NINTII PAIR OF NERVES (Nervi hypoglessi, tel gostotorii, Wi nslow ; Lingtudis, Vic d'Azyr; A inth nerve of Willis; 2'wellch of Scemmenng.) The ninth pair of nerves take their origin from the side of the medulla ob longata, cornmencimr by a variable number of small radicles in the fissure which separates the corpus olivare from the pyramidale.
The superior of these radicles are attached about the centre of this fissure, and the infe rior a little below its termination; they are placed on a line one below the other, which line describes a slight curve looking upwards and backwards, following the curved form of the oh vary body.
The origin of this nerve is superior to that of the first cervical, to which also it lies on a plane a little anterior ; it IS separated from the origins of the par vagum by the olivary body, and has lying immediately in front the corpus pyramidale and the vertebral artery.
The radicles which form the origin of this nerve vary in number from five to ten or twelve ; and if any of these radicles be ex amined closely, they will be found to consist of two or more minor filaments, so that it is very difficult to say exactly by how many roots or origins the ninth nerve is attached to the me dulla spinalis.* These filaments in general unite into two fasciculi, which pass in a direction downwards„ forwards, and outwards to the anterior condy loid foramen, through which the nerve escapes from the cavity of the cranium.
It rarely happens that these fasciculi unite in the cavity of the cranium ; in general, they pass on separately until they reach the foramen, wheie in passing through the dura meter they beconie united into one trunk, which is here invested with a strong neurilemma, derived from the dura mater.
The ninth pair of nerves, on emerging from the anterior condyloid foramen, is in close re lation to the eighth pair of nerves, the internal carotid artery, internal jugular vein and with 1 the superior cervical ganglion of t'he sympa I thetic.
' Ilere the nerve lies external to the vagus.
Connected to it by a dense cellular tissue, for a space about the eighth of an inch, it passes behind the internal carotid artery immediately before that vessel enters the carotid canal, and lies in front of the jugular vein ; here also the nerve is connected to the anterior and superior part of the superior cervical ganglion, in a manner to be presently described.
In this situation the ninth nerve lies deep in i ' the neck, being covered by the origins of the I styloid muscles, the postenor belly of the di . 'gastric, the sterno-mastoid, the skin, platysma, and fascia.
', The trunk of the nerve then passes down . ' wards, outwards, and slightly forwards, escapes from beneath the "posterior belly of the digas , , tric and anterior edge of the sterno-mastoid, becomes more superficial, is crossed in this part of its course by the occipital artery, and at a point in the neck corresponding to the level of the third cervical vertebra,* and opposite the angle of the jawit the nerve turns forwards and upwards, forming an arch, the convexity of which looks downwards and backwards; here the nerve is covered only by the skin, platysma, and fascia, crosses and lies in front of the origin of the occipital artery, the internal jugular vein, external ca rotid artery, and vagus nerve ; passing still in wards and upwards towards the posterior edge of the hyoglossus muscle, the nerve is crossed by the tendon of the digastric, lying here su perior to the lingual artery.
I t then passes between the mylo-hyoid and the hyo-glossus, and having reached the anterior edge of the last-named muscle, it enters and passes through the fibres of the genio-hyoglossus, the substance of which muscle it divides into its terminating branches, the connections and distribution of which shall be examined after we have considered the connections of this nerve and the branches which it gives off and receives in its course through the neck.