THE THORACIC RETE 341 



along them between the transverse processes and up into the neural canal, where it 

 surrounds their junctions with the venous sinuses running in the canal. The intraspinous 

 veins are thus embedded in the retial substance throughout their course from the neural 

 canal to the posterior thoracic vein, but the ascending arteries pass out between the 

 heads of the ribs considerably more laterally, and thus are only surrounded by retial 

 substance at their roots, where they pass between the ribs. 



These veins and arteries are in communication with the rete wherever they pass 

 through it by means of innumerable small arterioles and venules and by somewhat 

 stouter twigs. From many points on the main posterior thoracic trunks, also, small 

 arteries and veins run to various parts of the rete, particularly around the proximal 

 parts of the posterior thoracic vessels. The ascending arteries passing through the rete 

 diminish somewhat in calibre after leaving it — that is dorsal to it. The intraspinous veins 

 are correspondingly somewhat increased — that is ventral to the rete. 



The retial masses are confluent above and below the heads of the ribs, forming a 

 continuous enveloping mass as far as the 6th rib. However, in the 1-73 metre foetus 

 the masses of retial substance surrounding the last two ascending arteries and the 

 corresponding intraspinous veins were isolated from the main mass of the rete, and 

 formed two lobes quite separate from the rest of the organ between the heads of the 

 4th and 5th and the 5th and 6th ribs. In the smaller foetus these two lobes seemed to 

 be joined to the main body. 



In the abdominal region each of the segmental ascending arteries, corresponding to 

 those described above for the thoracic region, leaves the dorsal aorta as a single mid- 

 dorsal trunk, which soon bifurcates, its branches passing to the dorsal musculature on 

 either side of the vertebral column. The descending intraspinous veins from the neural 

 venous sinuses drain into the postcava, as has already been mentioned. The postcava 

 in the abdominal region consists of a pair of trunks, the venae iliacae (Daudt, p. 280), 

 which run forward parallel with one another as far as the anterior end of the kidneys. 

 Here they fuse to form a single median postcava, which has thus only a short course 

 within the abdominal cavity before it penetrates the diaphragm. The venae iliacae on 

 either side receive their intraspinous veins from the corresponding neural venous sinus 

 of that side of the body. Farther caudally the ascending arteries are given off by the 

 caudal artery and the intraspinous veins received by the caudal veins. Thus the neural 

 venous sinuses are connected to the postcaval system throughout the whole length of 

 the body and the precaval and postcaval venous systems are in open communication 

 with one another by way of the neural canal. 



The descending veins between the 6th and 12th lumbar vertebrae break up into a 

 complicated venous network on the ventral aspect of the vertebral column as shown in 

 Fig. 5. Presumably this network serves to facilitate the flow of blood from the post- 

 cava to the neural venous sinuses in the lumbar region of the body and in the opposite 

 direction also. 



The posterior thoracic system is not only responsible for the blood supply to the 

 rete mirabile and the dorsal musculature, but it also serves the anterior intercostal 



