(e.g., Saccoglossus), which burrow in sand and mud,
and pterobranchs (e.g., Rhabdopleura), tiny colonial
animals. The hemichordates have a tripartite (three-
fold) division of the body, pharyngeal gill slit, a form of
dorsal nerve cord, and a similar structure to a noto-
chord. Graptolites, which comprise the class Grap-
tolithina, are common fossils in Ordovician and
Silurian rocks and are now considered hemichordates.
hemochromatosis A genetic condition of massive
iron overload leading to cirrhosis and/or other tissue
damage attributable to iron.
hemocyanin A dioxygen-carrying protein (from
invertebrates, e.g., arthropods and mollusks), contain-
ing dinuclear TYPE 3 COPPERsites.
See alsoNUCLEARITY.
hemoglobin A dioxygen-carrying heme protein of
red blood cells, generally consisting of two alpha and
two beta SUBUNITs, each containing one molecule of
PROTOPORPHYRIN IX.
See alsoBLOOD.
hemolymph The body circulatory fluid found in
invertebrates, functionally equivalent to the blood and
lymph of the vertebrate circulatory system.
hemophilia An inherited clotting problem that
occurs, with few exceptions, in males. It delays coagula-
tion of the blood, making hemorrhage difficult to con-
trol. Hemophilia A, called classical or standard
hemophilia, is the most common form of the disorder
and is due to a deficiency of a factor called factor VIII
(FVIII). Hemophilia B, also called Christmas disease, is
due to a deficiency of factor IX (FIX). A person with
hemophilia does not bleed any faster than a normal per-
son, but the bleeding continues for a much longer time.
See alsoBLOOD.
hemorrhoids(piles) Abnormally enlarged or dilated
veins around the anal opening.
Hench, Philip Showalter(1896–1965) American
Physician Philip Showalter Hench was born in
Pittsburgh, Pennsylvania, on February 28, 1896, to
Jacob Bixler Hench and Clara Showalter. After
attending local schools he attended Lafayette Col-
lege, Easton, Pennsylvania, and received a B.A. in- The following year he enlisted in the U.S.
Army Medical Corps but was transferred to the
reserve corps to finish his medical training. In 1920
he received his doctorate in medicine from the Uni-
versity of Pittsburgh.
He became the head of the Mayo Clinic Depart-
ment of Rheumatic Disease in 1926, and by 1947 was
professor of medicine. During World War II he
became a consultant to the army surgeon general.
During this period with the Mayo Clinic he isolated
several steroids from the adrenal gland cortex and,
working with Edward KENDALL, successfully conduct-
ed trials using cortisone on arthritic patients. Hench
also treated patients with ACTH, a hormone pro-
duced by the pituitary gland that stimulates the
160 hemochromatosis
Scanning electron micrograph of a human erythrocyte, or red
blood cell, tangled in fibrin. Fibrin is the insoluble form of
fibrinogen, a coagulation factor found in solution in blood plasma.
Fibrinogen is converted to the insoluble protein fibrin when
acted upon by the enzyme thromboplastin. Fibrin forms a fibrous
meshwork, the basis of a blood clot, which is the essential
mechanism for the arrest of bleeding. Red blood cells contain the
pigment hemoglobin, the principal function of which is to transport
oxygen around the body. Magnification: ×6900 (at 10 ×8 in. size).
(Courtesy © Dr. Tony Brain/Photo Researchers, Inc.)