SPINAL NERVES. 455 



(communicans noni) from the second and third cervical nerves. From 

 the convexity of this loop branches are sent to the sterno-hyoideus, 

 sterno-thyroideus, and both bellies of the omo-hyoideus ; sometimes 

 also a twig is given off to the cardiac plexus, and occasionally one to 

 the phrenic nerve. If the descendens noni be traced to its origin it 

 will be found to be formed by a branch from the hypoglossal, and one 

 from the first and second cervical nerves ; occasionally it receives also 

 a filament from the pneumogastric. 



The Tliyro-hyoidean nerve is a small branch, distributed to the 

 thyro-hyoideus muscle. It is given off from the trunk of the hypo- 

 glossal near the posterior border of the hyoglossus muscle, and 

 descends obliquely over the great cornu of the os hyoides. 



The Communicating filaments, with the gustatory nerve, are several 

 small twigs, which ascend upon the hyoglossus muscle near its ante- 

 rior border, and form a kind of plexus with filaments sent down by 

 the gustatory nerve. 



SPINAL NERVES. 



There are thirty-one pairs of spinal nerves, each arising by two 

 roots, an anterior or motor root, and a posterior or sensitive root. 



The anterior roots proceed from a narrow white line, anterior lateral 

 sulcus, on the an tero- lateral column of the spinal cord, and gradually 

 approach towards the anterior longitudinal fissure as they descend. 



The posterior roots, more regular than the anterior, proceed from the 

 posterior lateral sulcus, a narrow grey stria, formed by the internal 

 grey substance of the cord. They are larger, and the filaments of 

 origin more numerous than those of the anterior roots. In the inter- 

 vertebral foramina there is a ganglion on each of the posterior roots. 

 The first cervical nerve forms an exception to these characters ; its 

 posterior root is smaller than the anterior ; it often joins in whole or 

 in part with the spinal accessory nerve and sometimes with the hypo- 

 glossal : there is frequently no ganglion upon it, and when the 

 ganglion exists it is often situated within the dura mater, the latter 

 being the usual position of the ganglia of the last two pairs of spinal 

 nerves. 



After the formation of a ganglion, the two roots unite and constitute 

 a spinal nerve, which escapes through the intervertebral foramen 

 and divides into an anterior branch for the supply of the front aspect 

 of the body, and a posterior branch for the posterior aspect. In the 

 first cervical and two last sacral nerves this division takes place 

 within the dura mater and in the upper four sacral nerves externally 

 to that cavity, but within the sacral canal. The anterior branches, 

 with the exception of the first two cervical nerves, are larger than the 

 posterior ; an arrangement which is proportioned to the larger extent 

 of surface they are required to supply. 



