756 THE NERVOUS SYSTEM. 



derived from a branch which passes from the first cervical nerve to the hypoglossal 

 nerve. 



The descendens hypoglossi is a long slender branch, which quits the hypoglossal 

 where it turns round the occipital artery. It consists mainly of fibres which pass 

 to the hypoglossal from the first and second cervical nerves in the above-mentioned 

 communication. It descends in front of or within the sheath of the carotid vessels, 

 giving off a branch to the anterior belly of the Omo-hyoid, and then joins the 

 communicating branches from the second and third cervical nerves, just below the 

 middle of the neck, to form a loop, the ansa hypoglossi. From the convexity of 

 this loop branches pass to supply the Sterno-hyoid, Sterno-thyroid, and the pos- 

 terior belly of the Omo-hyoid. According to Arnold, another filament descends 

 in front of the vessels into the chest, and joins the cardiac and phrenic nerves. 



The thyro-hyoid is a small branch arising from the hypoglossal near the poste- 

 rior border of the Hyo-glossus ; it passes obliquely across the great cornu of the 

 hyoid bone and supplies the Thyro-hyoid muscle. 



The muscular branches are distributed to the Stylo-glossus, Hyoglossus, Genio- 

 hyoid, and Genio-hyo-glossus muscles. At the under surface of the tongue numer- 

 ous slender branches pass upward into the substance of the organ to supply its 

 intrinsic muscles. 



Surgical Anatomy. The hypoglossal nerve is an important guide in the operation of 

 ligation of the lingual artery (see page 489). It runs forward on the Hyo-glossus just above 

 the great cornti of the hyoid bone, and forms the upper boundary of the triangular space in 

 which the artery is to be sought for by cutting through the fibres of the Hyo-glossus. 



THE SPINAL NERVES. 



The spinal nerves are so called because they take their origin from the spinal 

 cord, and are transmitted through the intervertebral. foramina on either side of the 

 spinal column. There are thirty-one pairs of spinal nerves, which are arranged 

 into the following groups, corresponding to the region of the spine through which 

 they pass : 



Cervical 8 pairs. 



Dorsal 12 " 



Lumbar . 5 " 



Sacral 5 " 



Coccygeal 1 pair. 



It will be observed that each group of nerves corresponds in number with the 

 vertebrae in that region, except the cervical and coccygeal. 



Each spinal nerve arises by two roots, an anterior or motor root and a pos- 

 terior or sensory root, the latter being distinguished by a ganglion, termed the 

 spinal ganglion. 



The Roots of the Spinal Nerves. 



The Anterior Roots. The superficial origin is from the antero-lateral columns 

 of the cord, corresponding to the situation of the anterior cornu of gray matter. 

 Each root is composed of from four to eight filaments. 



The deep origin can be traced through the antero-lateral column ; the roots, 

 after penetrating horizontally through the longitudinal fibres of this tract, enter 

 the gray substance, where their fibrils diverge in several directions : some passing 

 inward, are continued across the anterior commissure in front of the central canal, 

 to become continuous with the axis-cylinder processes of the large cells of the 

 anterior cornu of the opposite side; others terminate in the mesial group of cells 

 of the anterior column of the same side ; other fibrils pass outward, to become con- 

 tinuous with the axis-cylinder processes of the group of cells in the lateral part of 

 the anterior column. 



The Posterior Eoots. The superficial origin is from the postero-lateral fissure 

 of the cord. The real origin of these fibres is from the nerve-cells in the posterior 



