Circulatory System General. 101 



tlie superior and inferior cavao, subclavian and iliac veins, and 

 in the veins of the liver (both portal and hepatic), heart, lungs, 

 kidneys, brain, and spinal cord. Many of the veins within 

 the Cranium are included in spaces formed by the separation 

 of the laminae of the dura mater, and do not admit of being 

 dilated beyond a certain size ; these are termed ' sinuses/ In 

 all Mammals may be found the ' superior longitudinal sinus,' 

 united at the Uorcular Herophili'* with the lateral Sinuses. 

 It is very rarely that the veins have an outer coat of circular 

 fibres. In most higher Mammals there is one superior cava as in 

 Man, but there are instances, by no means uncommon, of two 

 anterior or descending vena3 cava) as in Birds. The posterior 

 or inferior cava is commonly dilated in diving animals 

 previous to entering the heart. The Portal circulation is 

 limited in Mammalia to the Liver, the Kidney being supplied 

 with arterial blood only. The veins of the Kidney are con- 

 tinued from the Renal artery, and communicate solely with 

 the inferior cava. The Iliac veins combine to form the 

 inferior cava in all Mammals, without conveying any part of 

 their blood to the Kidneys. The external jugular veins are 

 often very wide where they convey the blood from the brain 

 sinuses as well as from the neck ; in Man, the Quadrimiana, 

 and Sectorialia, the blood from the brain sinuses is conveyed 

 by a separate channel, viz. the Internal jugular veins. A 

 plexiform disposition of the veins is sometimes met with. 



The Blood of Mammalia is hot and red : the blood corpuscle 

 is, as a general rule, a circularf disc ; and, whether circular or 

 oval, instead of being swollen in the centre by a nuclear part, 

 it is there thinnerij:; the disc is consequently slightly biconcave. 

 The average diameter of the human blood disc is ^^ : they 



* In Man tlie sinuses which are contained in the several processes or folds of 

 the dura mater converge to a common point, which corresponds with the occipi- 

 tal protuberance, and is called the confluence of the sinuses, or torcular (a wine-, 

 or oil-press) Herophili. 



t Camelidae alone excepted. % Non-nucleated. 



