200 OUTLINES or EQUINE ANATOMY. 



artery, so that tlie vessel is largest and its walls thinnest 

 and most liable to rupture here. From the posterior and 

 also from the internal sinus Valsalvse a coronary artery 

 arises, going to supply the substance of the heart. The left 

 coronary artery, the shortest, runs directly down the left 

 ventricular furrow, and its terminal branches anastomose 

 with those of the right at the point of union of the two 

 ventricular furrows. It gives off a branch in a posterior 

 direction which passes in the posterior auriculo -ventricular 

 furrow to the vessel of the right side. The right coronary 

 artery, after winding through the anterior auriculo -ventri- 

 cular furrow, meets the posterior branch of the left artery 

 and then runs down the right ventricular furrow to meet 

 the left artery at the inferior extremity. The blood from 

 the heart is returned by the coronary or cardiac veins, 

 of which the left is the longest ; they terminate almost 

 by a common opening in the right auricle as already 

 described. 



The heart is supplied with nerve power from three 

 sources : By the ^neumogastric and gangliated cord of the 

 symjMtlietic through the medium of the cardiac plexus, and 

 by small symjKitlietic ganglia emhedded in its substance. The 

 latter endow it with its regularity of contraction (rhythmic 

 action), the pneumogastric serves to restrain, the glan- 

 gliated cord of the sympathetic to stimulate its action. 



The muscular fibres of the heart are involuntary ; they 

 nevertheless present slight striation and a modena red 

 colour, similar to that of voluntary muscle. They differ 

 from the latter, however, in the imperfection of their 

 striation, in the absence of a sarcolemmatous sheath, and 

 in frequent anastomosis of the constituent fibrillse. 



In the infero-anterior part of the thorax, at the lower 

 part of the anterior mediastinum, especially developed in 

 the foetus, in whom it extends a short way up the neck, is 

 the thymus gland, a body resembling a salivary gland in 

 appearance, and pouring the product of its lobules into a 

 central cavity, from which, as there is no duct, it seems to 

 be reabsorbed. It is a vascular gland, and is the true 

 " sweetbread." 



