538 



ANATOMY OF VERTEBRATES. 



Proceeding with the mammalian modifications of other parts of 

 the arterial system, I find that in all Lyencephala the carotids 

 are relatively smaller than in the better-brained groups : and that 

 the vertebral arteries give the main supply of arterial blood to the 

 brain. In the Monotremes the brachial artery emerges from the 

 thorax above the first rib, and passes between it and the coracoid : 

 the trunk is speedily reduced by the number of small branches 

 given off, and, with some of these, perforates the distal end of the 



421 



Femoral plexuses. Omithorhynchus. 



humerus nearly midway between the condyles. The phrenic, 

 coeliac, and mesenteric arteries are given off from the abdominal 

 aorta ; the renal artery is short, wide, and single ; there is no 

 inferior mesenteric artery, but the abdominal aorta, fig. 421, «, 

 terminates by dividing into the two common iliac, ib. b, and the 

 caudal, ib. A, arteries. The iliac trunks are unusually short : 

 they give off a * circumflex,' c, which soon is resolved into a plexus 

 bending over the ilium to supply the muscles on the back of the 

 pelvis : on the opposite side of the origin of the iliac is sent off 



