The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with respect to the main body of the organism. In medical disciplines, all references to a location on or in the body are made based upon the standard anatomical position.

A straight position is assumed when describing a proximo-distal axis (towards or away from a point of attachment). This helps avoid confusion in terminology when referring to the same organism in different postures. For example, if the elbow is flexed, the hand remains distal to the shoulder even if it approaches the shoulder. (Full article...)

Human knees

In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest joint in the human body. The knee is a modified hinge joint, which permits flexion and extension as well as slight internal and external rotation. The knee is vulnerable to injury and to the development of osteoarthritis.

It is often termed a compound joint having tibiofemoral and patellofemoral components. (The fibular collateral ligament is often considered with tibiofemoral components.) (Full article...)

Human placenta from just after birth with the umbilical cord in place

The placenta (pl.: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth (sometimes incorrectly referred to as the 'maternal part' of the placenta). Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.

Mammalian placentas probably first evolved about 150 million to 200 million years ago. The protein syncytin, found in the outer barrier of the placenta (the syncytiotrophoblast) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus: essentially a virus that helped pave the transition from egg-laying to live-birth. (Full article...)

Portrait attributed to Daniël Mijtens, c. 1627

William Harvey (1 April 1578 – 3 June 1657) was an English physician who made influential contributions in anatomy and physiology. He was the first known physician to describe completely, and in detail, the systemic circulation and properties of blood being pumped to the brain and the rest of the body by the heart, though earlier writers, such as Realdo Colombo, Michael Servetus, and Jacques Dubois, had provided precursors of the theory. (Full article...)