390 RESPIRATORY NERVES. 



The Descendens noni is a long slender twig, which quits the hypo- 

 glossal just as that nerve is about to form its arch around the occipi- 

 tal artery, and descends upon the sheath of the carotid vessels. Just 

 below the middle of the neck it forms a loop with a long branch 

 (communicans noni) from the second and third cervical nerves. 

 From the convexity of this loop branches are sent to the depressor 

 muscles of the larynx. If the descendens noni be traced to its con- 

 nexion with the hypoglossal nerve, and examined with care, it will 

 be found to be formed by two filaments, one from the lingual, the 

 other, of larger size, from the first and second cervical nerves. 



The Hyoidean branch, is a small twig distributed to the insertions 

 of the depressor muscles of the larynx, particularly to the thyro- 

 hyoid. 



The Communicating filaments with the gustatory nerve are two 

 or three small branches which ascend upon the anterior part of the 

 hyo-glossus muscle, and join corresponding branches sent down- 

 wards by the gustatory. 



RESPIRATORY NERVES. 



Under this head are grouped, by Sir Charles Bell, certain nerves 

 which are associated in the movements of respiration. They all 

 arise in the course of a distinct tract, situated between the corpus 

 olivare and corpus restiforme on each side of the medulla oblongata, 

 and which may be traced upwards to the corpora quadrigemina ; 

 hence this portion of the brain has been named the respiratory 

 tract. 



4th pair, PATHETICI (trochlearis). The fourth is the smallest 

 cerebral nerve; it arises from the valve of Vieussens and testis, and 

 winds around the crus cerebri to the extremity of the petrous por- 

 tion of the temporal bone, where it pierces the dura mater near to 

 the oval opening for the fifth nerve, and passes along the outer wall 

 of the cavernous sinus to the sphenoidal fissure. In its course 

 through the sinus it is situated at first below the motor oculi, but 

 afterwards ascends and becomes the highest of the nerves which 

 enter the orbit through the sphenoidal fissure. 



After reaching the orbit it crosses the levator palpebrse muscle 

 near to its origin, and is distributed upon the orbital surface of the 

 superior oblique or trochlearis muscle ; hence its synonyme trocli- 

 learis. 



Branches. While in the cavernous sinus the fourth nerve gives 

 off a recurrent branch; some filaments of communication to the 

 ophthalmic nerve ; and a branch to assist in forming the lachrymal 

 nerve. The recurrent branch passes backwards between the layers 

 of the tentorium and divides into two or three filaments, which are 

 distributed to the lining membrane of the lateral sinus. In a prepa- 

 ration before me this branch is very distinct; but I have not always 

 succeeded in finding it. 





