4i0 THE ANATOMY OF THE HORSE 



SuPERFiciALis COSTARUM lies on the back in the form of a thin layer of 

 aponeurosis, edged with fleshy slips, which indigitate with those of the 

 obliquus abdominis externus (see Fig. 61). Its action is to raise the ribs and 

 increase the capacity of the chest. 



TuAXSVERSALis COSTARUM is situated on the supero-lateral part of the 

 thorax. Origin — from the ribs close to the spine. Insertion — to the 

 transverse process of the last cervical vertebrse. Action — to aid the last- 

 named muscles. 



Levatores COSTARUM are fifteen or sixteen muscular slips, which con 

 nect the transverse processes of the dorsal vertebrfe with the anterior 

 borders of the ribs, in the spaces between their tubercles and angles 

 Action — to raise the ribs and enlarge the cavity of the thorax. 



DORSAL REGION 



LoNGissiMUS DORSi lies along the back beneath the muscles of the supe- 

 rior extremity (which have been removed). It is a large powerful muscle, 

 and forms the chief mass of the soft parts constituting the loins and back. 

 Origin — from the crest of the ilium, side of the sacrum, and spinous and 

 transverse processes of all the lumbar vertebrae. Insertion — to the angles 

 of the twelve posterior ribs, and to the transverse processes of all the dorsal 

 vertebrze, and of the three posterior cervical. Action — to bend the back, 

 and thus raise either the fore or hind-quarter, when the other is fixed. It 

 is the main agent in rearing and kicking, and is strongly called into play 

 in galloping and leaping. 



Spinalis dorsi is situated deeply on the sides of the withers. Origin 

 — it is closely connected posteiiorly with the last muscle, being attaclied 

 to the spinous processes of the posterior dorsal vertebrse. Insertion — to the 

 spines of the six or seven anterior dorsal vertebrae, and the three or four 

 posterior cervical. Action — to assist the longissimus dorsi in rearing and in 

 raising the fore-quarters in galloping. 



Semi-spinalis dorsi is deeply buried beneath the two last muscles, with 

 which it co-operates in its action. 



MUSCLES OF THE ABDOMEN 



These are naturally divided into two groups, according to the 

 positions which they occupy and the offices they perform. Thus the 

 superficial abdominal muscles form the lower walls of the cavity of the 

 abdomen, while the deep abdominal muscles bound it anteriorly and 

 superiorly. 



SUPERFICIAL ABDOMINAL REGION 



The abdominal muscles, four in number, constitute the lower walls of the 

 belly, and together form, as it were, a strong sheet, by means of which the 

 intestines and abdominal organs are kept in position. After reflecting the 



