Page 167 - Anatomy and Physiology of Farm Animals, 8th Edition

152 / Anatomy and Physiology of Farm Animals

inspiratory, causing air to enter the lungs When these muscles contract, they tend
to rotate the ribs upward and forward,
by increasing the size of the thorax.
VetBooks.ir inspiration. It is a dome‐shaped sheet of increasing the size of the thorax. The
The diaphragm is the chief muscle of
costales interni), which lie deep to the
skeletal (voluntary) muscle separating the internal intercostal mm. (mm. inter-
thoracic and abdominal cavities. It is external intercostal muscles, run from
attached ventrally at the sternum, laterally each rib to the next one in front, and the
along the most caudal ribs, and dorsally to fibers are directed ventrad and craniad.
the lumbar vertebrae; the dome or cupula Although not all studies are in agreement,
of the diaphragm projects deeply into the most anatomists describe their action as
thorax (Fig. 7‐19). Contraction of the fibers reducing the volume of the thorax and
of the diaphragm tends to flatten the dia­ therefore aiding in forced expiration.
phragm and force the abdominal viscera Some authorities believe that both the
caudad, further into the abdomen. This, in external and internal intercostal muscles
effect, increases the volume of the thorax may function in both inspiration and
and lowers intrathoracic pressure, drawing expiration.
air into the lungs. As mentioned previously, the abdomi­
The external intercostal mm. (mm. nal muscles are able to act as muscles of
intercostales externi) extend from each expiration by forcing the abdominal viscera
rib to the next rib behind. The fibers are against the diaphragm and by pulling the
directed ventrad and caudad similarly to ribs caudad, both of which decrease the
the external abdominal oblique muscle. size of the thorax.