I 62 



WHALES 



Figure gj. Histological secliun ihrougli the thoracic rete of a porpoise. The elastic tissue is 



stained dark, showing that the walls of the arterioles have only a very thin inner and outer 



elastic layer, and that the wall consists mainly of smooth muscle. Between the arterioles 



lie thin-walled venules and fat cells. 



a beautifully illustrated book on the subject in 1836. His copperplates 

 convey a very clear picture of the retia and even of their fine structure, 

 and this despite the fact that microscopy was still in its infancy in his day. 



The blood is carried to the vascular networks principally by the inter- 

 vertebral and intercostal arteries as well as by the costocervical and 

 supreme intercostal arteries which divide into innumerable branches in 

 a serpentine course. All the branches are of equal diameter, and the 

 terminal branches are, moreover, imperceptibly interwoven and are 

 anastomosed so as to form a complete network. From the functional point 

 of view, it is important to note that while the afferent vessels are pre- 

 dominantly of the elastic type, the vessels of the network itself are strongly 

 muscular, i.e. their wall consists mainly of a thick layer of smooth muscle 

 at the expense of elastic fibres (Fig. 93). 



A microscopic examination will show that, in addition to being built 

 up of arteries, the retia are also built up of veins, although to a much 

 lesser extent. In contrast to the arteries, these small veins have very thin 

 walls without muscle tissue. Moreover, they lack valves so that the blood 

 in them can flow in two directions. In a very few places, the arterial retia 

 are joined to the venous retia by capillaries. Erikson is quoted by Scholan- 

 der (1940) as stating that in these mixed networks, the arterial parts are 

 directly connected to the venous parts by arterio-venous anastomoses, but 

 no such anastomoses have ever been seen under the microscope. The retia 



