Page 372 - Anatomy and Physiology of Farm Animals, 8th Edition

The Respiratory System / 357

Trachea and Bronchi divide to the extent that they are less than
1 mm in diameter, the cartilage disap-
VetBooks.ir The trachea extends from the caudal end pears, and these airways are called bron-

chioles. The bronchiole eventually
of the larynx to the bronchi (Fig. 19‐7). It
is formed by a series of C‐shaped hyaline branches into several alveolar ducts,
tracheal cartilages that provide cross‐ which terminate in clusters of air sacs,
sectional rigidity to resist collapse and the alveoli. It is here that the exchange of
are joined one to another by elastic annu- gases with the blood takes place. Some
lar ligaments that permit the trachea terminal bronchioles have alveoli in their
considerable flexibility to follow move- walls, hence are called respiratory bron-
ments of the neck. The dorsal side of chioles (Fig. 19‐9).
the trachea is completed by connective
tissue and the m. trachealis, a smooth
muscle whose tone affects the diameter Thorax
of the trachea.
The trachea passes caudad as far as the The thorax is bounded cranially by the first
base of the heart, where it divides into pair of ribs, the first thoracic vertebra, and
two principal (primary) bronchi, one the cranial part of the sternum. This ring of
for each lung (Fig. 19‐7). The ruminants skeletal elements is the thoracic inlet. The
and pig have an additional tracheal bron- dorsal part of the thorax is defined by the
chus arising cranial to the principal bron- thoracic vertebrae and epaxial muscles,
chi; it supplies the cranial lobe of the and the ventral part, by the sternum. The
right lung (Fig. 19‐8). The principal bron- ribs and costal cartilages, linked by inter-
chi branch into secondary (also called costal muscles, create the lateral walls. The
lobar) then tertiary bronchi, subsequent overall shape of the thorax is that of a cone
branches becoming smaller and smaller. with the apex at the thoracic inlet. The
The walls of these bronchi are supported base of the cone is covered by the dome‐
by cartilaginous plates. When the airways shaped diaphragm.

Trachealis muscle
Tracheal ring

Mucosa
Annular ligament

Principal bronchi
Carina

Figure 19-7. Basic anatomy of trachea and principal bronchi.