THE INFRAHYOID MUSCLES. 299 



nerve. In most cases a third factor is added, the cleido-occipital, which arises from the 

 clavicle in front of or outside the cleido-mastoid and applies itself to the hinder border of the 

 sterno-mastoid, being inserted in continuation thereof along the superior curved line. The 

 clavicular origin of the muscle varies greatly in width, being sometimes as narrow as the 

 sternal, while in other instances it extends for three inches along the bone ; in such cases it 

 may be divided into separate slips. In animals without a clavicle the cleido-mastoid is 

 continued into the clavicular part of the great pectoral muscle or deltoid, thus forming a 

 cephalo-humeral muscle. 



The transversus nuchee is a fasciculus of frequent occurrence springing from the external 

 occipital protuberance in connection with the upper end of the trapezius, and inserted into 

 the aponeurosis of the sterno-mastoid. When absent, tendinous fibres often run-in its place. 



The siqwaclavicularis (Luschka) is a small muscle not unfrequently met with, lying behind 

 the origin of the sterno-mastoid. It arises by a slender tendon from the upper border of the 

 manubrium, passes upwards and outwards above the sterno-clavicular articulation, and is 

 inserted into the upper surface of the clavicle. When present on both sides the two muscles 

 may be joined in the middle line. 



The supraclavicularis proprius (Gruber) is attached to the clavicle at each end, forming 

 an arch above the middle of the bone. It has also been seen ending in the fascia over the 

 adjacent part of the trapezius. 



Nerves. The sterno-mastoid receives its principal nervous supply from the spinal 

 accessory, the offsets of which to the muscle are joined by a branch of the second cervical 

 nerve. 



THE INFBAHYOID MUSCLES. 



The sterno-hyoid muscle, a flat band of longitudinal fibres, arises variably, 

 from the back of the sternum and the posterior sterno-clavicular ligament, from 

 that ligament and the clavicle, or from the clavicle only, and sometimes, to a small 

 extent, from the cartilage of the first rib. It is inserted by short tendinous fibres 

 into the lower border of the body of the hyoid bone for the inner half. 



Relations. The muscle is concealed below by the sternum and the sterno-mastoid 

 muscle, higher up by the skin and fascia only ; it lies on the sterno- thyroid and thyro-hyoid 

 muscles, which it partly covers. The inner border approaches that of the corresponding 

 muscle towards the middle of its extent, but is separated from it by a slight interval 

 superiorly, and by a larger interval near the sternum ; the outer margin is in contact with 

 the omo-hyoid near the hyoid bone. 



Varieties. Doubling of the sterno-hyoid is occasionally met with, or more rarely an 

 accessory cleido-liyoid muscle, arising from the clavicle, and inserted into the hyoid bone in 

 front of the sterno-hyoid. Absence of the sterno-hyoid on one or both sides has been 

 seen. The muscular fibres are occasionally interrupted by a transverse tendinous intersection 

 at the level of the intermediate tendon of the omo-hyoid, or in rare cases opposite the insertion 

 of the sterno-thyroid. 



The sterno-thyroid, broader and shorter than the preceding muscle, behind 

 which it lies, arises from the thoracic surface of the first piece of the sternum, 

 lower down and more internally than the sterno-hyoid, and variably from the first 

 and second rib-cartilages ; it ascends, diverging a little from its fellow, and is 

 inserted into the oblique line on the ala of the thyroid cartilage. 



Relations. The greater part of its anterior surface is concealed by the sternum and the 

 sterno-hyoid muscle, as well as by the sterno-mastoid. By its deep surface it rests on the 

 innominate vein, the lower part of the common carotid artery, the trachea, and the thyroid 

 body. The inner margin is contiguous to the muscle of the other side in the lower part of 

 the neck. 



Varieties. The two sterno-thyroid muscles are frequently united across the middle line 

 at their origins. Absence of the muscle has been observed, also doubling. It is occasionally 

 partly crossed by transverse or oblique tendinous lines. At the upper extremity a few fibres 

 are often continued into the thyro-hyoid muscle or into the inferior constrictor of the pharynx. 

 From the outer border a slip is occasionally given off to the sheath of the carotid vessels, 

 covering them as high as the thyroid cartilage (costo-fascialis). 



The thyro-hyoid muscle forms a continuation upwards of the preceding ; it 



