x.] THE CIRCULATING SYSTEM. 421 



Very different proportions may exist between the external 

 and internal carotids, compared with what exists in man ; 

 and the course taken by each may vary in ways which 

 characterize different groups of Mammals respectively. 



The brachial artery may also break up into a number of 

 small branches running side by side, as in the Sloths and 

 Slow Lemurs. 



The femoral arteries of the same animals are also simi- 

 larly subdivided, and the same condition obtains in some 

 other animals, e.g. the Echidna. 



Great convoluted retia mirabilia may also be formed by 

 the intercostal arteries, as we see in the Cetacea, e.g. the 

 Porpoise. 



A small rete mirabile (in what is called as we shall see in 

 Lesson XII. a pseudobranchia) may be developed from the 

 first (or hyoidean) aortic arch, as e.g. in Lepidosiren and 

 osseous Fishes. 



The intercostal arteries may be less numerous than the 

 intercostal spaces, as in many Fishes. 



The artery of the pectoral limb may be given off from the 

 dorsal aorta, almost immediately after its formation, as in 

 Fishes. 



The dorsal aorta may dilate beneath each vertebral cen- 

 trum of the abdomen, as in the Carp. It may give off many 

 small branches to the kidneys, as in Fishes. 



The internal iliac arteries may be given off distinctly from 

 the external, as in the Kangaroo ; and, as in the same animal, 

 the middle sacral artery may be continued on of large size. 

 The internal iliacs may be larger than the external, as in Birds. 



The inferior mesenteric artery may abort in man's own class, 

 as e.g. in the Kangaroo ; and the two cceliac arteries may 

 ramify in a sort of rete mirabile, as in the Porbeagle Shark. 



8. The structure of the VEINS, their coats, valves, and the 

 primary facts as to their conditions, have been noted in the 

 first two Lessons of " Elementary Physiology," but the re- 

 capitulation of certain points is here necessary in order to 

 avoid obscurity. 



The veins, like the arteries, may be divided into two very 

 unequal categories. The first of these comprises all those 

 which aid in bringing back blood from all parts of the body 

 to the right side of the heart : this consists then of the 

 systemic veins. The second category includes only those 

 veins which bring back blood from the lungs to the left side 

 of the heart (the left auricle) the pulmonary veins. 



