MYOLOGY. 53 



side of the median line, and are twenty-one pairs in number, arranged iu 

 three layers, a superficial, middle, and deep. The superficial consists of 

 two pairs, the platysma myoides and sterno-cleido-mastoid. The middle 

 may be divided into two orders, the inferior and superior. • The inferior are 

 three : the sterno-hyoid, sterno-thyroid, and omo-hyoid ; the superior are 

 nine: digastric, mylo-hyoid, genio-hyoid, three styloid muscles, and the 

 hyo-glossus, genio-hyo-glossus, and lingualis muscles. The deep layer con- 

 sists of seven pairs : longus colli, rectus capitis, anticus major and minor, 

 rectus lateralis, and three scaleni. This arrangement excludes the muscles 

 of the palate, pharynx, and larynx. 



The platysma myoides, or latissimus colli, is a thin, pale, cutaneous muscle, 

 weak and indistinct in many subjects ; it is situated on the fore part and 

 side of the neck, extending from the chest and shoulder to the face ; it 

 arises by many fine fibres from the cellular membrane covering the upper 

 part of the deltoid and pectoral muscles. It is inserted, first, into the skin 

 and cellular tissue of the chin ; second, into the fascia along the sides of the 

 lower jaw ; third, into the fascia which covers the parotid, and which adheres 

 to the meatus auditorius. Its use is to depress the angle of the lip and the 

 lower jaw, as also to compress and support the several muscles, glands, and 

 vessels in the region of the neck. 



The sterno-cleido-mastoideus is situated at the anterior and lateral part of 

 the neck : it arises by a strong flat tendon from the upper and anterior part 

 of the first bone of the sternum, also from the upper edge of the clavicle, 

 and is inserted into the upper part of the mastoid process, and into the 

 superior transverse ridge of the occipital bone. The sternal portion can 

 rotate the head so as to turn the face towards the opposite side ; the clavi- 

 cular can bend the head and neck, so as to approximate the ear and shoulder. 

 Both portions acting together on each side will move the head downwards 

 and forwards. 



The steriw-hyoideus^ a long, flat, and thin muscle, arises within the thorax 

 from the posterior surface of the first bone of the sternum and sternal end 

 of the clavicle, and is inserted into the lower border of the body of the os 

 hyoides, internal to the omo-hyoid. Use: to depress the os hyoides, pharynx, 

 and larynx. 



The sterno-thyroideus is broader and shorter than the last : it arises from 

 the posterior surface of the sternum and cartilage of the second rib, and is 

 inserted into the oblique line on the ala of the thyroid cartilage. Its use is 

 to depress the larynx. 



The omo-hyoideiis is a long and slender muscle, situated obliquely along 

 the inferior, lateral, and fore part of the neck. It arises from the superior 

 costa of the scapula behind its semi-lunar notch, and sometimes from the 

 acromial end of the clavicle, and is inserted into the lower border of the os 

 hyoides. Its use, in conjunction with its fellow on the opposite side, is to 

 draw the os hyoides, pharynx, and larynx, downwards and backwards. 



The digastricus^ placed at the lateral and anterior part of the neck, thick 

 and fleshy at each extremity, round and tendinous in the centre, arises from 

 a groove in the temporal bone internal to the mastoid process, and is in- 



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