CHAP. XIX.] NINTH NERVE. 109 



the deep surface of the tongue, being closely related to the internal 

 carotid artery and the jugular vein, and winding round the external 

 carotid, in its course to the upper and anterior part of the neck. 



Immediately after its emergence, it forms anastomoses with the 

 vagus nerve, the superior cervical ganglion, and with the cervical 

 plexus, and then it gives off: 1. The descending branch, which 

 forms remarkable anastomotic arches with branches of the second 

 and third cervical nerves, either within or in front of the sheath of 

 the common carotid artery and jugular vein. From the convexity 

 of this arch or these arches, there pass branches to the sterno-hyoid, 

 and sterno-thyroid muscles and to the anterior belly of the omo- 

 hyoid. 2. It gives nerves to the thyroid-hyoid, genio-hyoid, hyo- 

 glossus, and stylo-glossus muscles. 3. On the inferior surface of 

 the tongue it breaks up into its terminal, or glossal branches which 

 pass into the muscular structure of the tongue. 



As the hypo-glossal nerve crosses the hyo-glossus muscle it forms 

 an anastomosis with the lingual branch of the fifth pair. 



Function of the Ninth Nerve. As this nerve has no other con- 

 nexion than with muscles, we cannot regard it in any other light 

 than as a motor nerve. The muscles to which it is distributed are 

 those of the tongue, and as this is the principal nerve to these 

 muscles, we may justly regard it as the motor nerve of the tongue. 

 The other nerves of the tongue, the lingual branch of the fifth and 

 the glosso-pharyngeal, are obviously traceable to the mucous mem- 

 brane. 



In the lower mammalia the ninth nerve is distributed precisely 

 as in man, and it is proportional in size to the muscular activity of 

 the tongue. 



Numerous experiments have been made on this nerve. Section 

 of it on one side paralyses the motor power of the tongue on that 

 side. And galvanic irritation of it throws the muscles on the same 

 side into convulsive action. 



Several instances have been observed of tumors compressing this 

 nerve, and causing paralysis of the muscles of the tongue on the 

 same side. The tongue participates in the hemiplegic paralysis 

 which results from an apoplectic clot in the brain, or other extensive 

 disease of that organ, along with all those parts whose nerves are 

 implanted in some part of the extended centre of volition, and are 

 ordinarily excited by a mental stimulus. In such cases, the loss of 

 power in the tongue is usually indicated by its deviation to the 

 paralysed side in its protrusion, though occasionally the tip is turned 

 towards the sound side. 



