590 THE BLOOD-VASCULAR SYSTEM 



They are distributed to the skin and give medial mammary branches [rr. mammarii mediales] 

 to the medial region of the breast. 



(6) The posterior branches [rami posteriores]. — These large branches are given off from the 

 intercostals opposite the quadrilateral space bounded by the transverse process of the vertebra 

 above, the neck of the rib below, the body of the vertebra medially, and the anterior costo- 

 transverse hgament laterally. Passing backward toward this space with the dorsal branch 

 of the corresponding intercostal nerve, they divide opposite the intervertebral foramen into a 

 muscular and a spinal branch, (i) The muscular branch [r. muscularis] passes backward 

 through the quadrilateral space, and soon subdivides into a medial and a lateral branch. The 

 former passes between the longissimus dorsi and iho-costalis, and, after supplying these muscles, 

 gives off medial cutaneous branches [rr. cutanei mediales]. The latter branch pierces the multi- 

 fidus spinse, and, emerging between the longissimus dorsi and semispinalis dorsi near the spinous 

 processes, gives off lateral cutaneous branches [rr. cutanei laterales]. It supphes the muscles 

 in its course. 



(ii) The spinal branch [r. spinalis] enters the intervertebral foramen with the spinal nerve 

 of the corresponding segment. The disposition of the spinal branch is similar to that of the 

 spinal branches entering the canalis vertebralis in other regions and may be described here: — 



ARTERIES OF THE VERTEBRAL CANAL 



Spinal arteries are derived from the vertebral, ascending cervical and costo-cervical arteries, 

 from the dorsal rami of the intercostal (fig. 478) and lumbar arteries, and from the ilio-lumbar 

 and lateral sacral arteries. The spinal branch in each case divides into three branches, post- 

 central, prelaminar and neural. 



Each post-central branch divides on the lateral part of the posterior longitudinal ligament 

 into an ascending and a descending branch by which means a bilateral series of anastomosing 

 arches are formed throughout the length of the canal. From the concavities of the opposite 

 arches transverse connecting stems are formed which are again connected by a median longitu- 

 dinal channel. 



The pre-laminar branches also divide and form an anastomosis in front of the laminae 

 and ligamenta fiava. This is similar in character to the post-central, but much less regular. 



The neural branches enter the dura mater and are usually small and end by supplying the 

 nerve roots. A variable number of these (5-10 on a side) are larger than the others and rein- 

 force the longitudinal anterior and posterior spinal arteries given off from the vertebrals within 

 the cranium. (For arteries of the spinal cord, see Section VII.) 



(2) The superior phrenic arteries [aa. phrenicse superiores], are small twigs 

 coming off from the thoracic aorta immediately above the diaphragm. They are 

 distributed to the vertebral portion of the diaphragm on its upper surface. 



(3) The arteria aberrans is a small twig which, arising from the thoracic 

 aorta near the right bronchial artery, passes upward and to the right behind the 

 oesophagus and trachea, and is occasionally found to anastomose on the oesophagus 

 with the arteria aberrans of the superior intercostal artery (see p. 568). It is 

 regarded as the remains of the right aortic dorsal stem (fig. 506). 



(4) The mediastinal branches [rami mediastinales], numerous, but small, are 

 distributed to the pleura, and the vessels, nerves and lymph-nodes of the posterior 

 mediastinum. 



THE ABDOMINAL AORTA 



The abdominal aorta [aorta abdominalis] (fig. 479), the abdominal portion of 

 the descending aorta, begins at the aortic opening in the diaphragm opposite the 

 lower broder of the twelfth thoracic vertebra, and ends usually opposite the middle 

 of the bod}' of the fourth lumbar vertebra by dividing into the right and left 

 common iliac arteries. It is at first centrally placed between the medial crura 

 of the diaphragm, but as it descends in front of the lumbar vertebrae it leaves the 

 middle line, and, at its bifurcation, lies a little to the left side of the spine. 



The place at which the aorta bifurcates may l)e somewhat roughly indicated on the surface 

 of the abdomen l)y a point about 2.5 cm. (1 in.) below and a little to the left of the umbilicus. 

 ''I'Ik! 1(!V('1 of its l)ifurcation may Ijc more accurately determined l)y drawing a straight line 

 across the front of the abdomen joining the highest points of the iliac crests. 



The inferior vena cava, which accompanies the abdominal aorta, lies to its 

 right side. lielow, the vein is in conta(;t with the artery and on a somewhat 

 posterior plane; but above, it is separated from the aorta by the right medial crus 

 of the diaphragm, and, in consc^quence of the caval opening in the diaphragm 

 being placed further forward than the opening for the aorta, is on an anterior 

 plane. 



