Page 363 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Common integument (integumentum commune) 345

The shaft is divided into the calamus and the rachis.
· skin glands (glandulae cutis),
VetBooks.ir · accessory skin structures (appendices integumenti), A small opening, the distal umbilicus (umbilicus dista-
lis) is located at the junction between these segments. The
· the skin folds of the wings (patagia) and
calamus is embedded in the skin; the rachis forms the visi-
· interdigital webs (telae interdigitales).
ble portion of the feather. Located at the tip of the calamus
is a round opening, the proximal umbilicus (umbilicus
Skin glands proximalis), into which the dermal papilla projects. The
Birds do not have sweat glands. Sebaceous glands are papilla is covered by a layer of living epidermal cells, from
found in only three locations: which the new feathers are formed after moulting.
The calamus is round in cross-section and, in the mature
· above the tail: uropygial gland (glandula uropygialis), feather, predominantly hollow. During feather develop-
· in the external acoustic meatus: auricular glands ment, the calamus is filled with pulp consisting of a loose
(glandulae auriculares) and mesenchymal reticulum surrounding a central artery and
· in the cloaca: the vent glands (glandulae venti). vein. During maturation, the pulp recedes leaving behind
a series of air-filled compartments.
The uropygial gland is consistently present in chickens and The rachis contains pith formed from epithelial cells.
water birds, but may be absent in psittacids and pigeons. Its upper surface is convex, while the ventral side (facing
It consists of two bilaterally symmetrical lobes, each with the body) bears a groove (sulcus ventralis). The rachis car-
an excretory duct. The ducts open on the unpaired median ries two rows of slender, rigid barbs (rami). Arising from
uropygial papilla (papilla uropygialis), upon which a each barb are two rows of fine barbules (radii, barbulae).
tuft of small down feathers (circulus uropygialis) is often Barbulae of adjacent barbs cross one another at right ang-
present. The oily holocrine secretory product is used in les. The distally directed barbulae (radii distales) possess
preening to cover the feathers in a fatty waterproof film. It tiny hooklets (hamuli) that interlock with the proximally
is thought that the uropygial gland may also play a role in directed barbulae (radii proximalis) (Figure 15.37).
the storage of vitamin D, allowing this nutrient to be taken Disruption of this interlocking arrangement by external
up by the beak during grooming. mechanical forces is corrected during preening, whereby
The glands of the external acoustic meatus produce the hooklets and barbules are reconnected (‘zipping the
a waxy secretion containing numerous sloughed cells. feathers’).
A mucoid substance is secreted by the glandulae venti Together, the interconnected barbs form the vane
located on the labia of the cloaca. (vexillum). The vexillae of neighbouring feathers over-
lap in a shingle-like arrangement such that one is covered
Feathered body regions (vexillum internum) while the other is exposed (vexillum
externum).
Feathers A small afterfeather (hypopenna) projects from the
Related phylogenetically to the scales of reptiles, fea- distal umbilicus. This may consist of just a tuft of umbili-
thers are the distinguishing feature of all birds. Through cal barbs (barbae umbilicales) or may – as is typical of
their many specialised characteristics (e.g. lightweight covert feathers – have an aftershaft (hyporhachis) with
construction, interlocking of the vanes, compliance and two hypovexillae. The calamus is set within a cylindrical
conformability), feathers confer upon birds the capacity for cutaneous pit known as the feather follicle. A close fit is
flight. Particularly in males, feathers are often brightly and formed between the calamus and the follicle, resembling
characteristically coloured. the relationship between the hair and the hair follicle in
mammals.
FEATHER STRUCTURE The feather follicle has an inner epidermal and outer
The following description is based on the structure of a dermal wall (Figures 15.35 to 15.37). The epidermis of
mature contour feather, consisting of the: the follicle is continuous with the calamus at the proximal
umbilicus. Here, the living epithelial cells of the follicle
· shaft (scapus), give way to the dead keratinised cells of the calamus. At
− calamus, the base of the feather follicle the small dermal papilla
− rachis (rhachis) and projects into the proximal umbilicus (Figure 15.36) and
· internal and external vexillum. blends with the mesenchymal pulp within the calamus of
the developing feather. In the young, growing feather, the
dermal papilla is well vascularised to provide adequate nut-
rition to the epidermis for the processes involved in feather
development.

Vet Histology.indb 345 16/07/2019 15:06