54 DISSECTION OF THE DOG 



Ventrictjltts sinister. — The left ventricle differs from the right in its 

 general shape. It is conical in conformity with the general form of the 

 ventricular part of the heart, and its apex is continued to the apex of the 

 heart as a whole. Owing to the thickness of its wall, coupled with the fact that 

 the inter- ventricular septum is concave towards the left, a transverse section 

 of the left ventricle would present an oval or rounded outline. 



Generally the trabeculse carnese are less conspicuous than in the right 

 ventricle ; and the papillary muscles are larger and only two in number. 



The atrio-ventricular opening (ostium venosum) is provided with a valve 

 of two cusps (valvula bicuspidalis) , each being larger and stronger than the 

 segments of the tricuspid valve. As on the right side, there are generally 

 smaller cusps intervening between the major segments. 



The exit from the ventricle is by way of the aortic opening (ostium arteri- 

 osum), around which are arranged three semilunar valves similar to those 

 guarding the entrance to the pulmonary vein, but stronger in structure. 

 The three semilunar segments are arranged so that one is caudal, one to the 

 right, and one to the left. 



Aorta. — The aorta — the main artery of the body as a whole — leaves the 

 left ventricle. The first part of the vessel forms an arch which may be divided, 

 for descriptive purposes, into (1) the ascending aorta, and (2) the aortic arch 

 proper. The ascending portion is short and runs in a cranio-dorsal direction 

 and towards the left (to the middle line). From it arise the two coronary 

 arteries for the supply of the substance of the heart. 



The aortic arch has its convexity looking dorso-cranial and towards the 

 left. From the convex side of the arch spring two vessels, the brachio-cephalic 

 and left subclavian arteries. The concave side of the arch is closely related 

 to the pulmonary artery, with which it is connected by a fibrous cord, the 

 ligamentum arteriosum, representing an embryonic communication between 

 the two vessels. 



Dorsally the aortic arch is related to the trachea and oesophagus. The 

 left vagus, cardiac, and recurrent nerves cross the ventral surface of the arch 

 near its termination. 



Dissection. — The vessels leaving the aortic arch should be followed to 

 the entrance to the chest. This is facilitated by cutting through the 

 cartilages of the first pair of ribs just where they join the bony segment 

 of the rib. The first segment of the sternum, with the cartilages attached, 

 may now be turned over towards the head. The cranial vena cava 

 should also be cut across. In cleaning the arteries great care must be 

 taken to preserve intact the numerous nerves — some of them small — 

 which are related to the vessels. 



A. brackeo-cephalica. — The brachio-cephalic artery is the larger of the 

 two vessels arising from the aortic arch, and is the first to take origin therefrom. 



