SUMMARY 361 



The fact, mentioned above, that the first and second intercostal spaces are not 

 supplied with blood from intercostal arteries or drained by veins, seems to be in ac- 

 cordance with this theory of the functions of the retia mirabilia. The first and second 

 intercostal spaces are in intimate contact with the fatty masses of the rete. They are 

 thus closely related to a source of oxygen supply during the long period of sustained 

 contraction during submergence. 



SUMMARY 



1. A description has been given of the retia mirabilia from the dissection of two 

 Fin Whale foetuses, o-86 metre (male) and 1-73 metres (female) in length. 



2. The thoracic rete is a fatty vascular mass lying at the anterior end of the thorax 

 and the base of the neck in the fork of the scalenus and rectus capitis anticus major 

 muscles. It is in contact with all the vertebrae from the first and second cervical to the 

 fifth and sixth dorsal. It extends between their transverse processes up into the neural 

 canal through the large foramina between the neural spines. 



A further vascular mass has been found lying against the basis cranii close to the 

 tympanic bulla. 



In the cervical region the neural canal of the vertebral column is filled with a dense 

 venous plexus. 



3. These vascular masses surround certain blood vessels, but do not block either 

 their arterial or venous channels. The vessels which the thoracic vascular mass surrounds 

 are the ascending branches of the posterior thoracic artery and the intraspinous veins 

 descending from the large neural sinuses to the posterior thoracic vein. The basicranial 

 vascular mass surrounds the cervico-facial artery and drains into, but does not surround, 

 the pterygo-maxillary vein. 



4. The postcaval and precaval venous systems in the Fin Whale have been found 

 to be in wide communication by means of a pair of sinuses in the neural canal of the 

 vertebral column, accompanying the spinal chord. There is a large venous plexus in 

 the lumbar region which probably facilitates this communication. 



5. The intercostal blood supply has been described to show: 



(i) That it is derived from four sources as follows : internal mammary arteries and 

 veins, azygos veins and segmental arteries, posterior thoracic arteries and veins, and the 

 thoracic rete. 



(ii) That the blood supply to the intercostal spaces becomes increasingly generous, 

 proceeding in a rostral direction until the first two intercostal spaces are reached. These 

 have no direct vascular supply, but are overlaid by the rete. 



(iii) That the return of blood from the intercostal muscles behind the 7th rib is 

 effected by a pair of small veins, thought to be homologous with the azygos veins of 

 other mammals. 



(iv) That the posterior thoracic arteries and veins of the Fin Whale are probably 

 homologous with the superior intercostals of other mammals. 



