56 DISSECTION OF THE DOG 



A. transversa colli. — The transverse cervical artery pierces the vertebral 

 extremity of the first intercostal space. 



Truncus omo-cervicalis. — The omo-cervical trunk, arising from the 

 subclavian within the first rib, at once leaves the thorax and has been followed 

 in the triangular space at the root of the neck, where it divides into ascending 

 cervical and transverse scapular arteries. 



A. mammaria interna. — The internal mammary artery leaves the sub- 

 clavian at the same level as the omo-cervical trunk, but from the opposite 

 side of the artery. The whole of its course has now been exposed. 



Dissection. — In order to examine the cardiac and pulmonary nerve-plexuses 

 in their entirety, the arch should be cut at its two ends. Then, if 

 the ligamentum arteriosum be also cut, the aortic arch can be turned 

 forward, thus exposing the trachea, oesophagus, and left recurrent nerve. 



Plexus cardiacus et plexus pulmonalis. — These two plexuses are 

 intimately connected with one another. They are formed by intercommuni- 

 cating branches from the sympathetic (ganglion thoracale primum), the vagus, 

 and the recurrent nerves. More or less independent branches (rami bronchiales) 

 leave the vagus about the level of the tracheal bifurcation and pass into the 

 hilus of the lung as constituents of its root. Nerves from the cardiac plexus 

 pierce the pericardium in the neighbourhood of the pulmonary artery and 

 the aorta. 



Dissection. — Remove the heart from the thorax. The vena azygos and the 

 caudal vena cava will be the only vessels requiring division. The aortic 

 valves should be displayed by slitting open the commencement of the 

 aorta. In doing so the cut should be made between two cusps so as to 

 avoid undue injury. 



Valvule semilunares. — The aortic semilunar valves (valvulse semilunares 

 aortse) are constructed on the same principle as those at the entrance to the 

 pulmonary artery, but, owing to the circumstance that they have to withstand 

 a greater pressure of blood, they are stronger. The valve segments are three 

 in number — right, left, and caudal — and are pocket-shaped or semilunar, with the 

 mouth of the pocket directed towards the aorta. A sinus occurs behind each 

 segment as is the case in the pulmonary artery, and in two of the sinuses the 

 openings into the coronary arteries will be seen about on a level with the free 

 edge of the valve segments. The left coronary artery leaves the left sinus, and 

 the right coronary artery leaves the right sinus. 



About the middle of the free edge of each segment a small fibrous nodule 

 can be demonstrated. The valve consists of a double layer of endocardium 

 with interposed fibrous tissue. This tissue, however, is not uniformly dis- 

 tributed. A firm cord lies in the free edge of each cusp, and fibrous strands 

 are numerous throughout the valve. 



