THORAX AND ABDOMEN OF THE HORSE 23 



covered by pleura (pleura pericardiaca), and, as has just been noticed, is 

 crossed by the phrenic nerves. 



The outer layer of the pericardium is composed of strong fibrous 

 tissue that is continued, as tubular investments, on to the caval veins, 

 vena azygos, pulmonary vessels, brachio-cephalic trunk, and aorta. 

 The adhesion of the membrane to these vessels begins about G cm. from 

 the point at which they enter or leave the heart. 



Dissection. — Make a crucial incision through the pericardium on 

 both sides of the chest. The two horizontal incisions should not be 

 made to meet. 



Within the fibrous pericardium is a serous membrane disposed after 

 the customary manner of its kind ; that is to say, the parietal part of 

 the membrane lines the fibrous pericardium, and is reflected from the 

 great vessels on to the heart itself as the visceral pericardium or 

 epicardium. It will be observed that the aorta and pulmonary artery 

 are enclosed within a common tubular sheath of serous pericardium, and 

 that caudal to these vessels there is a passage, the transverse sinus of 

 the pericardium {s'lnns tTa.nsyers\is pericardii). The serous membrane 

 endows the interior of the fibrous pericardium and the surface of the 

 heart with a smooth, glistening appearance, and, like all other serous 

 membranes, is always moist. 



A considerable amount of fat generally underlies the epicardium in 

 the grooves on the surface of the heart and around the roots of the 

 great blood vessels. 



Dissection. — The large veins that return the blood to the heart from 

 all parts of the body except the lungs are to be approached from the 

 right side of the thorax. The pleura must be stripped from their 

 surface and the fat that covers them must be removed. Every care 

 must be taken not to injure nerves that are related to the veins. 



V, CAVA CRANIALIS. — The cranial vena cava ^ is the large unpaired 

 vessel that drains the head and neck, both thoracic limbs, and a con- 

 siderable part of the wall of the thorax. It is formed at the entrance 

 to the chest by the union of the two jugular and the two subclavian 

 veins. Very commonly the jugular veins join together and enter the 

 vena cava as a common vessel. At its commencement the caval vein 

 is fixed by connective tissue to the medial surface of the first rib. 



The cranial caval vein traverses the precardial mediastinum slightly 

 to the right of the median plane, ventral to tlie trachea, and to the right 

 of the brachio-cephalic artery and trunk. Piercing the pericardium to 

 the ricrlit of the aortic arch, it terminates in the right atrium of the 

 heart. The exact point at which the vein ends and the atrium begins 

 1 Cavus [L.], hollowed ; deep-channelled (as applied to a river). 



