392 THE HORSE. 
RHOMBOIDEUS BREViSs, lying upon the side of the withers, has its 
origin from the spines of the second, third, and fourth dorsal vertebra. 
Insertion—to the inner part of the superior border and cartilage of the 
scapula. This muscle is clothed externally with yellow elastic tissue. 
Action—to elevate the scapula. 
PECTORALIS TRANSVERSUS is placed on the lateral, anterior, and posterior 
parts of the breast. Ovigin—trom the lateral and posterior parts of the 
cariniform cartilage, over which it plays, being lubricated by a bursa, 
and meets its fellow by a white tendinous line. Jnsertion—to the antero- 
inferior part of the body of the humerus, “scaber canalis,” and the 
fascia covering the leg. Action—it forms a sling to support the trunk, 
and to keep the arm close to the chest during locomotion. 
PECTORALIS MAGNUS is situated on the infero-lateral part of the thorax. 
Origin—from the fourth, fifth, and sixth bones of the sternum, ensiform 
cartilage, and the cartilages of the four posterior true ribs. It is attached 
by fascia to the external oblique muscle of the abdomen. Jnsertion— 
bifid : one portion to the internal tubercle at. the head of the humerus ; 
the other consists of a few fibres, which pass over to the external tubercle 
at the head of the humerus. Action—to draw the shoulder downwards 
and backwards. ; 
PECTORALIS PARVUS lies between the fore leg and side of the thorax. 
Origin—trom the lateral parts of the cariniform cartilage, and slightly 
from the four anterior bones of the sternum. Jnsertion—to the outer 
tubercle at the head of the humerus, and to the supero-anterior border of 
the scapula, reaching nearly to its antero-superior angle. A ction—to 
assist the last-named muscle. 
SERRATUS MAGNUS is situated between the shoulder-blade and side of 
the chest. Origin—from the transverse processes of the four posterior 
cervical vertebree, and from the entire length of the first, second, third, 
fourth, and fifth ribs as low as their cartilages ; from the sixth, seventh, 
and eighth as low as their middles; digitating with four portions of the 
external oblique muscle. Jnsertion—to the uperior border, and by a few 
fibres to the ventral surface and cartilage of the scapula. Actton—it 
tends greatly to support the trunk im a sling, especially when the weight 
of the body comes down with a sudden shock, asin jumping. The two 
muscles are more or less concerned in all the motions of the scapula, and 
will become dilators of th2z chest whenever they are contracted, while the 
limbs remain fixed points. 
MUSCLES OF THE THORAX. 
THE RIBS are approximated to each other by two layers of muscles, 
which cross each other, so that when acting together the greater length of 
fibre given by this arrangement increases their power. ‘These are the 
INTERCOSTALES EXTERNI and INTERNI. 
LATERALIS STERNI and STERNO COSTALES assist the intercostals in 
contracting the chest. 
SUPERFICIALIS COSTARUM lies on the back in the form of a thin layer of 
apaneurosis, edged with fleshy slips, which indigitate with those of the 
obliquus abdominis externus (see Fig. 2,37). Its action is to raise the ribs 
and increase the capacity of the chest. 
TRANSVERSALIS COSTARUM is situated on the supero-lateral part of the 
thorax. Origin—from the ribs close to the spine. Jnsertion—to tha _ 
