DISSECTION OF THE DOG 57 



The segments of the pulmonary semilunar valves (valvulse semilunares 

 a. pulmonalis) are right, left, and cranial in position. They are the same in 

 general character as those of the aortic valve, but are sometimes destitute of 

 fibrous nodules in their free edge. The sinuses behind them present no orifices 

 leading into arteries. 



Structure of the wall of the heart. — By far the greater part of the 

 thickness of the wall of the heart is composed of muscular tissue constituting 

 the myocardium : much more abundant in the ventricles than in the atria. 

 On careful dissection it is possible to show that the muscular fibres are disposed 

 in indefinite layers, and run circularly and obliquely in direction. 



At the junction of the ventricles and the atria are two fibrous rings, 

 surrounding the atrio-ventricular orifices, and affording attachment to the 

 bicuspid and tricuspid valves. Smaller rings of fibrous tissue surround the 

 pulmonary and aortic orifices, and give attachment to the valves of these 

 openings. 



Covering the heart externally is the visceral portion of the serous peri- 

 cardium, the epicardium, between which and the myocardium is a greater or 

 less amount of fat, particularly obvious in the grooves. 



The interior of the heart is lined by a smooth, shining membrane, the 

 endocardium, continuous with the lining of the blood-vessels. 



Arrangement of the cardiac orifices. — If the atria be removed the 

 relationship of the openings into and out of the ventricles will be evident. 

 Most cranial and towards the left is the pulmonary opening. Immediately 

 caudal to this and slightly to its right is the orifice leading into the aorta. The 

 right and left atrio-ventricular openings are not on the same transverse level ; 

 the left opening being the more caudal. 



Dissection. — Carefully clean the trachea, the bronchi, the oesophagus, and 

 the vagus nerves. In doing so bronchial lymph-glands (lympho- 

 glandulse bronehiales) will be observed in the neighbourhood of the 

 termination of the trachea, more especially between the diverging bronchi. 



Trachea et bronchi. — The trachea or tube by which air passes to and 

 from the lungs begins in the neck, passes through the thoracic inlet and 

 terminates in the two bronchi immediately dorsal to the commencement of the 

 aorta. The thoracic portion of the trachea is not exactly in the middle line 

 except at its termination. The presence of the oesophagus causes it to incline 

 to the right. 



The main ventral relations of the trachea in the chest are the two innominate 

 veins, the cranial vena cava, the right subclavian artery, the right and left com- 

 mon carotid arteries, the brachio-cephalic artery, and the aortic arch. Along 

 the left ventral border runs the left recurrent nerve. To the left the 

 trachea is related to the oesophagus, and to the right to the right phrenic and 

 vagus nerves. Dorsally it is in contact with the longus colli muscle, except close 



