THE HYPOGLOSSAL NERVE. 567 



Connecting branclies. — Connection tvitk the pneumogastric. — Close to the 

 skull the hypoglossal nerve is connected with the second ganglion of the pneumo- 

 gastric by separate filaments, or in some instances the two nerves are united so 

 as to form one mass. 



Union n-ith the si/m2)athctic and fir.st tn-o spinal nerves. — Opposite the first 

 cervical vertebra the nerve communicates by several twigs with the upper cervical 

 ganglion of the s^onpathetic, and with the loop uniting the first two spinal 

 nerves in front of the atlas. 



Descending branch of the ninth nerve.— This branch (r. de- 



scendens noni) leaves the ninth nerve where this turns round the occi- 

 pital artery, or, sometimes, higher up. It passes downwards on the sur- 

 face of the sheath of the carotid vessels, gradually crossing from the 

 outer to the inner side, gives a branch to the anterior belly of the omo- 

 hyoid muscle, and jomsabout the middle of the neck in a loop v?jth one 

 or two branches from the second and third cervical nerves, forming the 

 ansa hypoglossi. The concavity of this loop is turned upwards ; and 

 the connection between the nerves is effected by means of two or more 

 interlacing filaments, which enclose an irregularly shaped space. From 

 this interlacement of the nerves, filaments are continued backwards to 

 the posterior belly of the omo-hyoid, and downwards to the sterno-hyoid 

 and stemo-thyroid muscles. Occasionally a filament is continued to 

 the chest, where it joins the cardiac and phrenic nerves. 



Muscular branches. — The branch to the thyro-hyoid muscle is a 

 separate twig given off from the hypoglossal nerve as it approaches the 

 hyoid bone. 



The nerve supplies branches to the hyo-glossus, genio-hyoid, 

 and genio-hyo-glossus muscles as it becomes contiguous to each, 

 and, when arrived close to the middle of the tongue in company with 

 the ranine artery, gives off several long slender branches, which pass 

 upwards into the substance of the organ. Some filaments join with 

 others proceeding from the gustatory nerve. 



Varieties. — In animals the ninth nerve not unfrequently possesses a posterior 

 root furnished with a ganglion, in the same manner as that of a spinal nerve. 



It is not uncommon to find the descending branch of the ninth nerve within 

 the sheath of the large cervical vessels and in such cases it is placed either over 

 or under the vein. This nerve in some cases appears to be derived either alto- 

 gether from the pneumogastric, or from both the pneumo-gastric and hj-po- 

 glossal nerves. There is every reason, however, to believe that these varieties in 

 origin are only apparent, arising from the temporary adliesion of the filaments 

 of this branch to those of -the pneumogastric. It is probable, moreover, that 

 the descendens noni has little if any real origin from tlie hypoglossal nerve : 

 Luschka states, as the result of numerous researches on the human subject, 

 that the descendens noni usually contains no filaments from the hypoglossal, 

 but is a branch from the first and second cei'vical, temporarily associated -nath 

 the ninth nerve ; and this quite agrees with the circumstance that in some 

 animals the nerves supplied to those muscles to which the descendens noni of the 

 human subject is distributed come from the cervical plexus. 



A branch is described as uniting with its fellow of the opposite side in the 

 substance of the genio-hyoid muscle, or between it and the genio-glossus. This 

 loop, as also the ansa hj-poglossi, is recommended by Hyrtl as a particularly 

 favom-able example for the observation of nei-ve-fibres returning to the nervous 

 centres without distribution, to which he gives the name of '• nerves without 

 ends." (•' Nat. Hist. Review," Jan. 18G2.) It was observed by Cruveilhier that 

 in the ansa hj^^oglossi an interchange of fibres takes place, so that a filament of 

 the cervical spinal nerve is directed upwards along the branch of the hypoglossal, 

 and vice versa 



