444 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the elimination of a functional renal portal circulation even as a rule 

 from early stages of development, and in many mammals (Edentates, 

 Whales, Carnivores, Primates) by remarkable asymmetry which develops 

 in the region of the anterior venae cavae. These latter vessels (ducts of 

 Cuvier) show in such mammals the usual paired symmetrical condition 

 during early stages. Later however a cross connexion prises between 

 the right and left anterior cardinal veins a little in front of their opening 

 into the ducts of Cuvier. This channel gradually enlarges so as to drain 

 the whole of the blood from the left anterior cardinal vein into the 

 corresponding vein of the right side close to its opening into the right 

 duct of Cuvier. At the same time the''. left duct of Cuvier undergoes 

 gradual reduction until eventually it is represented merely by a short 

 stump (coronary sinus) into which open the veins of the wall of the heart. 



A striking peculiarity of the mammals is that except in the embryo 

 the red corpuscles are not complete cells but merely non-nucleated discs, 

 circular and biconcave in form except in the Camels where they are 

 elliptical. 



The lymphatic system reaches a high stage of development in the 

 mammal there being well-developed lymphatic vessels, provided with 

 muscular walls like the blood-vessels and draining into the venous 

 system by a longitudinally placed thoracic duct lying on the left side. 

 The lymphatic vessels at particular points break up into a spongework 

 (lymphatic glands) in which active multiplication of leucocytes takes 

 place. Here and there in the course of the lymphatic vessels, as is also 

 the case with veins, there are valves which ensure that the flow shall 

 take place in the right direction but the lymphatic hearts which occur in 

 the lower vertebrates have disappeared entirely in the mammals. 



In the adult mammal the cartilaginous elements of the skeleton 

 become almost entirely replaced by bone, the individual bones usually 

 containing marrow in their interior, — either yellow and fatty or red in 

 colour. It is in this last-mentioned red marrow that the chief formation 

 of new erythrocytes in the adult mammal takes place. 



Instead of being separated by smooth joint surfaces as is the rule in 

 the lower tetrapods, the adjacent vertebral centra are in the mammal 

 (except in rare cases, as in the neck of Ungulates) united together by 

 intervertebral discs of tough fibrous tissue. The vertebral column shows 

 marked differentiation into regions — cervical (usually 7 vertebrae), 

 thoracic or dorsal, lumbar, sacral, and caudal. The ribs are long and 

 well developed in the thoracic region, and the anterior ones reach the 

 sternum which is divided up into a number of segments. 



