VASCULAR SYSTEM. 



511 



5rrf 



tricle and auricle (sometimes as in Halicore the division is marked 



externally). It is enclosed in a pericardium, and sends off an 



arterial trunk, which forms a left aortic arch, from which two 



vessels frequently arise, viz. (1) a right anonyma, with the two 



carotids and right subclavian ; and (2) the left subclavian ; or, 



as in man, three vascular- 



trunks, viz. (1) a right 



anonyma, with the right 



carotid and right subcla- 



vian ; (2) the left carotid ; 



and (3) the left subcla- 



vian, all close to one 



another. As a rule, one 



superior vena cava (right) 



and an inferior vena cava 



open into the right aur- 



icle : more rarely, as in 



the rodents, monotremes, 



and elephants, etc., there 



are two superior venae 



cavae. Retia mirabilia 



have been recognised 



principally for the ar- 



terial vessels, and are 



found on the extremities 



of burrowing and climb- 



ing animals (Stenops, 



Myrmecophaga, Brady- 



pus, etc.) ; on the caro- 



tids round the hypophy- 



sis, and on the ophthal- 



mic arteries in tjie orbit 



in ruminants ; finally on 



the intercostal arteries 



and the iliac veins of the 



dolphin. The red blood 



corpuscles are devoid of a nucleus and are discoidal, except in the 



Camelidae in which they are elliptical. They vary in size : in 



man they are 3 2 * th of an inch : in the elephant 2 / th of an 



inch ; they are smallest in the chevrotains in which they are 



04- .' 



FIG. 268. Circulatory apparatus of man (from 

 Owen, after Allen Thomson). Ad right auricle ; 

 Ao aortic arch ; Aod descending aorta ; Ap pul- 

 monary artery ; As left auricle ; Br bronchi ; 

 Cd right carotid Cs left carotid ; D intestine; 

 Jd right jugular ; Jl common iliac artery ; J I 

 common iliac vein ; Js left jugular ; L liver ; 

 M mesenteric artery ; N kidney ; P lungs ; Sd 

 right subclavian artery ; Ss left subclavian artery ; 

 Srd right subclavian, Svs left subclavian vein ; 

 Tr trachea ; Va inferior vena cava ; Vd superior 

 vena ca\a ; Vd right ventricle ; Vp vena portae ; 

 Vpu pulmonary vein ; Vs left ventricle. 



