REPORT ON THE SPHENISCID^. 153 



The arrangement of the arteries of the anterior extremity of the Penguins is even more 

 exceptional, and indeed, so far as I can ascertain, is altogether confined to members of 

 the group of Spheniscidse. The existence of retia mirabilia in various species of 

 mammals is well known, and in birds they have been described in connection with the 

 orbital and anterior tibial arteries. I cannot, however, ascertain that anything at all 

 resembling the perfect arterial rete, above described in the Penguins, has hitherto been 

 observed in any other bird. A venous rete has been described by Van der Kolk ^ in 

 the wing of the Condor, but an alar arterial rete appears to occur in the wing of the 

 different members of the group of Spheniscidse, and in them alone. The occurrence of 

 this arterial rete in birds which spend a large portion of their existence beneath the 

 surface of the water, is of interest in connection with the occurrence of large retia in the 

 truly aquatic mammalia, such as the Cetacea. In the Penguins, as in the Cetacea, the 

 physiological raison d'etre is far from apparent. 



The characteristics of the group of the Spheniscidse, so far as the arterial system is 

 concerned, may be summed up shortly as follows : — Firstly, The possession of two 

 common carotid arteries of equal size, symmetrically placed with reference to the middle 

 line of the neck, and furnishing branches which are symmetrically distributed in the regions 

 of the head and neck. Secondly, The entire absence or marked degeneration of the sciatic 

 artery, and the substitution for it of a branch of the crural artery. TJiirdly, The existence 

 of an arterial rete mirabile in the region of the humerus and forearm. 



VENOUS SYSTEM. 



Pulmonary Veins. 



Of these there are two — a right and a left. Each is formed by the junction of two 

 branches, and passes transversely inwards above the corresponding innominate vein, to 

 open close to its fellow into the cavity of the left auricle. The pulmonary veins are 

 situated above (dorsad of) the other constituents of the root of the lung. 



Systemic Veins. 

 Tlie Jugidar Veins. 



The jugular veins (PI. XI. fig. 1) of the two sides are of equal size. Each is formed 

 by the union of a number of branches in the region of the head. These branches may 

 be divided into two sets — a superficial and a deep. The superficial branches are three in 

 number. 



' Amiales des Sciences Naturelles, ser. iv., torn. v. p. 141, pi. iv. 



(ZOOI-. CHALL. EXP. — PART XVIII. 1883.) S 20 



