PARTS AND FEATURES 



the transverse foramina of the cervical vertebrae become reduced, ac- 

 companying ehmination of the internal carotid and vertebral artery, 

 in most toothed whales these foramina are still more reduced, and in 

 Physeter and Hyperoodon they are entirely lacking, this detail thus ap- 

 parently being dependent upon the possible depths to which whales 

 descend. In Sirenia the transverse foramina are also rudimentary. 



The most spectacular detail of the vascular system of aquatic mam- 

 mals is the extent to which retia mirabilia, or networks of vascular 

 anastomoses, occur. They are found extensively in the Cetacea, Sirenia 

 (depicted with especial advantage in Murie's figures), in the Phocidae, 

 and to a lesser degree possibly in the Otariidae and Odobenidae. But 

 this is tiot in itself an aquatic adaptation, for extensive retia have also 

 been reported in monotremes, some marsupials, some lemurs, ant-eaters, 

 sloths, armadillos, some rodents, the Manidae, and others. Whatever 

 the function may be we are justified in assuming that retia are not a 

 secondary adaptation in aquatic mammals, but rather that such as ex- 

 hibit this character have lacked the stimulus for fusion of the retia 

 into larger blood vessels and that at least those of a diffuse pattern have 

 been retained from the primitive ancestral condition, or rather that the 

 embryonic condition is retained throughout life, as pointed out by von 

 Baer (1835). 



It is not easy to determine the extent of retia without specially in- 

 jected material. They were not striking in the sea-lion which I dis- 

 sected, while they were, over certain areas, in the seal. They may occur 

 in the arterial or venous system or both. Retia may be gathered in 

 single, sheathed bundles, which also convey lymphatic trunks, as in 

 sloths and armadillos, or a diffuse pattern as in monotremes and siren- 

 ians. They may occur in different areas dependent upon the type of 

 mammal considered. Many authors have noted the intracranial retia 

 of the internal carotids at the base of the skull in ruminants, located at 

 the sides and back of the sella turcica. These are said to be better de- 

 veloped in grazers than browsers, and least so in the giraffe. Some 

 vascular clusters seem clearly to function as reservoirs for blood, as in the 

 case of the psoadic plexus of Cetacea. This takes the form of numerous, 

 transverse, separated blood vessels posterior to the kidneys. 



Vrolik believed retia to be connected with aboreal habits, and Carlisle 

 that they were correlated with slow movements, but the circumstance 

 that they are present in some agile, nonarboreal mammals refutes these 

 hypotheses. Hunter and Cuvier assigned to those of the arterial system 

 the function of storing oxygenated blood, and Wilson, the storing of 



[323} 



