98 THE ANATOMY OF THE HORSE. 



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

 or lumbar nerves. 



The arteries and veins are branches of the dorso-spinal divisions of the 

 intercostal or lumbar vessels. 



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

 to the ilio-costalis and musculus accessories 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 arises fcom the 

 transverse processes of the first two lumbar vertebras, and from the 

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

 into the hinder edges of the ribs anterior to the 14th, 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 Longissimus 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 surface 

 of the ilium between the crest and the sacro-iliac 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 supraspinous 

 ligament. Its fibres are inserted into the lumbar transverse and 

 articular processes, the dorsal transverse 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 attach- 

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

 four dorsal and last four cervical vertebrae ; while the upper division, 

 getting many new fibres from the first four dorsal spines, becomes 

 inserted into the spinous processes of the four cervical vertebras 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 cervical 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 



