Bird Brain Structure

The 12 pairs of Cranial Nerves

The Brain of Birds - Anatomy and Bird Intelligence

Bird Brain Structure - The 12 pairs of Cranial Nerves

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There are twelve pairs of cranial or brain nerves which arise from the brain and leave the cranium through special holes in its structure. These 12 pairs, as in other Classes of Vertebrates, are frequently spoken of by their number, counting from the nasal region backwards to the occiput.

Diagram of ventral view of the brain structure of a Goose

I-XII.....The twelve pairs of cranial nerves
Ch.....Chiasma of the optic nerves cut across
L.o.....Optic lobe
F.S.....Sylvian fissure
Sp.I.....First spinal nerve

I. The Olfactory Nerve (Nervus olfactorius) forms the anterior and ventral continuation of the hemisphere of its side, but arises in reality from ganglionic cells in the thalamencephalon and the midbrain. It leaves the cranial cavity through a canal in the dorsal and median part of the orbit and ends in the ganglionic cells of the olfactory membrane of the nose.

II. The Optic Nerve (Nervus opticus) arises from the ganglionic cells of the mantle of the optic lobes. Immediately in front of the hypophysis is the optic chiasma, produced by the complete crossing of the fibres, which compose the two optic nerves, those from the right optic lobe passing over the left, and those from the left lobe to the right side. From the chiasma start the right and left optic nerves, each leaving the cranium by the large optic foramen between the orbitosphenoid and alisphenoid, entering the orbit near the posterior and ventral corner of the orbital septum and ultimately forming the retina of the eye.

III. The Oculomotor Nerve (Nervus oculomotorius) arises close behind the hypophysis, near the medio-ventral line, from the midbrain, enters the orbit behind or together with the optic nerve (II), and supplies most of the external muscles of the eye, namely the m. rectus superior, inferior, internus, and obliquus inferior. A ciliary, partly sympathetic, branch supplies the eyeball and the internal muscles (see EYE).

IV. The Trochlear Nerve (Nervus trochlearis or patheticus) is the only one which leaves the brain on its dorsal surface, namely as a thin thread winding its way from the midbrain upwards between the cerebellum and the optic lobes, and entering the orbit through a fine opening close to the optic nerve (II) in order to supply the m. obliquus superior of the eyeball.

V. The Trigeminal Nerve (Nervus trigeminus) is next to the optic the thickest nerve, and of a complex nature, being motory and sensory. It arises from the sides of the midbrain and hindbrain, forms the large Gasserian ganglion in the wall of the cranium, and leaves the latter in the form of three branches.

  • The first or ophthalmic branch comes directly out of the ganglion through a foramen behind the optic (II), runs along the dorsal corner of the orbital septum, and leaves the orbit at its inner anterior corner in order to supply the palate, the bill, forehead, and the lacrymal gland. It is chiefly sensory, and consequently strongest in birds with tactile bills, like Ducks and Snipes.
  • The second or upper maxillary branch runs along the ventral edge of the orbital septum, and besides the palatine and maxillary regions supplies the eyelids and Harder's gland.
  • The third or inferior maxillary branch is the strongest of the three; it leaves the cranium together with the second through a foramen between the basi-alisphenoid and petrosal bones and innervates all the masticatory muscles, the parotid gland, and the whole of the under jaw.
VI. The Abducens Nerve (Nervus abducens) is a very thin nerve arising from the hindbrain near the medio-ventral line, entering the orbit through a special foramen latero-ventrally from the optic foramen, and supplying the lateral rectus muscle and the two muscles of the nictitating membrane. It is entirely motory.

VII. The Facial Nerve (Nervus facialis) arises from the side of the hind brain, possesses a ganglion (known as the ganglion geniculatum), passes through the petrosal bone into the Fallopian canal, and sends the sympathetic sphenopalatine branch to the second branch of the trigeminal nerve (V). The facial nerve leaves the tympanic cavity behind the quadrate bone, supplies the digastric muscle or depressor of the mandible, the little stapedius muscle of the ear-bones, the mylohyoid and stylohyoid muscles of the tongne, and further on connects itself with branches from the first four cervical nerves and occasionally with branches from the glossopharyngeal nerve (IX), ultimately supplying the skin on the front of the neck. There are no branches, as in Mammals, to supply the face, nor is there in Birds a chorda tympani, i.e. a branch of the facial nerve joining the mandibular branch of the trigeminal nerve (V).

VIII. The Vestibulocochlear Nerve (Nervus acusticus) arises dorsally from the facial nerve (VII), of which it is the sensory portion. It is very short and thick, possesses a little ganglion, and spreads out in the cochlea of the EAR as the nerve of hearing.

IX.  The Glossopharyngeal Nerve  (Nervus glossopharyngeus) takes its origin from the dorso-lateral sides of the medulla oblongata, near the rhomboid fossa. It leaves the cranium through the foramen jugulare, which lies between the petrosal and the lateral occipital bones, and also serves as exit for the vagus nerve (X) and the jugular vein.  Here the ninth nerve forms a big swelling, the ganglion jugulare, and is connected with the ganglion of the vagus and with the large sympathetic ganglion cervicale supremum, receiving a strong branch from the stem of the vagus, and dividing into two branches:

  • One, the pharyngeal branch, supplying the upper portion of the pharynx and the gustatory papillae of the palate.
  • The other, or lingual branch, supplying the glottis, larynx, and the tongue, and acting chiefly as the nerve of taste.
X. The Vagus Nerve (Nervus vagus or pneumogastricus) arises behind the glossopharyngeal (IX), and passes likewise through the jugular foramen. Its ganglion is connected with that of the glossopharyngeal and with that of the sympathetic system. The stem of this nerve receives a branch from the hypoglossal (XII) and takes up the accessory (XI). It runs down the side of the oesophagus, enters the thoracic cavity between the brachial nerve plexus and the carotid artery, then passes between the bronchus and the subclavian artery to the ventral side of the proventriculus, and joining its fellow from the other side, spreads out to supply the stomach. Other branches leave the principal stem of each vagus at the level of the bronchi to supply the liver, heart, and lungs; and as the recurrent laryngeal branch also supply the distal portion of the trachea and oesophagus. Some fibres of the vagus often extend beyond the stomach, and are connected with the sympathetic nerves of the trunk, supplying part of the intestinal canal.

XI. The Accessory Nerve or Spinal Accessory Nerve (Nervus accessorius), a little nerve taking its origin between the dorsal and ventral roots of the third cervical nerve, runs upwards through the occipital foramen into the cranium, and joins the ganglion of the vagus (X), to leave the cranium with the latter and to supply the cucullaris muscle or constrictor colli.

XII. The Hypoglossal nerve (Nervus hypoglossus) arises ventro-laterally from the medulla oblongata, and leaves the cranium by two foramina in the lateral occipital bone, in front of and sidewards from the occipital condyle. It supplies the complexus muscle, forms a connecting loop with the first cervical nerve, innervates some of the cervical muscles, and divides into two branches - one of which supplies most of the muscles of the tongue and communicates with its fellow on the undersurface of the tongue, while the other innervates the muscles of the larynx, and then descends along the side of the trachea to the syrinx in order to supply the vocal muscles and membranes.
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