Circulatory System Special. 105 



appropriate stimulus of oxj^genated blood during the periods of long 

 submersion and consequent interruption of respiration to which the 

 Cete are subject. The brachial artery also of Phocaena has a plexi- 

 form distribution. 



Sirenia. The littoral and herbivorous Sirenia, which never 

 go so deep, or stay so long submerged as the whales, are said not to 

 possess either of the plexuses above described. The brachial artery 

 has a plexiform arrangement in Manatee. 



In-Enamellata. The brachial artery is remarkably modified in 

 the Sloths, where, after bending over the first rib and traversing the 

 axilla, it suddenly sends off, or seems to break up into a fasciculus 

 of minute longitudinal branches which surround and conceal the 

 main trunk which exists in the middle of the plexus. A femoral 

 plexus is also given off at the brim of the pelvis by one or two 

 branches which subdivide, but not, as in the axilla, by many ramus- 

 cules from one point. Rete have been noticed on the blood vessels of 

 the limbs in Myrmecophaga didactyla, and Dasypus sexcinctus also. 



Monotremata. The arterial system of both genera is characterised 

 by the subdivided plexiform disposition of many of the arteries. 



Origin of arteries of posterior extremities. 



Mursupialia. In the Kangaroo and Yulpine Phalanger the Aorta 

 gives off, opposite the interspace of the two last lumbar vertebrse, 

 the iliac arteries ; but these are afterwards resolved into the ordinary 

 branches of the external iliac of the placental Mammals, with the 

 addition of the ilio-lumbar artery. The trunk of the Aorta, much 

 diminished in size, maintains its usual course for a very short 

 distance, and then gives off the two internal iliacs, and is continued 

 as the ' arteria sacra media ' to the tail. The transitional character 

 of this part of the Marsupial sanguiferous system, between the Ovi- 

 parous and Placental types, is manifested in the large size of the 

 external"^' iliacs as compared with the internal iliacs, their greater 

 share in the supply of blood to the hinder extremities, and the 

 brevity of the Aortic trunk between their origins. In Marsupialia 

 the inferior mesenteric artery does not arise primarily from the Aorta. 



4. Venous system. 

 - Valves, 



* In most Birds the femoral, or external iliac, are smaller than the ischiadic, 

 or internal iliac, arteries subsequently given off. 



