THE THORACIC RETE 33 i 



itself. In B. rostrata and in all Mystacocetes, according to Bouvier, the plexus is formed 

 by intercostals given off from the thoracic artery. The anterior of these breaks up 

 entirely to form the plexus, while the more posterior ones do not, but send branches 

 outside it towards the region of the mammary artery. 



Bouvier had convinced himself by dissection and injection that the plexus is arterial. 

 Breschet and Owen were also emphatic in their statement of its arterial constitution. 

 Murie, as has been mentioned, considered that it is made up of both arterial and venous 

 components. Carte and Macalister, however, were definitely of the opinion that the 

 plexus is mainly venous. 



THE THORACIC RETE 



In order to make clear the structure and relationships of the vascular masses designated 

 the "Rete Mirabile" by Owen, two Fin Whale foetuses were dissected at South 

 Georgia — one o-86 metre (male) and the other 173 metres (female). The major part of 

 the dissection was carried out on the larger of these. However, many of the blood vessels 

 included in the following description, as well as the nerves in the neck region, were made 

 out only in the smaller foetus, since the other had to be decapitated before it could be 

 transported across to the laboratory from the whaling factory. 



APPEARANCE OF THE RETE 



During the dismemberment of the adult carcass on the flensing plan, the rete can 

 be seen as a series of fatty masses between the transverse processes of all the cervical 

 and the first six dorsal vertebrae. These fatty masses contain, embedded within their 

 substance, very numerous small thick- walled blood vessels. There can also be seen in 

 the centre of each mass a large vessel which has thin walls and is evidently venous — 

 it passes through the mass from the direction of the spinal cord. These large veins, 

 when opened with a knife, are found to receive numerous small vessels on all sides from 

 the substance of the fatty mass of the rete. 



In the smaller of the two foetuses dissected the fineness of the component threads of 

 the rete gave the organ the appearance of a diffuse gland, pink in colour and fairly 

 compact and dense in texture, immediately anterior to the thoracic cavity. In the 

 larger foetus it was seen quite clearly to be made up of a mass of minute blood vessels 

 of capillary size. The network enclosed very numerous fine nerve fibres in its meshes. 

 It was seen to be everywhere quite independent of surrounding muscles, but lay in 

 more intimate contact with the anterior two pairs of intercostals, as will be pointed 

 out later in this paper. It was invested by its own thin connective tissue integument 

 and could be easily dissected away from neighbouring tissues. Wherever an artery 

 passed through it, many smaller branches could be seen running from that vessel into 

 the rete, and similarly minute veins were seen running into the venous channels on 

 whose course the rete lay. In many places special retial arteries and veins extended 



