io6 THE NERVOUS SYSTEM 



In the upper part of the neck, the hypoglossal nerve receives 

 a branch of communication from the anterior ramus (primary 

 division) of the first cervical nerve. This communication 

 leaves the nerve in three parts, which are all distributed to 

 muscles acting on the hyoid bone. The first part constitutes 

 the ramus descendens hypoglossi, which unites with the ramus 

 descendens cervicalis (C. 2 and 3) to form the ansa hypoglossi. 

 From the loop thus formed the sterno-hyoid, the sterno- 

 thyreoid and the omo-hyoid muscles receive their nerve 

 supply. The second part supplies the thyreo-hyoid ; the third 

 part is distributed to the genio-hyoid. 



The sterno-thyreoid passes upwards from the posterior aspect of the 

 manubrium sterni to the lateral aspect of the thyreoid cartilage and, as it 

 ascends, it covers the lateral lobe of the thyreoid gland. 



The sterno-hyoid covers the medial part of the sterno-thyreoid and extends 

 upwards to the hyoid bone. 



The omo-hyoid is a digastric muscle. Its posterior belly runs medially 

 from the upper border of the scapula and, under cover of the sterno- 

 mastoid, ends in the common tendon, which is held down to the medial 

 end of the clavicle by a slip of the deep cervical fascia. Its anterior belly 

 runs upwards, superficial to the sterno-thyreoid and along the lateral 

 border of the slerno-hyoid, to reach the hyoid bone. 



The thyreo-hyoid may be regarded as the upward continuation of the 

 sterno-thyreoid to the hyoid bone. 



These four muscles depress the hyoid bone and larynx in the last stage 

 of the act of deglutition, and they have a steadying action when these 

 structures are being elevated. 



Paralysis of this group, combined with paralysis of the 

 tongue on the same side, is symptomatic of a lesion of the 

 hypoglossal nerve in the first part of its extra-cranial course. 

 Owing to paralysis of the depressors, the tonus of the un- 

 opposed elevators (rn.ylo-hyoid and digastric) causes the greater 

 cornu of the hyoid bone to lie at a higher level on the side of 

 the lesion. When the muscles become atrophied, the condition 

 is readily recognised on palpation of the thyreoid cartilage. 



The genio-hyoids are two short muscles which extend from the deep 

 surface of the mandible at the symphysis to the hyoid bone. They lie deep 

 to the mylo-hyoids and aid them in elevating the hyoid bone and larynx. 



