356 DISCOVERY REPORTS 



SUMMARY 



It seems that the plexus in the inguinal region is different in nature and function 

 from the others described in this paper. It is in intimate relation with the lymph 

 glands and their channels and has no innervation that could be detected in dissection. 

 It is far more diffuse in texture than the thoracic and basicranial Retia and has not the 

 connective tissue investment that was found in the latter. 



Murie (1873, PP- 290-1) has noted the close association of lymphatics and vascular 

 networks, and it may be that, as he has suggested, these networks afford interchange 

 between the blood and the lymphatic systems. 



DISCUSSION 



The following facts have emerged from the foregoing description of the retia 

 mirabilia and their relations. 



1. The retia are vascular masses closely investing certain arteries and veins, but not 

 forming an integral part of them. They do not impede the flow of blood in any direction 

 either away from or towards the heart. They are supplied with blood by their own 

 arteries and drained by their own veins. Thus they are independent organs, and not 

 merely constrictor mechanisms on the course of the main blood-system. It is therefore 

 incorrect to describe them as either arterial or venous. The blood, however, which 

 enters the thoracic rete must be mainly venous, entering it from the neural venous 

 sinuses. A smaller proportion will be arterial, entering the rete from the posterior 

 thoracic artery. The blood which enters the basicranial rete must be supposed to be 

 mainly arterial, derived from the main facial artery. 



2. The precaval and postcaval venous systems are in open communication by way 

 of the neural venous sinuses and the lumbar venous plexus. The two wide sinuses 

 accompanying the spinal chord would appear to have the function of reservoirs, since 

 they do not drain any organ or region of the body, but receive blood from the main 

 venous channels of the body. In whichever direction the blood flows in the sinuses it 

 reaches the heart. 



3. There is an increasingly generous blood supply to the intercostal muscles pro- 

 ceeding forwards. The two anterior pairs of intercostal spaces have no direct arterial 

 supply or venous drainage, but are in close contact with the capillaries of the thoracic rete. 



4. The thoracic rete is innervated by the somatic and sympathetic nervous systems 

 and the basicranial apparently by the trigeminal nerve only. 



The function of the retia would seem to be the retention of blood. They are blood 

 reservoirs but they do not interfere with the blood supply to or from any part. That the 

 retia were reservoirs of this sort was the suggestion of Hunter (1787, pp. 371-459), 

 Breschet (1836, pp. 1-82) and Owen (1868, vol. in, pp. 545-6). These authors believed, 

 however, that the structures are used for the distribution of arterial blood to the general 



