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4.8.13 Accessory and hypoglossal nerves (cranial nerves XI, XII)

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Let's move on, to look at the eleventh cranial nerve, the accessory. It's a motor nerve, supplying just two muscles. The spinal accessory nerve passes around the upper end of the internal jugular vein, then passes downward and backward behind the posterior belly of the digastric. It runs beneath the sternocleidomastoid muscle, which we'll add to the picture. The spinal accessory supplies the sternocleidomastoid muscle, sometimes running deep to it, sometimes through it.

Emerging near the posterior border of the sternocleidomastoid, the spinal accessory nerve runs downward and backward across the splenius muscle, and passes beneath the other muscle that it supplies, the trapezius.

Lastly we'll look at the twelfth cranial nerve, the hypoglossal. It's a motor nerve, supplying all the muscles of the tongue, and partly supplying the infrahyoid muscles.

The hypoglossal nerve emerges between the internal carotid artery and internal jugular vein. It runs downward and forward across the external carotid artery.

We've added the styloglossus muscle to the picture here. Missing from the picture are the stylohyoid muscle, and the posterior belly of the digastric.

The hypoglossal nerve gives off this descending branch, then turns and runs forward. It runs just below the styloglossus muscle, and passes forward into the gap between hyoglossus medially, and the mylohyoid muscle laterally: the same gap that the lingual nerve runs in. The hypoglossal nerve supplies all the muscles of the tongue including the intrinsic muscles, and also the geniohyoid muscle.

To look at the descending branch of the hypoglossal nerve, we'll go further down the neck. Here's the descending branch of the hypoglossal nerve. Its fibers in fact come from C1. It passes downwards and forwards in front of the internal jugular vein. It's joined by this branch from C2 and 3 to form a loop, called the ansa ...

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(3.04)

Let's move on, to look at the eleventh cranial nerve, the accessory. It's a motor nerve, supplying just two muscles. The spinal accessory nerve passes around the upper end of the internal jugular vein, then passes downward and backward behind the posterior belly of the digastric. It runs beneath the sternocleidomastoid muscle, which we'll add to the picture. The spinal accessory supplies the sternocleidomastoid muscle, sometimes running deep to it, sometimes through it.

Emerging near the posterior border of the sternocleidomastoid, the spinal accessory nerve runs downward and backward across the splenius muscle, and passes beneath the other muscle that it supplies, the trapezius.

Lastly we'll look at the twelfth cranial nerve, the hypoglossal. It's a motor nerve, supplying all the muscles of the tongue, and partly supplying the infrahyoid muscles.

The hypoglossal nerve emerges between the internal carotid artery and internal jugular vein. It runs downward and forward across the external carotid artery.

We've added the styloglossus muscle to the picture here. Missing from the picture are the stylohyoid muscle, and the posterior belly of the digastric.

The hypoglossal nerve gives off this descending branch, then turns and runs forward. It runs just below the styloglossus muscle, and passes forward into the gap between hyoglossus medially, and the mylohyoid muscle laterally: the same gap that the lingual nerve runs in. The hypoglossal nerve supplies all the muscles of the tongue including the intrinsic muscles, and also the geniohyoid muscle.

To look at the descending branch of the hypoglossal nerve, we'll go further down the neck. Here's the descending branch of the hypoglossal nerve. Its fibers in fact come from C1. It passes downwards and forwards in front of the internal jugular vein. It's joined by this branch from C2 and 3 to form a loop, called the ansa cervicalis. The branches that arise from the ansa cervicalis provide the motor supply to all four of the infrahyoid muscles.

In looking at the twelve cranial nerves we've concentrated on the main aspects of each nerve and omitted many of the smaller branches and cross-connections. We've also left out of the picture the complex autonomic nerve supply to the head and neck. The details of both the sympathetic and parasympathetic systems in the region are too small to be shown clearly in straightforward dissections. They're more readily understood from diagrams, to which I'm sure you'll have access.

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