The ninth nerve, on escaping from the an terior condyloid foramen, is connected to the par vagurn, as was before noticed, by dense cel lular tissue, but also by a nervous filament; further on, as the ninth nerve approaches the transverse process of the atlas, it receives a twig from the first cervical nerve, or from the nervous loop formed round the transverse process of the atlas by the communicating branches of the first and second cervical nerves.t In this situation, also, the ninth is connected by a small nerve with the superior cervical gan glion.
RdIMIS cervicalis descendens, seu descendens noni.—The next regular branch given off by this nerve is immediately before it turns in front of the jugular vein and carotid artery, when it gives off a large and regular branch called cer vicalis descendens,. or descendens noni.
The point at which the ninth nerve gives off this branch is immediately below the angle of the jaw, and where it escapes from under the edge of the sterno-mastoid muscle. The de scendens noni from this passes downwards and forwards to the inferior part of the neck ; at its origin this nerve frequently receives a twig from the par vagum ; it passes down the neck in front of the jugular vein and carotid artery, crossing these vessels obliquely, beinF,, in this course superficial to the cellular investment de rived from the cervical fascia which constitutes the sheath of these vessels.
Onw-hyoid bronch.—About the centre of the neck, the cervicalis descendens gives off a con siderable branch, which, passing in a direction upwards and inwards, enters the interior belly of the omo-hyoid muscle, in the substance of which it ramifies.
Plexus.—Immediately below the tendon of the omo-hyoid, the descendens noni, uniting with branches given off by the second and third cervical nerves, forms a nervous arch, the convexity of which looks downwards and for wards. This plexus lies under cover of the sterno-mastoid, and in front of the jugular vein.
Sterno-hyoid and thyroid branches.—From the convexity of the arch formed by this plexus two or sometimes more nerves proceed down wards and inwards, and ramifying on the super ficial surface of the sterno-hyoid and thyroid muscles, are distributed to them.
Cardiac branch.—Meekel states, that on the left side particularly he has been able to trace a branch from this plexus into the thorax along the pericardium as far as the heart. The cervi calis descendens is observed sometimes to vary from the above description, in its course down the neck, and in its relation to the great vessels; for, instead of lying anterior and external to the sheath, it is occasionally found to pass down within the sheath, and sometimes even behind it. I have also seen it pass for a short dis
tance within the sheath in the upper part of its course, becoming superficial about the centre of the neck, and then running down in front of the sheath in the usual manner.
These varieties in the course and relations of this nerve are not, however, very commonly met with.
Thyro-hyoid branch.—The next branch given off by the ninth pair is where the nerve is passing under the tendon of the diga.stric, a little above the comp of the os hyoides. Here it gives off a considerable branch named thyro hyoid, from its distribution. This nerve passes from its origin downwards and inwards, cross ing the lingual artery, to which it lies super ficial, and is distributed to the thyro-hyoid muscle.
From the origin of the thyro-hyoid branch the ninth • nem passes inwards between the hyoglossus and mylo-hyoid muscles, and at the anterior edge of the hyoglossus it plunges into the genio-hyoglossus, in the substance of which its terminating branches ramify. In this course the ninth nerve supplies filaments to the mylo hyoid, the hyoglossus, the genio-hyoid, genio liyoglossus, and lingualis.
In the substance of the genio-hyoglossus the bmnches of the ninth nerve form distinct anas tornoses with branches of the fifth (the gusta tory); with this nerve the branches of the ninth form nervous loops or arches, the COD vexities of which look forwards, and from which branches pass off which may be traced to the mucous membrane of the tongue. There can be little doubt that these nerves are to be con sidered as compound, containing filaments de rived both from the ninth and fifth pair. Most anatomists state, that the ultimate branches of the ninth can be traced no further than the structure of the muscles which enter into the formation of the tongue, and this appears to be true with respect to the branches which do not anastomose with the fifth pair ; but it is more than probable, although difficult to denion strate, that from the anastomosis spoken of above, a nerve, composed of filaments both of the ninth and hfth, proceeds, and is distributed to the mucous metnbrane of the tongue.