Agriculture Reference
In-Depth Information
the activity of other structures that do not fall under the
voluntary control. In general, the sympathetic and the para-
sympathetic systems are antagonistic.
The sympathetic nervous system originates from the
thoracolumbar spinal cord, and is associated with sympa-
thetic ganglia. Viscera of the thoracic, abdominal, and
pelvic cavity, as well as the iris of the eye, the muscles of
hair follicles, the arterial tree, all smooth muscles, the
cardiac muscle, and the glands are supplied by the sympa-
thetic system.
The parasympathetic nervous system is polarized into
the brain and the sacral spinal cord. Nuclei in the brain and
ganglia outside of the brain cavity supply organs of the
head via the CN III, VII, IX, and X. The vagus nerve (CN
X) travels up the descending colon, supplying all thoracic
viscera (including the lungs and the heart) and all
the rest of the gastrointestinal tract (up to the descending
colon). The pelvic nerves originating from the sacral spinal
cord supply the descending colon, viscera within the pelvic
cavity, and some of the external genital organs. The nerves
form the pelvic plexus, which contains pelvic ganglia.
The
cerebral epiphysis
(or the pineal gland) is located
opposite to the hypophysis and is anchored above the third
ventricle (see Figure 6.37). The epiphysis is known for
producing melatonin, with an antigonadotrophic effect.
The
thyroid gland
is paired, consisting of two separate
lobes inconstantly connected by an isthmus. The gland lies
on the right and left lateral aspects of the fi rst fi ve to seven
tracheal rings, and the isthmus, when present, surrounds
the trachea ventrally. The thyroid hormones regulate
metabolism and growth.
The
parathyroid glands
, usually two on each side of the
trachea, are small structures located close to, or embedded
within, the thyroid lobes. The parathyroid hormone
(parathormone) regulates various aspects of calcium
metabolism.
The
adrenal glands
, paired and small, lie against the
roof of the abdominal cavity, and close to the cranial pole
of the kidneys. A cortex and a medulla are evidently sepa-
rated from one another. The cortex produces the corticoid
hormones and some sex steroids. The medulla is develop-
mentally associated with the sympathetic nervous system
and produces the sympathetic transmitters epinephrine and
norepinephrine.
The
thymus
is a paired lymphoid organ extending from
the larynx to the caudal third of the neck. It continues as
an asymmetrical structure and enters the thoracic cavity in
the cranial mediastinum. The thymus has a lobulated
appearance. A cortex and a medulla can be isolated from
each other. The cervical part of the thymus is unpaired in
the lower neck and is connected with the thoracic part. The
cervical part regresses rapidly and disappears, whereas the
thoracic part can last up to 5 years. It is well developed in
young animals and plays a distinct role in the immune
system (see The Immune System).
THE ENDOCRINE GLANDS
Ductless glands, the endocrine glands secrete hormones
into the bloodstream. Due to the participation in maintain-
ing the balance of the internal environment of the body, it
is sometimes described with the nervous system in the so-
called neuroendocrine system. The anatomy, physiology,
and pathology of the endocrine glands are altogether called
endocrinology. Detailed information about all of these
aspects can be found in endocrinology topics.
There are three categories of endocrine glands/organs:
the fi ve typical endocrine glands (hypophysis, cerebral
epiphysis, thyroid, parathyroid, and adrenal glands)
the organs that have endocrine and exocrine glands
(pancreas, testicles, ovaries, and placenta)
the thymus
THE SENSE ORGANS
The eye, the ear, the olfactory organ, the gustatory organ,
and the touch organ (the Common Integument) are the
sense organs.
The Eye
The eye is the organ of vision, consisting of the eyeball
and the adnexa. Paired organs, the eyes are located and
protected within the bony orbits (Figure 6.40).
The eyeball
(Figure 6.41) consists of three tunics and
the lens. From the external to the internal tunics, they are
the fi brous, the vascular, and the nervous tunics. Only a
small area of the major part of the fi brous tunics, the sclera,
can be seen from outside. It is an opaque structure, pierced
on the posterior end by numerous holes allowing the optic
The
hypophysis
(or the pituitary gland) is part of the
diencephalon (see Figure 6.37), lies on the fl oor of the
brain cavity, and has three lobes: glandular, neural, and
intermediate. It hangs from the ventral aspect of the brain
by a funnel-shaped stalk called infundibulum. The hypoph-
ysis is also called the endocrine brain, because it functions
by governing the activity of the other endocrine glands
and many more structures by specifi c hormones. Some of
its hormones will be described in Chapter 7, Applied
Reproductive Physiology.
