The first nerve fibres have been observed in silver-impregnated specimens. e.g. Carnegie No.6097. Short fibres can be observed in the trigeminal(V) and vestibulocochlear (VII,VIII) ganglia.
Intramural fibres and motor roots originate in the trigeminal motor nucleus
The three divisions of the trigeminal ganglion become more obvious, and in more advanced embryos of this stage. The three divisions of the trigeminal nerve are present as short stumps depicted in a reconstruction of Carnegie No. 4245-6. A section through the trigeminal ganglion, showing nerve fibres entering the rhombencephalic wall can be seen in O’Rahilly R., Müller F.,(1994) Fig 14.8. These afferent fibres join the common afferent tract. The trigeminal motor nucleus gives rise to intramural fibres some of which leave the brain. Müller F., O’ Rahilly R., (1988a). Motor roots exit at level of sulcus limitans.
The motor root of the trigeminal is identified leaving the brain wall ventral to the sensory root. It runs close to the surface of the ganglion but does not fuse with it. It joins the mandibular division of the trigeminal and continues into the first arch. In an advanced specimen of this stage the mandibular nerve is observed to possess one branch only and this is unnamed. Barniville H.L., (1915) (Keibel Embryo No. 1495)
The maxillary nerve, in the same embryo, extends into the maxillary process and breaks into several fibre bundles.
The ophthalmic division runs forward over the optic cup and down, following the circumference of the cup. In an advanced embryo of this stage, a possible naso-ciliary branch has been identified and described by Barniville H.L., (1915) (Keibel Embryo No. 1495)
The fibres of cells whose bodies are in the trigeminal ganglion run into the pons via the nervus intermedius where they may enter either the future chief sensory nucleus or turn down in the spinal trigeminal tract and synapse in the nucleus of the spinal trigeminal tract. In an advanced specimen of this stage fibres from the trigeminal ganglion have been traced into the brain where they follow one of three directions; 'directly to the mantle cells of the first and second neuromere’, ‘longitudinally backwards constituting the anlage of the spinal root V’, and finally ‘some distance forwards’ Barniville H.L., (1915)(Keibel Embryo No. 1495). Another group of sensory nerve fibres are the unipolar neurons of the mesencephalic trigeminal nucleus. These fibres travel in the mesencephalic tract of the trigeminal.
The ophthalmic division follows the circumference of the optic cup. It branches into the frontal and nasociliary nerves. Streeter G.L., (1908b) The nucleus of the mesencephalic tract is present in 27 of 28 embryos studied of this stage. O’Rahilly R., Müller F.,(1994)but the chief sensory nucleus is not present until stage 18.
Described by Thyng F.W., (1914), the ophthalmic nerve travels to the orbit giving small sensory branches to the oculomotor and trochlear as it passes dorsal to the optic stalk. It then branches into the naso-ciliary and frontal nerves which continue either side of the superior rectus and oblique muscles. The frontal nerve gives off several branches including the supraorbital while the naso-ciliary continues to the lateral nasal process. The maxillary nerve extends into the maxillary process. The mandibular nerve receives sensory fibres from the semilunar ganglion and also the motor output of the trigeminal. It divides into a buccal nerve which passes the parotid on its way to the epithelium in the mouth region and a larger trunk which gives rise to the auriculo-temporal branch, the inferior alveolar branch and the lingual branch. which joins the chorda tympani of the facial and innervates the submaxillary gland and also the tongue. However no description of these branches has been found yet.
The infra-orbital and pterygopalatine parts of the maxillary division , the recurrent meningeal branch, auriculotemporal branch , and the sensory buccal branch of the mandiblar nerve and the frontal and nasociliary branches of the ophthalmic nerve can be seen in a reconstruction of a 17.5 mm embryo Blechschmidt E., (1963) Plate 35.