MUSCLES OF THE THORAX 439 



spine. Insertion — to the rough tubercle on the inner side of the body of 

 the humerus. Action — to draw the humerus upwards and backwards. 



RiiOMBOiDEUS BBEVis, lying upon the side of the withers, has its orvjin 

 from the spines of the second, tliird, and fourth dorsal vertebrae. Insertion 

 — to the inner ptu-t 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. Origin — from 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. Insertion — 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. Insertion — 

 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 downwai-ds 

 and forwards. 



Pectoralis parvus lies between the fore-leg and side of the thorax. Origin 

 — from the lateral parts of the cariniform cartilage, and slightly from the 

 four anterior bones of tlie sternum. Insertion — 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. Action — 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 vertebrae, 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. Insertion — to the superior border, and by a few 

 fibres to the ventral surface and cartilage of the scapula. Action — it 

 tends greatly to support the trunk in a sling, especially when the weight of 

 the body comes down with a sudden shock, as in jumping. The two muscles 

 are more or less concerned in all the motions of the scapula, and will become 

 dilators of the 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 intercos- 

 tales externi and interni. 



Lateralis sterni and sterxo costales assist the intercostals in contract- 

 ing the chest. 



G G 



