ANATOMY IN A NUTSHELL. 451 



LESSON CXLI. 



(5) The intercostal arteries are eleven in number. Those of the first and 

 second spaces come from the subclavian, and the remaining nine come from the 

 thoracic aorta. The last one, which passes along the lower border of the Last 

 rib, is called the subcostal. 



The right intercostal arteries are longer than the left ones because the 

 aorta is situated on the left side of the spine. In each intercostal space the 

 artery lies upon the External intercostal muscle and has in front of it the pleura 

 and a thin fascia. From here it passes between the two layers of intercostal 

 muscles and passes to the lower border of the rib above to pass forward in the 

 groove on the lower border of this rib to anastomose with the anterior inter- 

 costals which are branches of the internal mammary. The first intercostal, 

 which comes from the aorta, anastomoses with the superior intercostal, while 

 the last three intercostals which come from the aorta pass between the abdom- 

 inal muscles to anastomose with the epigastric in front and with the phrenic 

 and lumbar arteries behind. In each intercostal space there is a vein, artery, 

 and nerve, and their position from above down is vein, artery, and nerve, ex- 

 cept in the upper intercostal spaces where the nerve is first above the artery. 

 Fibrous arches keep the intercostal muscles from pressing upon the intercostal 

 arteries during respiration. The lower intercostal arteries pass into the ab- 

 dominal wall and into the sheath of the Rectus muscle where they anastomose 

 with the internal mammary and the deep epigastric artery. The branches of 

 the intercostal arteries are (a) the posterior or dorsal branch, which passes 

 backward to the inner side of the anterior costo-transverse ligament where it 

 is distributed to muscles and integument of the back after dividing into an ex- 

 ternal and internal branch, (b) The spinal branch, which passes through 

 the intervertebral foramen to be distributed to the spinal cord and its coverings 

 and the bodies of the vertebrae, (c) The collateral intercostal branch 

 arises from the intercostal artery close to the angle of the rib and then passes 

 downward to the upper border of the rib below, along which it passes to anas- 

 tomose with the anterior intercostals from the internal mammary. 



The thoracic aorta commences on a level with the fourth dorsal vertebra 

 and descends on the left side of the spine to pass through the abdominal open- 

 ing in the Diaphragm after which it is called abdominal aorta. 



The abdominal aorta commences at the aortic opening in the Diaphragm 

 extends to the front of the body of the fourth lumbar vertebra where it divides 

 into tiih right and lkft common iliac arteries. (Plate XCI | 



LESSON CXLI I. 

 The Relations of the Abdominal Aorta. 

 In Front. — (1) Lesser omentum, (2) Stomach. (3) Branches of the 

 cceliac axis, (4) Solar plexus, (5) Splenic vein, ((>) Pancreas, (7) Left renal 

 vein, (8) Transverse duodenum, (9) Mesentery, (10) Aortic plexus. 

 Behind. — (1) Left lumbar veins, (2) Vertebral column. 

 Right Side. — (1) Right cms of Diaphragm, (2) Inferior vena cava, (3) 

 Yena azygos major, (4) Thoracic duct, (5) Right semilunar ganglion. 



