor fibres andsmall components of general visceromotor ones.
The investigation of the serial sections of the anguillid species studied has not succeeded in demonstrating a ciliary ganglion; although, it elucidates the presence of a nerve arising from the nervus oculomotorius having the same characteristicsof or homologous to the ciliay nerve. A completely lackingof the ciliary ganglion was, also mentioned in Salmo and Cyclothone acclinidens (Gierse, 1904), in Dipnoi (Jenkin, 1928) and in the ray fish Dasyatis Rafinesque (Chandy, 1955). Again, Burr (1933) denied the presence of the ciliary complex in Opistroproctus soleatus, but he found a ganglion on the third cranial nerve.Therefore, this reflects the fact that the eye is not well developed in the studied fish and this fish not depends on vision during its feeding. So, it may be considered as bottom fish. A similar observation was found also by Young (1988), Ali (2005) in Tilapia zillii, Taha (2010) in Hypophthalmichthys molitrix andin Gambusia affinis affinisMattar (2012) andDakrory et al. (2012). Also, Stramke (1972) mentioned that, due to the absence of lens muscles, accommodation is not possible; furthermore, the eel does not have a corpus chorioidae nor a musculus ciliaris.Thus, the presence of this nerve appears to be controlling the smooth muscles of the choroid and the iris blood vessels. The same was mentioned by Radzimirska (2003) in the domestic turkey.Again, Biometric studies on the nucleus of the oculomotor nerve have shown that the yellow eel probably makes relatively little use of its eyes (Kirsche, 1966).
In most amphibians, the trochlear nerve exits from the cerebral cavity through a special foramen (Herrick, 1894;Norris, 1908;Stadtmüller, 1925;Aoyama, 1930;De Beer, 1937;Paterson, 1939;Sokol, 1977Sokol, & 1981Mostafa and Soliman, 1984;Shaheen, 1987;Trueb and Hanken, 1992;Haas, 1995;Dakrory, 2002). In most cases, this foramen is found in the orbital cartilage. However, Van-Eeden (1951) mentioned that the trochlear foramen, in Ascaphus truei, does not pierce the orbital cartilage at all; but the nervus trochlearis passes over its margin. This author added that Ascaphus truei shares this feature with some Urodela. Sokol (1977) reported that the trochlear foramen in the anuran Pipa cadvalhoi is very small and presumably lies above the oculomotor foramen as in other tadpoles. In this respect, the trochlear foramen Amblystoma punctatum (Herrick, 1894) and Necturus (McKibben, 1913) was found to be located in the parietal bone. Sheil (1999), dealing with Pyxicephalus adspersus, stated that the trochlear foramen is located ventral to the lamina perpendicular to the frontoparietal bone or pierces it. On the other hand, a large optic-prootic foramen, for the exit of the nervi opticus and facialis was described by Trueb and Cannatella (1982) in Rhinophrynus dorsalis and Pipa pipa. Haas and Richard (1998) revealed that the nervi opticus and trochlearis leave the cranial cavity together through a large foramen opticum in Boophis.
It is clear from the detailed anatomical studies of the head serial sections of Anguilla anguillathat the nervus trochlearis carries special somatic motor fibres.
In jawless fishes, the nervus abducens emerges from the cerebral cavity together with the optic, oculomotor and trochlear nerves, through the optic fenestra (Johnels, 1948). On the other hand, Jollie (1968) reported that in lampreys the nervus abducens passes out the cranium together with the trochlear and trigeminal nerves through a large opening in the lateral side of the skull. However, Kent (1978) stated that lampreys seem to lack an abducens nerve or may be represented by small bundle emerging from the hind brain on the anterior surface of the trigeminal nerve.
In this study, the nervus abducens shows no connection with other cranial nerves. This is the case mentioned in many fishes (Allis, 1903;Bal, 1937;Ray, 1950 Almalki, 2017). However, two connections between the nervus abducens and the profundus nerve were recorded by Piotrowski and Northcutt (1996) in Polypterus senegalus.
Generally and in the present work, the nervus abducens, as in all vertebrates, innervates the rectus lateralis muscle. This condition was reported by many authors in some fishes (Bauchot et al., 1989;Dakrory, 2000;Ali, 2005;Nakae and Sasaki, 2006;Ali and Dakrory, 2008;Taha, 2010;Mattar, 2012;Almalki, 2017). In Tridentiger trigonocephalus, Kassem et al., (1988) stated that the rectus lateralis muscle consists of two kinds of fibres and is innervated by two distinct nerve bundles. However, in Latimeria chalumnae (Northcutt and Bemis, 1993) and in many tetrapoda, the abducens nerve innervates the rectus lateralis and the retractor oculi muscles. In Cyclostomata, Edgeworth (1935) stated that the nervus abducens innervates the rectus lateralis and the rectus externus inferior muscles. Fritzsch et al., (1990) found that two of the six ocular muscles are innervated by the nervus abducens in Petromyzon marinus. Pombal et al., (1994) confirmed this finding. The abducens nerve innervates one muscle (external rectus) in chondrichthyan and osteichthyan fishes but two muscles in the lamprey and in most tetrapods (Young, 2008).
In the present study, there is no posterior myodome (the eye muscle chamber). Some authors recorded the presence of this myodome as in Ctenopharyngodon idellus (Dakrory, 2000), Tilapia zillii (Ali, 2005 It is clear from the detailed anatomical study of the head serial sections of Anguilla anguilla that the nervus abducens carries special somatic motor fibres.