96 THE ANATOMY OF THE HORSE. 



edge of the longissiinus doi-si, and another perforating its substance. 

 Both sets are derived from the superior primary branches of the dorsal 

 or himbar nerves. 



The arteries and veins are branches of the doi'so-spinal divisions of the 

 intei-costal or lumbar vessels. 



The Transversalis Costarum (Plate 21). (This muscle corresponds 

 to the ilio-costalis and musmlns accessorius of man.) This is a composite 

 muscle extending across the entire series of ribs, being five or six inches 

 removed from the spine posteriorly, but close to it in front. Its fibres 

 are directed forwards and slightly downwards, and it possesses two 

 series of tendons. One set, forming slips of origin, is concealed at the 

 upper edge of the muscle ; the other, serving as slips of insertion, is 

 visible at its lower edge. By the upper set of tendons it aHses from the 

 transverse processes of the first two lumbar vei'tebra;, and from the 

 anterior borders of the ribs. By the lower set of tendons it is imerted 

 into the hinder edges of the ribs anterior to the llth, and to the trans- 

 verse process of the last cervical vertebra. 



Action. — To pull the ribs backwards, and thus assist in expiration. 

 Both muscles acting together may also assist in extending the spine ; 

 or acting singly, they may incline it laterally. 



The LoxGissiMUS Dorsi (Plate 21). This is the longest and 

 strongest muscle in the body, and it is also the most complex. It 

 extends along the spine, from the sacrum to the neck. In the loins it 

 forms a great muscular and tendinous mass (the common mass of man) ; 

 and anteriorly it is bifurcate, the trachelo-mastoid and complexus 

 muscles getting origin between its two branches. Its fibres arise from 

 the sacral sui-face of the ilium between the crest and the sacro-iiiac joint, 

 and from a strong, glistening fascia covering the surface of the muscle, 

 this fascia being fixed to the lumbar and dorsal spines, or to the supra- 

 spinous ligament. Its fibres are inserted into the lumbar transverse and 

 articular processes, the dorsal transvei-se processes, and the ribs as far 

 outwards as the edge of the transversalis costarum. About the 5th rib it 

 divides ; and the lower branch, continuing the outer series of attachments, 

 is inserted into the ribs, and the transverse processes of the first four 

 dorsal and last four cei-vical vertebrai ; while the upper division, getting 

 many new fibres from the first four dorsal spines, becomes inserted into 

 the spinous processes of the four cervical vertebi*a3 in front of the last. 



Action. — Acting with the opposite muscle, it is the great extensor of 

 the dorso-lumbar portion of the spine, being, in this respect, the chief 

 antagonist of the sublumbar and abdominal muscles. By its costal 

 attachments it may also assist in expiration. By its cei-vical attach- 

 ments it raises the neck. Acting singly, it inclines the spine to the 

 side of the acting muscle. 



Retractor Cost^^e (Plate 45). This is a small triangular muscle 



