THE NERVOUS SYSTEM 1499 



the cranial ca\'itv, and they join the spinal part of the accessory 

 nen-e (which has entered the cranial ca\ity through the foramen 

 magnum), the accessory nerve subsequent!}- emerging from the 

 cranial ca\'ity through the jugular foramen. 



Spinal Part. — ^The fibres of this part arise as the axons of the 

 cells of the spinal portion of the accessory nucleus. They are 

 directed at first dorsalwards, and then outwards through the lateral 

 column of the spinal cord, from which they emerge as a series of 

 funiculi which succeed to the funiculi of the bulbar part, the lowest 

 spinal funiculus being on level with the fifth cer\'ical ners'^e. The 

 funiculi of the spinal part ascend, l\Tng in the subdural space 

 between the ligamentum denticulatum and the posterior roots of 

 the upper five cervical nerves. They enter the cranial cavity 

 through the foramen magnum, and join the funiculi of the bulbar 

 part, to form the accessory nerve, which, as stated, emerges from 

 the cranial ca\-ity through the jugular foramen. 



Distribution of Accessory Nerve. — After emerging from the jugular 

 foramen, the accessory nerve divides into two branches — internal 

 and external — ^the internal branch containing the fibres of the bulbar 

 part, whilst the external branch contains the fibres of the spinal part. 



Spinal Distribution. — ^The external or spinal branch supplies the 

 sterno-cleido-mastoid and trapezius muscles. 



Bulbar Distribution. — The internal or bulbar branch passes over, 

 and in close contact with, the ganglion of the trunk of tiie pneumo- 

 gastric nerve. Its fibres are continued into (i) the phar^iigeal and 

 superior lar\Tigeal branches of the ganglion of the trunk of the 

 vagus, and (2) the trunk of the vagus beyond the ganglion. 



The bulbar fibres, through their connection with the vagus nerve, 

 are probably distributed to ^) the muscles of the soft palate, 

 excluding the tensor palati; (2) the constrictor muscles of the 

 phar^Tix; and (3) the intrinsic muscles of the lar^-nx. The bulbar 

 fibres ma}' also furnish (a) the inhibitory fibres to the heart, (b) the 

 motor fibres to the oesophagus, and (c) the motor fibres to the 

 stomach. 



The bulbar fibres of the accessory nerve are regarded by some 

 authorities as arising from the column of cells which constitutes the 

 ventral vago-glo5so-phar\-ngeal nucleus, or nucleus ambiguus. 



Twelfth or Hypoglossal Nerve. — The fibres of the hypoglossal 

 ner\-e arise from the axons of the cells of the hypoglossal nucleus, 

 which is situated within the medulla oblongata. This nucleus 

 represents a column of large multipolar motor-cells, which extends 

 from the level of the striae acusticae superiorly to the level of the 

 upper part of the decussation of the p\Tainids inferiorly. The lower 

 part of the nucleus is situated within the lower or closed part of 

 the medulla oblongata, and its upper part lies within the upper, 

 open, or ventricular part. The lower part lies in the grey matter 

 which forms the ventro-lateral aspect of the central canal of the 

 medulla oblongata. The upper part hes in the grey matter which 

 covers the bulbar part of the floor of the fourth ventricle, and is 

 imdemeath the area known as the trigonum hypoglossi. 



