544 ANATOMY OF VERTEBRATES. 



divide ; not, as in axilla, by many ramuscules from one point. 

 Other branches are sent off from the commencement of the femoral 

 artery, which rapidly subdivide and conceal the whole course of 

 the femoral trunk as far as the ham. Some branches from the 

 internal iliac plexus pass over the brim of the pelvis and anastomose 

 with arteries of the femoral plexus ; but there is no re-entering 

 of any of the members of the plexus into the main trunk. They 

 are distributed, successively quitting the plexus, to the femoral 

 muscles : a numerous fasciculus perforates the proximal part of 

 the interosseous space, between the heads of the gastrocnemius; 

 the remaining branches accompany the main trunk down the 

 back of the leg, some as far as behind the inner malleolus, whence 

 the main trunk passes to the scapho-calcaneal joint and divides 

 into the two digital arteries for the interspaces of the three toes. 

 The continuation of the trunk may be thus explained : — The foot 

 and hand being the first segments developed in their respective limb, 

 the artery supplying them is first established ; the subsequently 

 formed segments are supplied by branches sent off from this. 



In the Unau (Bradypus didactylus) the aorta sends off the right 

 brachial and both carotids by a common trunk, half an inch long: 

 the left brachial has a course of more than an inch in the thorax. 

 Each brachial sends off, after winding over the first rib, many 

 arteries : the main trunk perforates the entocondyloid hole of the 

 humerus, as does likewise part of the brachial plexus. The con- 

 tinued trunk forms no palmar arch, but terminates by bifurcating 

 at the interspace of the two digits. The abdominal aorta gives 

 off the common iliacs opposite the last lumbar vertebrae ; and then 

 sends off the last pair of lumbar arteries and the i sacra media : ' 

 this distributes a pair at each sacral vertebrae until it is resolved 

 into a pencil of arterioles at the fourth vertebra. The iliacs send 

 off, first, the epigastrics ; then the internal iliac plexus ; after 

 which the external iliac trunk, bending over the brim of the 

 pelvis, sends off the arterioles of the plexus, which conceals the 

 continuation of the femoral trunk ; this plexus supplies the femoral 

 muscles, and also a large proportion to the interosseous space of 

 the leg. The main trunk passes down the back of the leg, and 

 divides at the middle of the sole into two branches for the inter- 

 spaces of the three toes of the hind foot. 



A closely similar disposition of the arteries, but with less nume- 

 rous arterioles, obtains in the Anteaters 1 and Armadillos 2 : and 

 the analogy of this with the arterial system of the Monotremes is 

 worthy of note. 



1 cxcv". 2 cxcix". 



