THE VAGO-ACCESSORY MUSCLES. 499 



overlapping the lower part of the superior constrictor and extending in some cases 

 almost to the occipital bone, while the lower fibres are overlapped by the inferior 

 constrictor. 



Nerve-Supply. — From the pharyngeal plexus, probably by fibres from the 

 anterior portion of the spinal accessory nucleus. It is said to be supplied also by 

 the glosso-pharyngeal nerve. 



Action. — To compress the pharynx. 



Variations. — As in the case of the superior constrictor, the fibres from different parts of 

 the origin may have considerable independence. Thus the fibres from the greater cornu of the 

 hyoid have been recognized as a muscle, the ceraio-pharyngeus, distinct from the remainder, to 

 which the term chotidro-pharyngeus has been appUed. 



8. Constrictor Pharyngis Inferior (Figs. 501, 510). 



Attachments. — Like the middle constrictor, the inferior is also a fan-shaped 

 sheet and arises from the outer surface of the thyroid and cricoid cartilages of the 

 larynx. The fibres radiate backward to be inserted into the pharyngeal raphe, the 

 upper ones overlapping the lower part of the middle constrictor, while the lower 

 ones mingle with the musculature of the oesophagus. 



Nerve-Supply. — From the pharyngeal plexus, probably through fibres from 

 the anterior part of the nucleus of the spinal accessory. It is said to receive also 

 fibres from the vagus through both the superior and inferior laryngeal nerves. 



Action. — To compress the pharynx. The three constrictors of the pharynx 

 play important parts in the final acts of deglutition, forcing the food towards the 

 oesophagus. They are also important agents in producing modulations of the voice, 

 since the pharynx may be regarded as forming a resonator, alterations of whose form 

 will naturally result in modifications of voice. 



» 



Variations. — The portions of the muscle arising from each of the two laryngeal cartilages 

 may be more or less distinct and have been termed the thyro-pharyngeus and crico-pharyngeus. 



{b\ THE MUSCLES OF THE LARYNX. 



The muscles of the larynx will be considered in connection with the description 

 of that organ (page 1824). 



(f) THE TRAPEZIUS MUSCLES. 



I. Sterno-cleido-mastoideus. 2. Trapezius. 



This group includes but two muscles, the trapezius and sterno-cleido-mastoid, 

 which extend from the skull to the pectoral girdle. Both are in reality compound 

 muscles, formed by the fusion of fibres derived from the branchiomeres supplied by 

 the spinal accessory with portions of the myotomes supplied by the second, third, 

 and fourth cervical nerves. Strictly speaking, therefore, they belong only partially 

 to the series of branchiomeric muscles, but the union of the elements derived from 

 the two sources is so intimate that any attempt to distinguish them in a brief descrip- 

 tion of the muscles would tend to confusion. 



I. Sterno-Cleido-Mastoideus (Fig. 541). 



Attachments. — The sterno-mastoid is attached below by two heads to the 

 sternum and the clavicle. The sternal head arises by a strong rounded tendon from 

 the anterior surface of the manubrium Sterni, while the clavic7ilar head is more band- 

 like, and takes origin from the upper surface of the sternal end of the clavicle. 

 The two heads are directed upward and backward, the clavicular head gradually 

 passing beneath the sternal one, and the two, eventually fusing, are inserted into the 

 mastoid process of the temporal bone and into the outer part of the superior nuchal 

 line. 



