PRACTICUM V. THE SHEEP'S HEART. 



PLATES REQUIRED : XII, XIII, XIV. 



I. The heart has been prepared by filling with alcohol after the removal of the blood 

 The vessels are cut short. Bits of the lungs are left attached. The pericardium has some- 

 times been mutilated by the butcher ; when it remains there are usually attached bits of 

 fat which may be torn off with the fingers. 



2. Removal of the Pericardium, At about the middle of the length 

 of the heart slit the pericardium and "girdle" it completely so as to sep- 

 arate the apical (caudal) from the basal (cephalic) portion. Remove the 

 former ; note the smoothness of the ental surface. It and the apposed 

 ectal surface of the heart are covered by serosa and are moistened during 

 life by the secreted serum. 



a. Turn the basal portion cephalad, inside out, like the ringer of a 

 glove. At varying distances from the base it is attached to the heart and 

 vessels, and the ental serosa is reflected thereon ; recall the relations of 

 the two layers of the thoracic serosa, the pleura, Practicum III, 21 ; the 

 cardiac serosa in like manner consists of a VISCERAL LAYER, the EPICAR- 

 DIUM, covering the heart, and a PARIETAL LAYER, lining the pericardium, 

 and the two are continuous near the base of the organ ; see Figs. 9 and 



10, the PERICARDIAL LINE. 



b. Trim the pericardium along or near the line of attachment. 



3. General Topography of the Heart. Recognize the several re- 

 gions by comparison with that of the cat and PI. XII, and by the follow- 

 ing features : 



a. The APEX or caudal end is regular, conical, smooth, firm and 

 fleshy ; it is formed by the muscular VENTRICLES. 



b. The BASE or cephalic end is irregular and wider, and presents 

 not only the thin-walled AURICLES, but also vessels and fat and rem- 

 nants of lungs. 



c. At about the middle of the length, one diameter, the dextro-sin- 

 istral, is decidedly the greater, so that the heart is depressed, i. e., as if 

 from dorso- ventral pressure. 



d. Of the two wider aspects, the convex is ventral, while the dorsal 

 is, as a whole, concave. Of the two narrower sides, the right is the more 

 convex. 



e. Make sure of these aspects, so as to recognize them with the eyes 

 closed. 



f. Remember always that right and left and other descriptive terms apply to certain 

 regions of the heart without reference to the right and left of the observer and irrespect- 

 ive of the way in which the organ is held. For example on PI. XII the right of the heart 

 corresponds with the observer's in the lower figure, but in the upper it is at his left. 



4. The Vessels. These may^ be recognized from the cat and from 

 Figs. 9 and 10. Cut off the tied ends. Note the following points : 



a. The arteries, AORTA (A\ its CHIEF BRANCH (B}, and the PULMON- 

 ARY ARTERY, maintain a cylindrical form, and their cut ends are naturally 

 circular ; the great VEINS, POSTCAVA and PRECAVA, have thinner walls in 

 proportion to their size and collapse more or less completely. 



b. The POSTCAVA forms nearly a right angle with the long axis of 

 the heart, indicating that in the sheep the heart lies more obliquely than 

 in the cat. 



c. The right AZYGOUS VEIN, which in the cat (and man) opens into 

 the precava near its root, is rudimentary and may not be found. But a 

 LEFT AZYGOUS should be looked for as represented in PI. XII, As.; the 



