774 THE XERVOrS SYSTEM 



posk'rior nerve roots and the attacluncnt of the liganu'ntuni denticulatuni. The 

 .superficial origin of the nerve is tiius constituted by a row of filaments which extend 

 from the tiftli or sixth (rarely the seventh) to the first cervical nerve. The lowest 

 of this series of roots is close to the ligamentum denticulatuni; the highest is near 

 to the i)oint of emergence of the posterior roots of the first cervical nerve. The 

 roots unite to form a rounded cord which courses upwards, increasing in size as it 

 ascends, and enters the cranial cavity by passing through the foramen magnum. 

 Here it is joined, for the time being, l)v tlie accessory portion, and passes with the 

 latter, in the form of a single trunk, through the middle compartment of the jugular 

 foramen. At the base of the skull it runs downwards, outwards, and l>ack wards 

 between the occipital artery and the internal jugular vein, to pass under cover of 

 the sterno-mastoid muscle; it })ierces the clavicular fil;)res of the muscle and com- 

 municates with a branch derived from the second cervical nerve. From the jilexus 

 so formed the sterno-mastoid is supplied. The nerve then emerges at the ])()sterior 

 l)order of the muscle, crosses the floor of the posterior triangle of the neck obliijuely, 

 and ])asses under cover of the trapezius; on the deep surface of that muscle the 

 spinal accessory is joined by branches from the third and fourth cervical nerves, 

 and forms with them the subtrapezial plexus, from wliich the trapezius is supplied. 



TWELFTH OR HYPOGLOSSAL NERVE 



The hypoglossal nerve arises from an elongated column of nerve-cells which 

 extends from the stride medullares in the floor of the fourth ventricle downwards to 

 the level of the lower end of the olivary body. This colunni contains nerve-cells 

 (tf large size. It represents the neck of the anterior horn of the grey crescent in 

 the spinal cord, and is continued into that structure below. In its lower part the 

 column is placed ventro-laterally to the central canal, but, as the central canal 

 expands into the floor of the fourth ventricle, the grey column is displaced laterally, 

 and thus apparently occupies a more dorsal position. It corresponds to the area 

 described as the trigonum hypoglossi, and is separated from the ependyma of th(^ 

 ventricle only by a thin layer of medullated fibres. From the cells embedded in 

 tliis grey column the nerve-fil)res arise. These fibres pass in bundles which are 

 directed ventrally and outwards, internal to the olivary nucleus, between the latter 

 and the internal accessory olive. Finally they emerge in a row of from ten to 

 twelve filaments in the furrow between the olivary and pyramidal bodies, consti- 

 tuting the superficial origin of the hypoglossal nerve. 



An accessoiy nucleus is also described which is placed ventro-laterally to the above described 

 (or chief) nucleus. This accessory nucleus represents a portion of the liead of the anterior liorn. 



Fibres pass from the inner side of the hypoglossal nucleus, and cross in the raphe from where 

 they ascend brainwards to establish the cortical origin of the nerve. 



The filaments unite to form two fasciculi, each of which pierces the dura mater 

 separately at a point opposite to the anterior condyloid foramen. As these fasciculi 

 traverse the foramen, they unite to form a single trunk, which is ]>laced at first on 

 the inner side of the pneumogastric, but then winds spirally round the ganglion of 

 the trunk of the latter nerve, passing behind, external to, and finally in front of the 

 ganglion. In this situation the hypoglossal receives two or three conimunicating 

 l)ranches from the first and second cervical nerves, and some minute twigs from 

 tlie vagus and sym]iathctic. The hypoglossal then runs forwards and inwards, 

 describing a curve, the convexity of which looks downwards and outwards. At the 

 conunencement of this ])art of its course it crosses the internal carotid artery and 

 hooks round the occipital artery, passing immediately below that vessel. It then 

 crosses external to the external carotid artery, the lingual artery, the middle con- 

 strictor of the pharynx, and the hyo-glossus muscle, and, passing under cover of 

 the mylo-hyoid, breaks uj) into its terminal branches. As it lies on the midilh' 

 constrictor the great cornu of the hyoid bone is below it and the tendon of the 

 digastric muscle is above it. As it passes on to the hyo-glossus it is crossed on its 

 superficial surface by the digastric and stylo-hyoid muscles. It is also overlapped 

 in this situation by the submaxillary gland. 



