The =organs= of =respiration= consist of the =nasal cavity=, the
=nasopharynx=, the =pharynx= (also a food-passage), the =larynx= (also
the organ of the voice), the =trachea=, the =bronchi=, and the =lungs=.
With them are usually described also the =thyroid= and =thymus glands=.
1. =The Nasal Cavity. Cavum nasi.=--The osseous framework of the nasal
cavity has already been described (page 59), and in connection with this
description the boundaries of the cavity and its connections with other
cavities have been given. It consists essentially of a large cavity
bounded by the facial bones and divided by a longitudinal partition into
two lateral halves. The two cavities thus formed are nearly filled by
(1) the labyrinths of the ethmoid (ethmoturbinals), (2) the superior
nasal conchæ or nasoturbinals, projecting into the dorsal part from the
ventral surface of the nasal bones, and (3) the inferior nasal conchæ,
or maxilloturbinals, projecting into the ventral portion from the medial
surfaces of the maxillaries.
There remain to be considered, in addition to the bones, the
=cartilaginous= framework of certain parts of the nose, and the =mucous
membrane=. The lamina perpendicularis is continued by cartilage,
especially craniad, in such a way as to make a complete septum
separating the two cavities. This septum extends from the septum of the
external nose caudad to the lamina cribrosa, and from the internasal
suture ventrad to the vomer and the suture of the premaxillaries. All
parts of the nasal cavity are lined by mucous membrane. This is
continuous at the nares with the integument, while at the choanæ it
passes into the mucosa of the pharynx. It covers the conchæ nasales and
the labyrinths of the ethmoid, passing into the cellules of the latter.
Owing to the crowding together of the conchæ nasales and the labyrinths
the nose is almost completely filled, only three narrow passageways
being distinguishable. The ventral one of these, known as the =ventral=
or =inferior meatus= of the nose, lies ventrad of the inferior nasal
concha, next to the nasal septum. It passes caudad beneath the
horizontal plate formed by the vomer and ethmoid, and opens caudad at
the =choanæ= into the nasopharynx. The =dorsal= or =superior meatus= of
the nose lies just ventrad of the superior nasal concha, next to the
median septum; it leads to the frontal sinus, the lamina cribrosa, and
the caudal parts of the ethmoid. The =middle meatus= of the nose has
almost disappeared in the cat; it is simply the narrow space between the
superior and inferior meati. It is practically filled by the ethmoid,
into the cells of which it leads. The mucosa is continued from the nasal
cavity into the sinuses of the frontals and presphenoid.
At the sides of the nasal septum, near the ventral edge, and about one
to one and a half centimeters caudad of the nares, there is on each side
a small curved cartilaginous tube, about one centimeter or less in
length. This, the =vomeronasal= organ, or =organ= of =Jacobson=, lies
against the septum, between it and the mucosa. It begins at the incisive
canal or anterior palatine foramen, in the roof of the mouth, curves
thence caudodorsad close against the side of the nasal septum, and ends
blindly in the nasal cavity.
[Illustration: FIG. 103.--CROSS-SECTION OF THE CARTILAGES OF THE
EXTERNAL NOSE.
_a_, cartilage of the median septum; _b_, “wings”; _c_, ridge formed
by internal portion of wing.]
The =nares= or cranial openings of the nasal cavity are supported by a
number of cartilages which form the framework of the =snout= or external
nose. The cartilaginous continuation of the lamina perpendicularis
extends some distance craniad of the tips of the nasal and premaxillary
bones, forming the septum of the external nose (Fig. 103, _a_). From the
dorsal edge of this projecting cartilaginous septum, two thin
cartilaginous wings (_b_) extend laterad, forming the dorsal wall of the
narial opening. Each then turns ventrad to form the lateral wall of the
opening, but does not form the ventral floor; instead it curves mediad
and finally dorsad, thus extending from the floor of the opening as a
prominent ridge (_c_) within the nares. The cartilaginous wing is thus
rolled into a sort of spiral, ending with a free edge within the narial
opening. From the ventral edge of the median cartilaginous septum there
are likewise lateral extensions, which form part of the ventral boundary
of the nares; these do not quite reach the ventral parts of the dorsal
wings, however, so that a small part of the narial opening is not
bounded by cartilage. A section of the narial cartilages is shown in
Fig. 103. The wings from the dorsal edge of the septum do not extend
quite to the cranial tip of the nose, so that a notch is formed on the
ventrolateral side of the nares; a section in this region would
therefore differ from that figured.
The framework of the external nose thus formed is covered externally by
thick hairless skin, containing many glands. From the ventral end of the
internarial septum a groove passes ventrad, partly dividing the upper
lip.
The inner surfaces of the cartilages are covered by the =mucosa=, which
forms a number of ridges. The narial opening is almost completely
divided by the prominent ridge which is supported by the free edge (_c_)
of the cartilaginous wing above described. This free edge is covered by
a thick layer of mucosa, and the entire ridge so formed is continuous
caudad with the inferior nasal concha. On the medial side of the partial
partition formed by this ridge and near the ventral side there begins a
slight distance caudad of the outer opening another ridge, supported by
the ventrally incurved portion of the cartilaginous ring. This soon
becomes a thick swelling; beneath it opens the lachrymal canal, and that
part of the passageway that lies ventromediad of it is the beginning of
the inferior meatus of the nose. About one or two centimeters caudad of
the external opening a third ridge projects from the dorsolateral wall
of the cavity toward the large ridge first described; dorsomediad of it
is a narrow passage which is the entrance to the =superior meatus= of
the nose.
The =olfactory mucosa=, or that part to which the olfactory nerve is
distributed, and which therefore acts as the sensory surface, is
confined to the dorsocaudal parts of the nasal cavity, in the region
occupied by the cells of the ethmoid. The air penetrates to this region
probably only by a definite act of snuffing, the inferior meatus serving
as the usual passageway of air to the lungs.
The =nasopharynx= and =pharynx= are considered under the alimentary
canal (page 231).