[ 520 ] THE BLOOD SUPPLY OF THE FACIAL NERVE BY MICHAEL J. BLUNT Department of Anatomy, Royal Free Hospital School of Medicine The precise aetiology of Bell's palsy, or idiopathic facial paralysis, has not been defined, but the opinion most generally held is that the paralysis is due to ischaemia of the facial nerve trunk. It is therefore surprising to find no general agreement on the gross form of the blood supply to the facial nerve, and no information in the literature on its intrinsic vascular anatomy. The first account of the blood supply of the nerve is that of Bartholdy (1897), who collated the observations made by earlier authors and built up from them a complete picture of the pattern of arterial supply. As might be expected, this account con- tained several inconsistencies and items of conflicting evidence. The arterial supply of the terminal branches was investigated by Tobin (1943), and Guerrier (1951) gave a further account of the gross pattern of arterial branches to the nerve which did not agree, in several respects, with the earlier observations quoted by Bartholdy. It therefore seemed necessary to re-investigate the gross features of the blood supply of the facial nerve as well as to study its intrinsic vascular arrangements. MATERIAL AND METHODS Thirty human temporal bones were removed at post-mortem from twenty-seven subjects whose ages ranged from 14 to 85 years: the heads of two full-term foetuses were also available. The foetal heads were injected, one with neoprene, the other with indian ink in double strength plasma, and the extra-petrous portions of the nerves were dissected in both specimens. One temporal bone was removed from each and sectioned serially. In eight of the adult specimens the basilar artery was injected, and in fourteen specimens injection was made into the stem vessel giving rise to the stylomastoid artery. In both instances the injection mass used was either 5 % indian ink in double-strength plasma, or coloured neoprene. In a further group of eight specimens no injection was made. The facial canal was opened under a binocular microscope and those specimens which had been injected with neoprene were studied to find the gross form of the blood supply to the nerve. Those injected with indian ink in plasma were similarly studied; the nerve was afterwards removed from its canal and either mounted flattened on a slide, or else embedded and sectioned before mounting, in order to show the intrinsic vascular pattern. In the uninjected specimens the nerve was stained by the sodium nitroprusside- benzidine technique. Some nerves were mounted flattened out on slides, and others were embedded in gelatin and frozen sections were cut at 100z before staining. Details of preparation did not differ from those described by Pickworth (1934) except in regard to the methods of embedding and final mounting. Nerve segments were embedded in gelatin blocks which were left unfixed and unhardened: after The blood supply of the facial nerve 521 sectioning, the gelatin dissolved away in the warmed staining solutions without adversely affecting the benzidine reaction. Sections mounted in polystyrene mounting medium remained unfaded for 3 months on an average; when neutral balsam was used the stain survived for less than 2 weeks. THE GROSS BLOOD SUPPLY (a) From the pons to the floor of the internal auditory meats Between its exit from the pons and the internal auditory meatus the facial nerve receives branches directly from the anterior inferior cerebellar artery (PI. 1, fig. 1). This vessel runs laterally from the basilar artery and forms a loop which lies between the facial and auditory nerves and projects towards, and often into, the internal auditory meatus (Stopford, 1916; Nabeya, 1923; Konaschko, 1927). The loop usually gives rise directly to the internal auditory artery but, in one of the eight adult specimens and in one of the foetal temporal bones, the anterior vestibular and vestibulo-cochlear branches took separate origin from the loop of the anterior inferior cerebellar artery. In all of the specimens examined fine vessels passed from both main branches of the internal auditory artery to supply the facial and auditory nerves. Text-fig. 1. A diagrammatic representation of the right facial canal opened in its whole length. The arteries are shown as solid black lines which are broken where the vessels pass on the medial side of the facial nerve. (b) In thefacial canal In its course in the canal the facial nerve receives branches from both the petrosal branch of the middle meningeal artery and the stylomastoid artery (Text-fig. 1). The Jpetrosal artery Immediately after it has entered the skull through the foramen spinosum the middle meningeal artery gives off a short stem vessel which divides into the petrosal artery laterally and a branch to the trigeminal ganglion medially. The petrosal artery is sometimes double and when this is the case the second vessel may arise either from the same common trunk or from the accessory meningeal: each of these 522 Michael J. Blunt arrangements has been met with twice in a series of eighteen dissections, and in all of the four specimens an anastomosis linked the two vessels. The petrosal artery runs laterally in company with the greater superficial petrosal nerve to enter the temporal bone through the hiatus Fallopii (PI. 1, fig. 2). It gives off a short, stout branch which divides into a spray of fine vessels to the geniculate ganglion, and is then continued along the facial canal below the horizontal part of the facial nerve to anastomose with the stylomastoid artery. The anastomotic branch gives off a vessel which crosses the lateral side of the facial nerve and ramifies in the roof of the middle ear cavity. The stylomastoid artery The stylomastoid artery is usually described as taking direct origin from the posterior auricular artery but that arrangement has not been met with in eighteen dissections in this series. Below the stylomastoid foramen either the posterior auricular artery or the occipital artery gives off a common stem vessel which divides into a number of branches below the base of the skull; these branches pass to the tympanic plate, styloid process and mastoid process and to the post-auricular skin and the facial nerve (P1. 1, fig. 3). A further branch, which is the stylomastoid artery, enters the stylomastoid foramen on the medial side of the nerve. It was indirectly derived from the occipital artery twelve times and from the posterior auricular artery six times. The stylomastoid artery forms a loop in the lower part of the fa. ial canal, the returning limb of which leaves the canal with the posterior auricular nerve. From the convexity of the loop two branches usually arise: the largest ofthese accompanies the facial nerve and the other rapidly breaks up into branches which pierce the posterior meatal wall in company with filaments from the auricular branch of the vagus. The main ascending branch (PI. 1, fig. 4) lies on the medial side of the facial nerve as far as the junction between its horizontal and vertical parts and then loops around the inferior aspect of the bend, usually passing somewhat on to its lateral side, to reach the infero-medial aspect of the horizontal part of the nerve. It anastomoses directly with the petrosal artery to form a complete arterial arcade in the facial canal. From the arcade branches are given to the nerve close to the origin of the chorda tympani and at the level of the second bend; a separate branch accompanies the chorda tympani, and one or two branches run through the posterior wall of the canal to supply the mastoid air cells. From the horizontal part of the arcade a vessel leaves the facial canal and ramifies in the roof of the middle ear cavity. The lowest branch from the stylomastoid artery to the facial nerve is therefore at the level of the origin of the chorda tympani. (c) The extra-petrous course Below the stylomastoid foramen the nerve is supplied by a constant branch from the stem vessel of the stylomastoid artery (PI. 1, fig. 3), and, in the parotid gland, it receives twigs from either the posterior auricular artery or the occipital artery, and also from the superficial temporal and transverse facial arteries. The terminal branches of the nerve as they leave the parotid gland are each accompanied by a The blood supply of the facial nerve 523 fine arteria comitans which gives branches into the nerve and accompanies it until close to its termination. All the arterial branches to the facial nerve below the stylomastoid foramen are linked longitudinally by free anastomoses in the epineurium. Venous drainage In the internal auditory meatus the nerve is loosely fascicular in structure and contains little connective tissue between its fasciculi; it is widely separated from the periosteum of the meatal walls and its veins drain into branches of the internal auditory vein. At and below the geniculate ganglion the nerve is surrounded by a tough connective tissue sheath which is continuous on the one hand with the periosteum of the facial canal and on the other with the epineurium. The sheath encloses both the facial nerve and the arterial arcade, and in its substance lies a well- marked venous network which drains, anteriorly, into veins accompanying the petrosal artery and, inferiorly, into the venae comitantes of the stem vessel from which the stylomastoid artery is derived. Venous channels in the sheath also com- municate with those in the surrounding bone (P1. 1, fig. 5) and in adult material injected with indian ink venous channels in the walls of the canal were always filled through these connexions.