HEART, CIRCULATION, AND BLOOD 



163 



Figure 9^. The vascular networks with their afferent and efferent blood vessels in the anterior 

 thorax, the neck, and cranium of a Fin Whale foetus. Ao = aorta; C = costo-cervical artery 

 carrying blood from the aorta to the retia; V = costo-cervical vein returning blood from the 

 retia and the spinal veins to the anterior vena cava; R = thoracic and cervical retia; 

 Rb — basi-cranial rete ; Rs == spinal rete ; Vs = spinal vein; P = spinous process of a 

 thoracic vertebra; S = spinous process of a cervical vertebra. {Walmsley, 1938.) 



tnirabilia lie embedded in adipose tissue consisting of a large mass of fat 

 cells separated by connective tissue septa. 



This type of tissue can be found in other animals as well, particularly 

 when the fat has a mechanical function as, for instance, in the pads of a 

 dog's foot. Here the fat is not so much a food reserve as a shock-absorber. 

 The very structure of the tissue, which resembles a quilt, would seem to 

 bear out this contention. Thus the structure, the serpentine course of the 

 small vessels, the many branches, the anastomoses, and the thick muscular 

 wall of the small arteries, all strongly point to the conclusion that the retia 

 are subjected to very great and quick changes of volume. By quick dis- 

 tensions and contractions of their muscular wall, the small arteries enable 

 the retia to absorb and expel large quantities of blood very quickly, and 

 some form of shock-absorber is obviously essential if they are to function 

 smoothly. Moreover, it is quite possible that the venous retia also act as 

 shock-absorbers for the arterial retia. 



We have discussed the retia mirabilia of a porpoise because it has the 

 most highly developed system of vascular networks of all Cetaceans. 

 Common Dolphins, too, have considerable thoracic retia, but in other 

 Cetaceans they are neither so thick nor do they extend so far or so wide. 

 In Ziphiids, including the Bottlenose Whale, they are found only in the 

 cervical region and half-way down the thorax, while in Sperm Whales 

 they extend down to the lumbar region. Rorquals show vascular networks 

 in the cervical region, and in the thoracic region down to the sixth rib. I 



