MAMMALIA, 475 



adioulna or tibiofibula. The proximal ends of the radius and ulna 

 )oth take part in the formation of the facet or sigmoid notch in which 

 he condyle of the humerus moves, and the ulna is always produced 

 backwards as an olecranon process for the insertion of the triceps muscle. 

 This olecranon part of the ulna remains in cases where the ulna 

 itrophies, hence the radioulna or ulna has its facet deep and not 

 juite at the proximal end of the bone. Distally both radius and ulna 

 lave shallow facets for articulation with the proximal carpals. 



Tibia and Fibula. — The proximal ends of the tibia and fibula both 

 usually take part in the formation of the shallow facet of the knee-joint 

 apon which moves the distal end of the femur. 



Their proximal end therefore has a shallow facet which is at the 

 zxtreme end. Notice that of the two girdle-joints ; that of the fore-limb 

 Dr the glenoid joint is less deep than that of the hind-limb or the 

 cetabulum. But in* the case of the limb-joint that of the fore-limb or 

 Ibow-joint is much deeper than that of the knee-joint. By keeping 

 hese points in mind there should be no difficulty in recognising a 

 •adioulna from a tibiofibula or an ulna from a tibia. 



Manns and Pes. — The wrist-bones or carpus and the ankle-bones 



r tarsus require special study to be distinguished one by one, but the 



stragalus and calcaneum are always fairly characteristic, the former 



bearing a well-developed sigmoid head for the tibia and the latter being 



produced into the heel in which is inserted the tendon of Achilles. 



The metacarpals and metatarsals are remarkably developed in the 

 Ungulata. In correlation with a reduction in the number of the toes, 

 those remaining are correspondingly increased in size, forming the 

 cannon bones of the horse and ox. These have the appearance of the 

 true long-bones of the limbs, but they may at once be recognised by 

 having a hollow facet at one end (proximal) and a bilateral condyle at 

 the other end (distal). 



Development. — In studying mammalian development 

 we have to keep in mind that the larval and lecithal nutri- 

 tions have been given up and that there is a succession of 

 three forms of nutrition — the albuminal, the haemal and 

 the lacteal. The Prototheria are oviparous, i.e.^ the young 

 are discharged from the body as eggs surrounded by a shell, 

 and further development takes place outside the body of the 

 parent ; but the great majority of the Mammalia are vivi- 

 parous, i.e.^ the young are retained during early stages in 

 a special part of the oviduct, called the uterus, and are 

 '' born " later. 



Maturation and Production of the Ovum. — The 

 eggs arise in the ovaries which are paired. The outer epi- 

 thelial layer of the ovary is the germinal epithelium, and 

 from it the eggs sink into the underlying connective tissue 



