DISSECTION OF THE BACK AND THORAX. 99 



which lies. under cover of the last slip of the serratus posticus. It is 

 thin and aponeurotic at its upper edge, where it arises from the first 

 two or three lumbar transverse processes. The remainder of the muscle 

 is fleshy, and it is inserted by its anterior edge into the posterior border 

 of the last rib. Its lower edge is parallel to the highest fibres of the 

 internal oblique muscle of the abdomen, and it is generally described as 

 a part of that muscle. 



Action. — To assist in expiration. 



Directions. — Two sets of muscles lie under cover of the longissimus 

 dorsi, viz., the semispinals of the back and loins, and the levatores 

 costarum. A segment of the longissimus, from the 13th to the 17th 

 rib, should be excised after the fashion of Plate 21 ; or, if it be desired 

 to expose the whole of each series, the longissimus dorsi must be 

 entirely removed. 



The Levatores Costarum (Plate 21). These form a series of small 

 muscles, each occupying the extreme upper part of an intercostal space, 

 and at that point taking the place of the external intercostal muscle. 

 Each arises from the transverse process of a dorsal vertebra; and, 

 passing downwards and backwards, it expands, and becomes inserted 

 into the outer surface of the rib posterior to the vertebra from which it 

 takes origin. In the first two or three spaces the muscles are rudi- 

 mentary or absent. 



Action. — To assist in inspiration. 



The Semispinals of the back and loins (Plate 21). This is a com- 

 posite muscle, covering the sides of the vertebral spines from the sacrum 

 to the neck, and consisting of numerous fasciculi directed obliquely 

 upwards and forwards. Anteriorly these fasciculi are in series with 

 the semispinals colli, and posteriorly with the curvator coccygis. The 

 fasciculi take origin from the lateral lip of the sacrum, from the articular 

 tubercles of the lumbar vertebras, and from the transverse processes of 

 the dorsal vertebras. They become inserted into the vertebral spines, 

 ^ each fasciculus being inserted into the 3rd or 4th vertebra anterior to the 

 one from which it takes origin. In the forepart of the dorsal region (Fig. 

 20, page 160) the insertion is into the sides of the spines, but elsewhere 

 it is into or near the summits of the processes. 



Action. — It is an extensor or a lateral flexor of the spine, according as 

 the right and left muscles act together or singly. 



Directions. — Clean the outer surfaces of a few of the external inter- 

 costal muscles about the middle of the series, and at the side of the 

 sternum define the lateralis sterni muscle. 



The Lateralis Sterni. This is a thin, flat muscle, a few inches 

 broad. Arising from the outer surface of the 1st rib above its car- 

 tilage, it passes obliquely downwards and backwards over the 2nd 

 chondro-costal joint, and over the 3rd and 4th costal cartilages, and 



