632 NERVOUS SYSTEM. 



branch of the spinal accessory which goes to these muscles has a certain synchronism 

 with the action of the branch going to the larynx and the pharynx ; the one fixing the 

 upper part of the chest so that the expulsion of the air through the glottis may be more 

 nicely regulated by the expiratory muscles, and the other acting upon the vocal cords. 



In what is known to physiologists as muscular effort, the glottis is closed, the thorax 

 is fixed after a full inspiration, and respiration is arrested so long as the effort, if it be 

 not too prolonged, is continued. The same synchronism, therefore, obtains in this as in 

 prolonged vocal efforts. In experiments in which the muscular branch only has been 

 divided, shortness of breath, after violent muscular effort, is observed ; and this is proba- 

 bly due to the want of synchronous action of the sterno-cleido-mastoid and trapezius. 

 The irregularity in the movements of progression in animals, in which either both branch- 

 es or the muscular branches alone have been divided, is due to anatomical peculiarities. 

 Bernard has observed these irregularities in the dog and the horse, but they are not so 

 well marked in the cat. There have been no opportunities for illustrating these points 

 in the human subject. 



Sublingual^ or Hypoglossal Nerve. (Ninth Nerve.) 



The last of the motor cranial nerves is the sublingual ; and its functions are inti- 

 mately connected with the physiology of the tongue in deglutition and articulation, 

 although it is also distributed to certain of the muscles of the neck. 



Physiological Anatomy of the Sublingual Nerve. The apparent origin of the sublin- 

 gual is from the medulla oblongata, in the groove between the olivary body and the 

 anterior pyramid, on the line of the anterior roots of the spinal nerves. At this point, 

 its root is formed of from ten to twelve filaments, which extend from the inferior por- 

 tion of the olivary body to about the junction of the upper with the middle third. 

 These filaments of origin are separated into two groups, superior and inferior. From 

 this apparent origin, the filaments have been traced into the gray matter of the floor 

 of the fourth ventricle, between the deep origin of the pneumogastric and the glosso- 

 pharyngeal. Although there is much difference of opinion upon this point, it is probable 

 that some of the filaments of origin of these nerves decussate in the floor of the fourth 

 ventricle. The superior and inferior filaments of origin of the nerve unite to form two 

 bundles, which pass through distinct perforations in the dura mater. These two bundles 

 then pass into the anterior condyloid foramen and unite into a single trunk as they 

 emerge from the cranial cavity. 



After the sublingual has passed out of the cranial cavity, it anastomoses with several 

 nerves. It sends a filament of communication to the sympathetic as it branches from 

 the superior cervical ganglion. Soon after it has passed through the foramen, it sends a 

 branch to the pneumogastric. It anastomoses by two or three branches with the upper 

 two cervical nerves, the filaments passing in both directions between the nerves. It 

 anastomoses with the lingual branch of the fifth, by two or three filaments passing in 

 both directions. 



In its distribution, the sublingual presents several remarkable peculiarities : 



Its first branch, the descendens noni, passes down the neck to the sterno-hyoid, ster- 

 no-thyroid, and omo-hyoid muscles. From its relations with important vessels and 

 nerves, this branch possesses considerable surgical interest. 



The thyro-hyoid branch is distributed to the thyro-hyoid muscle. 



The other branches are distributed to the stylo-glossus, hyo-glossus, genio-hyoid, and 

 genio-hyo-glossus muscles, their terminal filaments going to the intrinsic muscles of the 

 tongue. 



It is thus seen that the sublingual nerve is distributed to all of the muscles in the 

 infra-hyoid region, the action of which is to depress the larynx and the hyoid bone after 



