512 THE CIRCULATORY APPARATUS. 



2. External serous membrane. This is the visceral lining membrane of 

 the pericardium, the description of which follows. 



5. The Pericardium. (Fig. 234, c.) 



Preparation. Place the animal in the second position, and remove the sternal ribs by 

 separating the cartilages and luxating their costo-vertubral articulations. This pro- 

 cedure permits the study of the situation and general disposition of the htart and 

 pericardium. But in order more easily to examine the reciprocal arrangement of these 

 two parts, it is necessary to extract them from the thoracic cavity by tearing through the 

 sternal insertion of the pericardium. 



The pericardium, or proper serous membrane of the heart, is a membranous 

 sac inclosing that organ, fixing it in the thoracic cavity, and favouring its 

 movements by its polished surface. 



This sac is formed by a fibrous layer, within which is spread a serous 

 membrane, divided into two parts one parietal, the other visceral. 



The fibrous layer of the pericardium presents somewhat the general form 

 of the heart Its internal surface is covered by the parietal portion of the 

 serous membrane. The external surface corresponds to the two laminae of 

 the mediastinum. Its summit (or apex), depressed on each side, and elon- 

 gated from before to behind, is firmly attached to the superior face of the 

 sternum, from the fourth rib to the origin of the xiphoid cartilage. By 

 its base, it is fixed to the large vessels going to and leaving the heart, where 

 it is continuous with their cellular sheath, and where it sends some fibres to 

 the longus colli. 



The serous membrane of the pericardium has been well compared by 

 Bichat to a cotton night-cap, the external part of which would represent the 

 parietal layer, and the inverted part the visceral division of that membrane. 

 The parietal layer adheres in the most intimate manner to the internal face 

 of the fibrous tunic, and is seen to be reflected, to form the visceral portion, 

 around the pulmonary arteries and the aorta to a certain distance from 

 their origin, and on the pulmonary veins. The visceral layer envelops 

 in common the two arterial trunks, covers a small part of the venae cavae, 

 particularly the anterior, spreads over the insertion of the pulmonary veins, 

 and then descends on the auricles and ventricles. The free face of this 

 layer is in contact with that of the parietal layer ; the adherent face is 

 applied to the tissue of the heart or that of the large vascular trunks, except 

 at the horizontal and vertical grooves, where it rests on the coronary vessels, 

 and on the mass of adipose tissue constantly accumulated on their track. 



In tbe living animal, the cavity of the pericardium is never entirely 

 filled by the heart, whose movements are, therefore, allowed much more 

 liberty. Otherwise, as it does not contain any gas, nor a sensible proportion 

 of fluid, 1 its walls are immediately applied to the surface of the heart. 



Blood reaches the pericardium by the mediastinal arteries. Its walls 

 receive some sympathetic nerve-fibres. 



(The pericardium is composed of a fine network of elastic fibres adhering 

 to the muscular structure of the heart by one surface, and covered by a 

 single or double layer of tesselated epithelium on the other. Gurlt, in 1867, 



1 With horses in health, the fluid exhaled into the pericardium is barely sufficient to 

 moisten and luhrify the free surface of its serous membrane. But in those worn- out and 

 enfeebled by age, privations, or disease, it is not rare to see it accumulated in greater or 

 le.-s quantity. To verify this, however, an examination ought to take place immediately 

 after death, as the accumulation of fluid in the serous cavities by cadaveric exhalation is 

 common in all animals. 



