HYPOGLOSSAL NERVE. THE SPINAL NERVES. 713 



XII. HYPOGLOSSAL NERVE. 



The elongated nucleus of origin of the hypoglossal nerve consists of large 

 cells, and is a continuation of the anterior horn of the spinal cord. It is situated 

 in the depth of the lowermost portion of the floor of the fourth ventricle. It 

 receives anastomotic fibers from the cerebral cortex of the opposite side. The 

 nuclei of both sides are connected by a commissure. 



The nerve arises as a bundle of neurites of from ten to fifteen filaments, and 

 makes its exit in a direction parallel with that of the anterior roots of the spinal 

 nerves (Fig. 242). In its development the hypoglossal shows itself to be in part 

 a spinal nerve. 



Purely motor at its root, the hypoglossal is the motor nerve of all 

 of the muscles of the tongue, including the geniohyoid and thyrohyoid. 

 The trunk of the hypoglossal nerve anastomoses with : i . The superior 

 cervical ganglion of the sympathetic, through which it receives vaso- 

 motor fibers, for division of the hypoglossal (together with that of the 

 lingual) is followed by redness of the corresponding half of the tongue. 

 2. Muscle-sense fibers enter the hypoglossal from the gangliform plexus 

 and from the small lingual branch of the vagus, also from anastomoses 

 with the cervical nerves and through those with the lingual beneath the 

 tongue. After division of the lingual nerve the tongue still possesses 

 dull sensibility. 3. The loop of the hypoglossal nerves anastomoses 

 with the two upper cervical nerves. These anastomoses pass further 

 through the descending branch (through which also muscle-sense fibers 

 from the lingual descend) as motor branches for the sternohyoid, omo- 

 hyoid and sternothyroid. Irritation of the roots of the hypoglossal 

 affects the muscles named only rarely and in slight degree. 



Division of both hypoglossal nerves paralyzes the tongue. Dogs are no longer 

 able to drink and they bite the flabby, pendulous tongue. Frogs, which catch 

 their prey with the tongue, must starve; in hanging out of the mouth the tongue 

 prevents closure of this cavity and as a result the animals die from asphyxia, 

 because they are able to pump air into the lungs only when the mouth is closed. 



Pathological. Paralysis of the hypoglossal nerve (glossoplegid) is generally 

 of central origin, and it causes derangement of speech. The deviation of the 

 tongue in case of unilateral paralysis is described on p. 276. Paralysis of the 

 tongue renders chewing difficult, prevents formation of the bolus and swallowing 

 in the mouth. In consequence of deficiency in the friction-movement of the 

 tongue, the sense of taste is impaired. The singing of high notes and of falsetto 

 notes, in the production of which special positions of the tongue appear to be 

 necessary, is interfered with. 



Spasm of the tongue, causing aphthongia, is generally of reflex origin, and. in 

 any event, is extremely rare. Cases of idiopathic spasm of the tongue have also 

 been described, the tongue being moved with great violence. The seat of irritation 

 has been either in the cerebral cortex or in the medulla oblongata. 



THE SPINAL NERVES. 



The thirty-one spinal nerves are connected with the spinal cord by means of 

 two roots: The anterior roots arise as neurites of the ganglia of the anterior horns; 

 the posterior roots have in reality grown into the spinal cord from without. They 

 arise from the pear-shaped bipolar cells of the ganglia of the posterior roots, one 

 fiber of which enters the gray matter of the spinal cord as a neurite and here 

 enters into contact with ganglion-cells (Fig. 251); and the other fiber of which 

 passes to the ganglion as a dendrite from peripheral areas endowed with sensi- 

 bility. The posterior roots make their exit from the sulcus between the posterior 

 and lateral columns of the spinal cord; the anterior roots, from the groove 

 between the lateral and anterior columns. The posterior forms the spindle- 

 shaped spinal ganglion. Then the two roots enter into intimate union and form, 

 still within the vertebral canal, a mixed trunk. The two branches originating 



