546 ANATOMY OF VERTEBRATES. 





to the provision of a reservoir of arterial blood, especially in 

 behoof of the brain. 



In the Porpoise the aorta, after giving off the two coronary- 

 arteries, forms the usual arch from the convexity of which are sent 

 off three primary trunks : the first is the largest, and gives off, 

 first, the ' posterior thoracic,' then the right carotid, and after- 

 wards divides into the right brachial and internal mammary 

 arteries. The second trunk sends off the left carotid, left brachial, 

 and internal mammary arteries. The third primary branch is the 

 left posterior thoracic artery. 1 The common carotid sends off an 

 inferior thyroid before dividing into external and internal carotids : 

 the subordinate branches of both these vessels form the plexuses in 

 various parts of the head, especially at the basis cranii and around 

 the optic nerve. The posterior thoracic, on each side, bends down 

 and gives off branches to the five anterior intercostal spaces : the 

 succeeding ones derive their arteries directly from the thoracic 

 aorta, and some of them by a short common trunk, which bifur- 

 cates. The intercostals send off the dorsal branch, and that which 

 accompanies the rib : but they also, and chiefly, divide into a vast 

 number of branches, forming by their close tortuous interlace- 

 ment a thick substance, compared by Tyson to a gland, 2 and, more 

 truly, by Hunter, to the plexiform mass of the spermatic artery in 

 the bull. 3 This arterial structure lines the sides of the thorax 

 from the ninth or tenth pair of ribs, forwards, penetrating between 

 the ribs near their joints and behind the costal ligaments, and 

 there anastomosing with corresponding productions from contiguous 

 intercostal spaces : branches pass therefrom into the neural canal, 

 surrounding the myelon with a similar plexus, increasing in thick- 

 ness near the skull and about the macromyelon, and anastomosing 

 freely with the myelonal meningeal arteries. Thus the neural 

 axis can receive its appropriate stimulus of oxygenated blood 

 during the periods of long submersion and consequent interruption 

 of respiration, to which the Cetacea are subject. Any convoluted 

 intercostal artery, contributing to this reservoir, can be unravelled 

 and traced to a great length, without sending off branches or 

 changing its calibre. In the Piked Whale (Balcenoptera), the 

 external thoracics and internal mammaries combine with the inter- 

 costals in supplying this huge and singular plexiform reservoir. 

 The brachial trunk gives off an external thoracic to the pectoral 

 muscles, a subscapular artery, one to the supraspinal fossa and a 

 circumflex branch: these supply the muscles of the fin. The 

 trunk then divides into two, each of which subdivides, and forms 



1 xciv. p. 365, note (1837). See also cxcvm". (1841), p. 

 • xciv. p. 365. s xciv. p. 365. 



383. 



