Page 539 - Anatomy and Physiology of Farm Animals, 8th Edition

524 / Anatomy and Physiology of Farm Animals

formation is stimulated by the effects of on the breasts and wings of ducks when
compared with chickens.
estrogen on osteoblasts, and just prior to
VetBooks.ir the beginning of an egg production period,
estrogen levels are relatively high as ovar-
ian follicles mature and begin secreting Gastrointestinal System
estrogen. Medullary bone provides a ready
source of calcium that can be mobilized for The complete gastrointestinal system of
use in eggshell production. Eggshell for- poultry is illustrated in Figure 30‐6.
mation and calcium metabolism will be A distinguishing characteristic of the
discussed in more detail in a later section class Aves is the modification of the jaws
of this chapter. into a beak, the shape of which is adapted
to the foraging behaviors and diet of
individual species. Gallinaceous birds
Musculature have pointed beaks for picking up individ-
ual food items, and Anseriformes have a
The most remarkable variations in the flattened, spoon‐shaped beak that is
skeletal muscles of birds are not surpris- somewhat more flexible and is adapted for
ingly associated with the musculature of straining through water for foodstuffs.
the wing. The m. pectorales produce the Beaks are featherless, covered by a thick
downbeat of the wings that is essential to keratinous epidermis.
provide lift during flight. These muscles The mouth of birds differs from that of
also constitute the breast of the bird bred mammals in that the palate is incompletely
for meat. The musculature of a bird can fused; this results in a longitudinal opening
also be further classified based on the con- between oral and nasal cavities called the
tent of myoglobin, a richly pigmented pro- choanal cleft. The tongue is variable in
tein that binds oxygen and facilitates shape, conforming as it does to the con-
oxygen delivery to muscle tissue. Muscles tours of the oral cavity. In domestic spe-
that are used for longer periods of activity, cies, the tongue lacks musculature and is
with high oxygen demands and myoglobin not much more than a bone (entoglossal
content, have slow‐twitch muscle fibers bone) covered with a thick, cornified
and are referred to as “dark” meat when mucous membrane. Taste buds are also
consuming poultry. During cooking, the present and vary significantly between
myoglobin proteins are denatured with species and breeds.
heat, losing their ability to bind oxygen and The pharynx begins caudal to the
their bright red color, resulting in a brown- tongue. The pharyngeal floor is character-
ish color. The “white” meat is then com- ized by a raised laryngeal mound, which
prised of fast‐twitch muscle fibers for features the aditus laryngeus (laryngeal
short bursts of anaerobic activity. The dis- slit). The opening into the esophagus is
tribution of dark versus white meat on a caudal and somewhat dorsal to this. Within
bird varies with species rather than diet. the neck, the esophagus features a distensi-
For example, chickens are primarily flight- ble dilation called the crop or ingluvies. It
less birds, spending a great deal of time is a small fusiform enlargement in ducks
walking or standing; therefore, the leg and geese, but is extremely well developed
muscles of chickens are comprised primarily in gallinaceous birds. In these birds, the
of slow‐twitch fibers whereas the pectoral crop lies on the left side of the neck cranial
muscles are made up of fast‐twitch fibers. to the furcula, and when full of food, it is
Conversely, flighted birds such as ducks readily palpable and visible. Like the
require considerable oxygen delivery to the esophagus, the crop is lined by a kerati-
muscles supporting flight; the higher myo- nized stratified squamous epithelium.
globin content in those slow‐twitch fibers Within the body cavity, the esophagus
results in a higher proportion of dark meat leads to a bipartite stomach. The first