Webster's New World Medical Dictionary

part of the heart. The autonomic nervous system
controls the firing of the SA node to trigger the start
of this cardiac cycle. The autonomic nervous system
can transmit a message quickly to the SA node, so it
in turn can increase the heart rate to twice the nor-
mal rate within only 3 to 5 seconds. This rapid
response is important during exercise, when the
heart has to increase its beating speed to keep up
with the body’s heightened demand for oxygen. Also
known as sinus node.
sinus 1
An air-filled cavity in a dense portion of a
skull bone. The sinuses decrease the weight of the
skull. The sinuses are formed in four right-left pairs.
The frontal sinuses are positioned behind the fore-
head, while the maxillary sinuses are behind the
cheeks. The sphenoid and ethmoid sinuses are
deeper in the skull behind the eyes and maxillary
sinuses. The sinuses are lined by mucous-secreting
cells. Air enters the sinuses through small openings
in the bone called ostia. If an ostium is blocked, air
cannot pass into the sinus, and likewise, mucous can-
not drain out. See also 
sinusitis.
2
A channel per-
mitting the passage of blood or lymph fluid that is not
a blood or lymphatic vessel, such as the sinuses of the
placenta.
3
A tract or fistula leading to a cavity
which may be filled with pus.
sinus, cavernous
See 
cavernous sinus.
sinus barotrauma
See 
aerosinusitis.
sinus headache
See 
headache, sinus.
sinus node
See 
sinoatrial node.
sinus rhythm
The normal regular rhythm of the
heart that is set by the sinoatrial (or sinus) node,
which is located in the wall of the right atrium (the
right upper chamber of the heart). Normal electri-
cal impulses of the heart start there and are trans-
mitted to the atria and down to the ventricles (the
lower chambers of the heart). The lack of normal
sinus rhythm is an arrhythmia (abnormal heart
rhythm). Sinus arrhythmia refers to the normal
increase in heart rate that occurs during inspiration
(breathing in). It is a normal response and is more
accentuated in children than in adults. See also
sinus tachycardia.