Agriculture Reference
In-Depth Information
musculo-mucosal continuation of the hard palate and has
a defi nite role in deglutition (see the oropharynx in the
next section). The laryngopharynx is the smallest part of
the pharynx, located dorsal to the larynx.
The thoracic cavity is divided into two halves by a con-
nective tissue partition called mediastinum. The thoracic
cavity is lined by a connective tissue called endothoracic
fascia and a serous membrane called pleura. The pleura
associated with the walls of the thoracic cavity, on both
sides of the mediastinum and covering the pericardium, is
called parietal pleura. The pleura covering the lungs is
called visceral or pulmonary pleura. The two categories of
pleurae are in continuation with each other at the level
where the two primary bronchi branch from the trachea
and are heading toward the lungs.
Each lung has an apex (toward the thoracic aperture), a
base (lying against the diaphragm), a costal surface and a
mediastinal surface, and borders. Both lungs are divided
into lobes separated by fi ssures. On the mediastinal aspect
of each lung, there is an indentation called the hilus, where
the primary bronchi and pulmonary vessels connect with
the lungs. The primary bronchi and the pulmonary vessels
form the roots of the lungs. The following are the pulmo-
nary lobes: cranial and caudal right and left lobes (the right
cranial lobe is divided into a cranial and a caudal parts);
the right lung is provided by a middle lobe; and on the
ventral aspect of the caudal lobe, an accessory lobe can be
identifi ed on the right lung. The accessory lobe is separated
from the right caudal lobe by the passage of the caudal
vena cava. On the surface of the cranial lobes and the
middle lobe, a polyhedral design may be seen, due to the
abundance of connective tissue separating the pulmonary
lobules (Figure 6.23 and Figure 6.24 ).
The internal confi guration of the lungs shows a segmen-
tation of the primary bronchi (lobar and segmental bronchi,
bronchioles, and respiratory bronchioles), ending into the
alveolar sacs. The alveolar mucosa supports a vascular rete
(network). Here the gas exchange takes place between the
venous blood leaving the carbon dioxide replaced by the
oxygen and becoming arterial blood, a process called
hematosis. The functional anatomy of the lungs is self
explanatory.
T
HE
L
ARYNX
The larynx is an air passage, a musculo-cartilaginous organ
between the pharynx and the trachea, and is also involved
in phonation. It consists of paired and unpaired cartilages,
joints, muscles, and a mucosal membrane. The muscles
move the cartilages between each other and the entire
organ rostrally and/or caudally. Some of the muscles regu-
late the amount of air entering the larynx, and others
produce sounds. The sound/voice is produced in the middle
of the larynx, by the vibration of the vocal cords.
T
HE
T
RACHEA
Consisting of incomplete cartilages, the trachea looks like
a pipe, allowing the air to fl ow toward the lungs and from
the lungs, and yet is collapsible. The cartilages are con-
nected by annular ligaments. On the dorsal aspect, where
the two ends of the cartilages face each other, the tracheal
muscle is shown on the inner surface of the trachea. The
trachea travels throughout the neck, ventral to the cervical
vertebrae and the esophagus, and enters the thoracic cavity.
Still with the esophagus on top of it, the trachea bifurcates
into the primary bronchi, dorsal to the heart. The primary
bronchi enter the lungs. Before the bifurcation, the trachea
sends an accessory bronchus to the right lung.
The Essential Respiratory Organs (the Lungs)
The lungs are located within the thoracic cavity. The latter
is a space outlined dorsally by the thoracic vertebrae, ven-
trally by the sternum, and laterally by the ribs and the
intercostal muscles. The thoracic vertebrae are lined in the
fi rst half by a muscle, whereas the sternum is covered by
a ligament and a muscle. The entrance into the thoracic
cavity (the thoracic aperture) is outlined by the fi rst pair
of ribs, the fi rst thoracic vertebra, and the fi rst sternebra.
The caudal wall of the thoracic cavity is the diaphragm, a
vital organ, which functions as the primary muscle of
inspiration. The diaphragm consists of peripheral muscles,
which send toward the center intermingled aponeuroses
collectively called the central tendon (the former phrenic
center). The diaphragm is pierced by three holes for allow-
ing important organs to pass from the thoracic cavity into
the abdominal cavity and vice versa. In a dorso-ventral
order, they are the following: the aortic hiatus (a large
foramen [opening]), the esophageal hiatus, and the foramen
of the caudal vena cava.
THE DIGESTIVE SYSTEM
In all species, the digestive system provides nutrients from
food and eliminates the wastes. However, in all ruminants
including the goat, the digestive system is adapted to allow
a quick storage of forage, which is digested slowly in safe
locations, to avoid predators in the wild. Therefore, the
digestive organs are different from those of the horse, pig,
and carnivores; and their function will be detailed in
Chapter 8, Digestive Physiology and Nutrient Metabolism.
The digestive system consists of a succession of structures
starting from the oral cavity and ending at the anus. To
