THORAX AND ABDOMEN OF THE HORSE 51 



permits an examination of the relative position of the various openings 

 associated with the ventricles. Cranial and to the left is that leading 

 into the pulmonary artery, and immediately caudal and to the right of 

 this is the aortic opening. The right atrio-ventricular opening is to the 

 right of the pulmonary orifice, while the left atrio-ventricular opening 

 is immediatel}^ caudal to the entrance into the aorta. 



Referred to the wall of the thorax, it may be said in general terms 

 that the pulmonary orifice is opposite the third rib and the third 

 interspace, the aortic orifice is opposite the fourth rib and the fourth 

 interspace, the right atrio-ventricular orifice is opposite the fourth and 

 fifth ribs, and the left atrio-ventricular orifice is opposite the fifth rib 

 and the fifth interspace. 



Dissection. — The sternum should now be removed in the following 

 manner. Cut through the first rib on each side about the middle of 

 its length. Cut across the internal thoracic artery (and vein) close to 

 its origin. Cut through the fifteenth, sixteenth and seventeenth ribs 

 at the same level as that at which the other ribs have been previously 

 severed. Run the knife through the intercostal muscles between the 

 seventeenth and eighteenth ribs and their cartilages. 



If now the diaphragm be incised about an inch from its sternal and 

 costal attachments, the sternum, &c., will be free. 



(In this operation it is assumed that the dissection of the abdomen 

 has been proceeding at the normal rate.) 



M. TRANSVERSUS THORACIS. — The transverse thoracic is a thin 

 sheet of muscle, triangular in outline, lying on the inner (dorsal) surface 

 of the sternum and costal cartilages and covered to a great extent by 

 pleura. Its origin is from the internal proper sternal ligament ; and its 

 insertion, by serrations, is to the costal cartilages from the second to 

 the eighth and the sternal ends of the corresponding rib bones. 



Dissection. — Follow the internal thoracic vessels b}^ incising the 

 transverse thoracic muscle along the line of their course. Occasionally 

 a few small lymph glands may be found along the vessels. 



A. THORACICA INTERNA. — The origin of the internal thoracic artery 

 from the subclavian about the caudal border of the first rib has already 

 been noted. After following the inner surface of the rib, the artery 

 disappears by passing under the edge of the transverse thoracic muscle. 

 It now runs towards the diaphragm beneath the muscle and along the 

 line of articulations of the costal cartilages and the sternum, and at the 

 seventh or eighth cartilage it divides into the musculo-phrenic and 

 cranial epigastric arteries. Its branches are as follows : — 



(1) Rami inter cost ales. — Intercostal branches are distributed to the 

 sternal ends of the first seven intercostal spaces, where they anastomose 

 with the terminations of the corresponding intercostal arteries. 



(2) Rami perforantes. — Perforating branches supply the transverse 



