THE HYPOGLOSSAL NERVE. 615 



backwards above the transverse process of the atlas, and terminates as a motor 

 nerve in the sternomastoid and trapezius (fig. 433). The latter muscle usually 

 receives motor fibres also from the cervical plexus (fig. 429). 



The external branch communicates with several cervical nerves. These fibres either partici- 

 pate in the innervation of the above-named muscles, or the accessorius returns part of the 

 sensory fibres supplied by the posterior roots of the two upper cervical nerves. 



Pathological. Stimulation of the outer branch causes tonic or clonic spasm of the above- 

 named muscles, usually on one side. If the branch to the sternomastoid be affected alone, the 

 head is moved with each clonic spasm. If the affection be bilateral, the spasm usually takes 

 place on opposite sides alternately, while it is rare to have it on both sides simultaneously. In 

 spasm of the trapezius the head is drawn backwards and to the side. Tonic contraction of the 

 flexors of the head causes the characteristic position of the head known as caput obstipum 

 (spasticum) or wryneck. In paralysis of one of these muscles, the head is drawn towards the 

 sound side (torticollis paralyticus). Paralysis of the trapezius is usually only partial. 



Paralysis of the whole trunk of the spinal accessory (usually caused by central conditions), 

 besides causing paralysis of the sternomastoid and trapezius, also paralyses the motor branches 

 of the vagus already referred to (Erb, Frankel). 



354. XII. NERVUS HYPOGLOSSUS. Anatomical. It arises from two large-celled nuclei 

 within the lowest part of the calamus scriptorius, and one adjoining small-celled nucleus {Roller), 

 while additional fibres come from the brain ( 378), and also perhaps from the olive (fig. 427, 12). 

 It springs by 10 to 15 twigs in a line with the anterior roots of the spinal nerve (fig. 420, IX.). 

 In its development part of the hypoglossal behaves as a spinal nerve (Froriep). 



Function. It is motor to all the muscles of the tongue, including the 

 geniohyoid and thyrohyoid. 



Connections. The trunk of the hypoglossal is connected with (1) the superior cervical 

 ganglion of the sympathetic, which supplies it with vaso-motor fibres for the blood-vessels of the 

 tongue. After section of the hypoglossal and lingual nerves, the corresponding half of the 

 tongue becomes red and congested {Schiff). (2) There is also a branch from the plexus ganglii- 

 formis vagi, its small lingual branch to the commencement of the hypoglossal arch. These 

 fibres supply the hypoglossal with sensory fibres for the muscles of the tongue, for even after 

 section of the lingual the tongue still possesses dull sensibility. It is uncertain whether fibres 

 with a similar function are partly derived from the cervical nerves or from the anastomosis 

 which takes place with the lingual. (3) It is united with the upper cervical nerves by means 

 of the loops known as the ansa hypoglossi. These connecting fibres run in the descendens noni 

 to the sternohyoid, omohyoid, and sternothyroid. Cervical fibres do not, as a rule, enter the 

 tongue ; stimulation of the root of the hypoglossal acts upon the above-named muscles only 

 very rarely and to a very slight extent {Volkmann). (Compare 297, 3, and 336, III.). 



Bilateral section of the nerve causes complete motor paralysis of the tongue. 

 Dogs can no longer lap, they bite the flaccid tongue. Frogs, which seize their 

 prey with the tongue, must starve ; when the tongue hangs from the mouth, it 

 must prevent the closure of the mouth, so that these animals must die from 

 asphyxia, as air is pumped into the lungs only when the mouth is closed. 



Pathological. Paralysis of the hypoglossal (glossoplegia), which is usually central in its 

 origin, causes disturbance of speech ( 319). [In unilateral palsy, the tongue lies in the mouth 

 in its normal position, but the base is more prominent on the paralysed side. When the 

 tongue is protruded, it passes to the sound side by the genio-hyoglossus ( 155).] Paralysis of 

 the tongue also interferes with mastication, the formation of the bolus in the mouth, and degluti- 

 tion in the, mouth. Owing to the imperfect movements of the tongue, taste is imperfect, and 

 the singing of high notes and the falsetto voice, which require certain positions of the tongue, 

 appear to be impossible {Bennati). 



Spasm of the tongue, which causes aphthongia ( 318), is usually reflex in its origin, and is 

 extremely rare. Idiopathic cases of spasm of the tongue have been described ; the seat of the 

 irritation lay either in the cortex cerebri or in the oblongata (Berger, E. Remak). For Pseudo- 

 motor Action, p. 601. 



355. THE SPINAL NEKVES. Anatomical. The thirty-one pairs of spinal nerves arise 

 by means of a posterior [superior, gangliated] root (consisting of a few large rounded bundles), 

 from the sulcus between the posterior and lateral columns of the spinal cord, and by means of 

 an anterior [inferior, non-gangliated] root (consisting of numerous fine flat strands), from the 

 furrow between the anterior and lateral columns, fig. 435. The posterior roots, with the 

 exception of the 1st cervical nerve, are the larger. Occasionally the roots on opposite sides are 

 not symmetrical ; one or other root, or even a whole nerve, may be absent from the dorsal 

 region {Adamkiewicz). On the posterior root is the spindle-shaped spinal ganglion ( 321, II., 



