io82 A MANUAL OF ANATOMY 



The lateral lobes of the thyroid body may be felt on either side of 

 the larynx and trachea as low as about the fifth ring ; and, in young 

 children, the thymus body extends upwards upon the trachea for 

 some distance above the manubrium sterni. 



The great cornu of the hyoid bone is the guide to the lingual artery 

 for operative purposes, the vessel lying just above the great cornu. 



The upper border of the thyroid cartilage is on a level with the disc 

 between the bodies of the third and fourth cervical vertebrae. 



The narrow anterior part of the cricoid cartilage is on a level with 

 the disc between the bodies of the fifth and sixth cervical vertebrae, 

 at which level the pharynx terminates in the oesophagus. In this 

 situation a foreign body, when swallowed, is liable to become 

 impacted. 



Above the middle third of the clavicle, between the sterno- 

 cleido -mastoid and trapezius muscles, there is a depression known 

 as the supraclavicular fossa, which indicates the position of the sub- 

 clavian or supraclavicular triangle, with the third part of the sub- 

 clavian artery and the trunks of the brachial plexus of nerves. 



Platysma Myoides — Origin. — ^The superficial fascia which covers 

 the clavicular parts of the pectoralis major and deltoid muscles. 



Insertion. — (i) The outer aspect of the base of the inferior maxilla 

 from the symphysis to the anterior border of the masseter muscle ; 

 and (2) the angle of the mouth, where the fibres blend with those 

 of the depressor anguli oris and orbicularis oris. 



Nerve- supply. —The inframandibular (cervical) branch of the 

 cervico-facial division of the facial nerve, which communicates 

 with branches of the superficial cervical nerve. 



The fibres are directed upwards and forwards or inwards over the 

 clavicle and side of the neck. 



Action. — (i) To draw the angle of the mouth downwards and out- 

 wards ; (2) to act as a feeble depressor of the inferior maxilla ; 

 (3) to raise the skin of the neck and upper pectoral region, as far 

 out as the acromion process, throwing it into obliquely-disposed 

 folds. By diminishing pressure upon the external jugular vein 

 during forced inspiration the muscle contributes to the return of 

 blood through that vessel. 



The muscle forms an extensive, thin, pale sheet, which is 

 embedded within the superficial fascia. In the region of the 

 symphysis of the inferior maxilla the innermost fibres decussate 

 across the middle line with those of the opposite side, the fibres of 

 the right muscle being superficial. The muscle covers the external 

 and anterior jugular veins, the superficial branches of the cervical 

 plexus of nerves, the subclavian triangle, and the sterno-cleidc- 

 mastoid, sterno-hyoid, omo-hyoid, and digastric muscles. 



The platysma myoides in man is a remnant of a subcutaneous muscular 

 sheet, called the panniculus carnosus, which exists in many animals, and by 

 which the twitching of the skin is produced. 



Jugular Veins. — ^The jugular veins are four in number on eithei" 

 side, namely, anterior, external, posterior, and internal. 



