THE HYPOGLOSSAL NERVE. 1277 



under the stylo-hyoid and mylo-hyoid muscles and over the hyo-glossus (Fig. 1082). 

 It terminates by piercing the genio-hyo-glossus and breaking up into a number of 

 fibres for the supply of the lingual muscles. 



Communications. Immediately after emerging from the anterior condyloid foramen, (a] 

 a tiny branch connects with the superior cervical ganglion of the sympathetic, (I)) one or two 

 filaments pass to the loop between the first and second cervical nerves and (c) several fibres 

 associate the nerve with the ganglion of the trunk of the vagus. At the point where the hypo- 

 glossal nerve and the occipital artery cross, (d) the lingual branch of the vagus joins the twelfth ; 

 and as the nerve lies beneath the mylo-hyoid and upon the hyo-glossus muscle, it communi- 

 cates with (e) the lingual branch of the mandibular nerve. 



Branches. The branches of the hypoglossal nerve are : (i) the meningeal, 

 (2) the descending, (3) the thyro-hyoid and (4) the lingual. 



1. The meningeal branch (r. meningeus) consists of one or two minute 

 filaments which supply the dura mater of the posterior cranial fossa and the diploe 

 of the occipital bone. As the hypoglossal is motor in function, it is likely that these 

 twigs are contributed to the nerve by the loop between the first and second cervical 

 nerves. 



2. The descending branch (r. descendens), or r. descendens hypoglossi, is 

 in reality only to a limited extent a branch of the twelfth, since the greater number 

 of its fibres are accessions to the hypoglossal from the first and second cervical 

 nerves. There is reason, however, to believe that these cervical nerves are not the 

 exclusive source of the fibres of the descendens hypoglossi, but that some arise from 

 the cells of the hypoglossal nucleus. The descending branch arises near the point 

 where the hypoglossal nerve hooks around the occipital artery and runs downward 

 and inward in front of or within the carotid sheath. It gives off a branch to the an- 

 terior belly of the omo-hyoid and, about the middle of the neck, joins the descend- 

 ing cervical nerve, or n. communicans hypoglossi, from the second and third 

 cervical nerves. A loop or plexus, termed the ansa hypoglossi, is thus formed and 

 from it filaments are supplied to the sterno-hyoid and sterno-thyroid muscles and to 

 the posterior belly of the omo-hyoid (Fig. 1082). 



3. The thyro-hyoid nerve (r. thyreohyoideus) is also only an apparent 

 branch of the hypoglossal, as its fibres can be traced back to the cervical 

 plexus. It is given off before the nerve dips beneath the stylo-hyoicl muscle and 

 passes down behind the greater cornu of the hyoid bone to reach its distribution to 

 the thyro-hyoid muscle. 



4. The lingual branches (rr. linguales) with one exception, comprise the 

 real distribution of the hypoglossal. As the nerve lies beneath the mylo-hyoid 

 muscle filaments are given off to the hyo-glossus, the stylo-glossus and the 

 genio-hyoideus. The fibres going to the genio-hyoid are in all probability de- 

 rived from the cervical plexus and are not of true hypoglossal origin. After giving 

 off the above-named branches, the hypoglossal nerve breaks up into the terminal 

 filaments which pierce the genio-hyo-glossus to supply it and the lingualis muscle. 



Variations. Occasionally the hypoglossal has been found to possess a posterior root bear- 

 ing a ganglion. This condition is to be regarded as a persistence of the temporary embryonal 

 stage during which the nerve is provided with a posterior root and a ganglion of Froriep 

 (page 1380). In one case the superficial origin was located at the posterior aspect of the me- 

 dulla. Quite frequently the vertebral artery passes between the rootlets of origin and in rare 

 instances behind them. Sometimes a cross filament, situated either between the genio-hyo- 

 glossus and genio-hyoid muscles or in the substance of the latter connects the two hypoglossal 

 nerves. Rarely the hypoglossal has been seen to send a filament to the mylo-hyoid, the .digas- 

 tric or the stylo-hyoid muscle. Occasionally the r. descendens hypoglossi seems to be derived, 

 either entirely or in part, from the vagus, but in these instances the fibres can be traced back to 

 their true origin from the cervical nerves. A filament from the Descending nerve sometimes 

 passes into the thorax, where it joins the vagus or the sympathetic ; in such cases the aberrant 

 branch is probably derived originally from either the sympathetic or the vagus. The r. descen- 

 dens hypoglossi may send a branch to the sterno-mastoid muscle. 



Practical Considerations. Involvement of the hypoglossal nerve, usually 

 together with other cranial nerves is frequent in bulbar disease. The most character- 

 istic symptom is a deviation of the tongue, when protruded to the affected side, caused 



