612 HANDBOOK OF PHYSIOLOGY. 



fibres, and to be destined entirely to the trapezius and extending from 

 the back of the fourth ventricle to the level of the olivary bodies close 

 to the middle line, inside the combined nucleus of the ninth, tenth, and 

 eleventh nerves. 



The Xllth Nerve (Hypoglossal) . 



Origin and Connections. The nerve arises from a large-celled and 

 very long nucleus in the bulb, extending from the back of the fourth 

 ventricle to the level of the olivary bodies close to the middle line, inside 

 the combined nucleus of the ninth, tenth, and eleventh nerves. Fibres 

 from this nucleus run from the ventral surface through the reticular 

 formation in a series of bundles passing between the olivary nucleus lat- 

 erally and the anterior pyramid and accessory olive medially, to gain 

 the surface. The nerve emerges from a groove between the anterior 

 pyramid and olivary body. The fibres of origin are continuous with 

 the anterior roots of the spinal nerves. It is connected with the vagus, 

 the superior cervical ganglion of the sympathetic and with the upper 

 cervical nerves. 



Distribution. This nerve is the motor nerve to the muscles con- 

 nected with the hyoid bone, including those of the tongue. It supplies 

 through its descending branch (descendens noni), the sterno-hyoid, 

 sterno-thyroid, and omo-hyoid; through a special branch, the thyro- 

 hyoid, and through its lingual branches, the genio-hyoid, stylo-glossus, 

 hyo-glossus, and genio-hyo-glossus and linguales. 



Functions. The function of the hypoglossal is exclusively motor. 

 As a motor nerve, its influence on all the muscles enumerated above is 

 shown by their convulsions when it is irritated, and by their loss of 

 power when it is paralyzed. The effects of the paralysis of one hypo- 

 glossal nerve are, however, not very striking. Often, in cases of hemi- 

 plegia involving the functions of the hypoglossal nerve, it is not possible 

 to observe any deviation in the direction of the protruded tongue; prob- 

 ably because the tongue is so compact and firm that the muscles on either 

 side, their insertion being nearly parallel to the median line, can push 

 it straight forward or turn it for some distance toward either side. 



The Pons Varolii. 



The pons Varolii is generally spoken of as a great commissure of 

 fibres; of fibres which connect the two halves of the cerebellum and of 

 fibres which connect the bulb and spinal cord with the upper part of the 

 brain. Although this is true it must not be forgotten that the pons 

 contains several masses of gray matter, and also in addition smaller col- 

 lections of nerve-cells. It is found that on section the following parts 



