62 DISSECTION OF THE DOG 



colli muscle on the one hand, and the first intercostal muscle and the second 

 rib on the other. Communicating branches connect it with the last two 

 cervical and the first three or four thoracic spinal nerves. A filament passing 

 between the seventh cervical nerve and the ganglion arises in common with 

 the nerve which supplies the thoracic part of the longus colli muscle. By 

 means of the ansa subclavia {Vieussenii] a connection is established between 

 the first thoracic and the caudal cervical ganglia. One or two cardiac rami 

 (accelerator to the heart) leave the ganglion. 



A nerve cord leaves the first thoracic ganglion and can be followed along 

 the dorsal wall of the chest into the abdomen. At first the cord occupies a 

 deep groove to the side of the longus colli muscle, but later it runs across the 

 heads of the ribs and the intercostal vessels. 



Ganglia on the cord, at first small but afterwards larger, begin at the fourth 

 intercostal space, and continue, one at each intercostal space, throughout 

 the rest of the thorax. Each ganglion is connected with the ventral primary- 

 branch of the corresponding spinal nerve by means of a communicating branch, 

 the ramus communicans. 



Dissection. — The wall of the thorax should now be examined from within. 

 Strip the pleura and follow the intercostal vessels. The arrangement of 

 the intercostal muscles may be again examined. 



Aa. intercostales. — The intercostal arteries are twelve in number on 

 each side of the body. Of these, the first three or four arise from the supreme 

 intercostal artery : the rest are branches of the thoracic aorta. Each inter- 

 costal artery divides into a dorsal and a ventral branch. The ramus dorsalis 

 is distributed to the muscles and skin of the back and to the spinal cord. The 

 ramus ventralis runs along the caudal border of a rib (in the sulcus costalis), 

 at first between the two intercostal muscles, and then between the internal 

 intercostal muscle and the pleura in association with the corresponding vein 

 and nerve, and ends by anastomosing with the intercostal or musculo-phrenic 

 branches of the internal mammary artery. In its course along an interspace, 

 several small branches leave the intercostal artery and cross the inner surface 

 of the rib. 



Vv. intercostales. — The intercostal veins are disposed after the manner 

 of the arteries, but all except the first two or three terminate in the vena 

 azygos. 



V. azygos. — The vena azygos begins in the abdomen by the union of the 

 first, or first and second lumbar veins. Entering the thorax to the right of 

 the aorta, it passes along the vertebral bodies to the right of the median plane 

 as far as the fifth or sixth thoracic vertebra. Finally, curving towards the 

 heart, the vein opens into the cranial vena cava, or possibly into the right 

 atrium itself. The vena azygos drains much the greater part of the wall of 

 the thorax. 



