Part 19

The =anterior= or =nasal wall= is formed by the cribriform plate of the ethmoid, which separates the cranium from the nasal cavity. It is perforated by numerous foramina for the passage of the olfactory nerve-bundles.

FIG. 36.—MEDIAN SECTION OF SKULL OF HORSE WITHOUT THE MANDIBLE.

The septum nasi is removed, but the mucous membrane on the turbinal bones is retained. a, a′, Superior turbinal bone, dotted line indicating limit between anterior coiled part and posterior uncoiled part; b, b′, superior turbinal folds, inclosing bars of cartilage; c, fold of mucous membrane formed by union of b and b′; d, d′, anterior coiled and posterior uncoiled part of inferior turbinal, dotted line indicating septum between them; e, f, inferior turbinal folds, former (alar fold) inclosing bar of cartilage; g, h, i, superior, middle, inferior meatus; o, o′, frontal sinus; partial septum between o and o′; q, nasal part of frontal sinus (nasal sinus); r, lateral mass of ethmoid bone; s, sphenoidal sinus; t, cranial cavity; u, opening made in superior turbinal bone at point where drainage of frontal sinus may be obtained. (After Ellenberger, in Leisering’s Atlas.) ]

THE NASAL CAVITY

The =nasal cavity= (Cavum nasi) is a longitudinal passage which extends through the upper part of the face. It is divided into right and left halves by a median =septum nasi=. The =lateral walls= are formed by the maxilla, premaxilla, and the perpendicular part of the palate bones. Attached to them are the turbinal bones, which subdivide each nasal fossa into three =meatuses= (Meatus nasi). This wall is crossed obliquely by the =canal= and =groove= for the =naso-lacrimal duct=, and its posterior part is perforated by the =sphenopalatine foramen=. The =superior wall= or =roof= is formed by the frontal and nasal bones. It is concave from side to side, and nearly straight longitudinally, except in the posterior part, where it curves downward. It presents a median elevation, the =nasal crest=. The =inferior wall= or =floor= is formed by the palatine processes of the premaxillæ and maxillæ, and the horizontal parts of the palate bones. It is wider but considerably shorter than the roof. It is concave transversely, and nearly horizontal from before backward, except in the posterior third, where there is a slight declivity. The anterior part presents a median groove for the cartilaginous septum, and a furrow for the organ of Jacobson on either side. On either side of the palatine processes of the premaxillæ is the =palatine cleft=.

The =septum nasi= is incomplete in the macerated skull. It is formed by the perpendicular plate of the ethmoid behind, and the vomer below. In the fresh state it is completed by a plate of cartilage.

The =superior meatus= (Meatus nasi superior) is a narrow passage between the roof and the superior turbinal bone. It ends at the cribriform plate of the ethmoid. The =middle meatus= (Meatus nasi medius) is the space between the two turbinal bones. In its posterior part is the very narrow opening into the maxillary sinus. The =inferior meatus= (Meatus nasi inferior) is the channel along the floor which is overhung by the inferior turbinal bone. It is much the largest and is the direct path between the anterior and posterior nares.

The external aperture is bounded by the nasal bones and the premaxillæ.

FIG. 37.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION PASSES THROUGH THE ANTERIOR END OF THE FACIAL CREST, AND BETWEEN THE THIRD AND FOURTH CHEEK TEETH.

a, Superior, b, inferior turbinal bone; c, d, cavities of a and b; e, common meatus; f, g, h, superior, middle, inferior meatus; i, k, passages to cavities of turbinal bones; l, naso-lacrimal duct; m, infraorbital canal; n, anterior end of maxillary sinus; o, septal cartilage. (After Ellenberger, in Leisering’s Atlas.) ]

FIG. 38.—CROSS-SECTION OF NASAL REGION OF SKULL OF HORSE; THE SECTION IS CUT ABOUT HALFWAY BETWEEN THE ORBIT AND THE ANTERIOR END OF THE FACIAL CREST, AND PASSES BETWEEN THE FIFTH AND SIXTH CHEEK TEETH.

a, Superior, b, inferior turbinal bone; c, d, cavities of a and b; e, common meatus; f, superior, g, middle, h, inferior meatus; i, placed over ridge in maxillary sinus; k, communication between outer and inner (turbinal) part of maxillary sinus; l, naso-maxillary opening; m, naso-lacrimal canal; n, infraorbital canal. (After Ellenberger, in Leisering’s Atlas.) ]

The posterior extremity or fundus is separated from the cranial cavity by the cribriform plate of the ethmoid, and is largely occupied by the lateral masses of that bone.

THE PARANASAL SINUSES

Connected directly or indirectly with the nasal cavity, of which they are diverticula, are four pairs of air-sinuses (Sinus paranasales), viz., maxillary, frontal, sphenopalatine, and ethmoidal.

The =maxillary sinus= (Sinus maxillaris) is the largest. Its external wall is formed by the maxilla, the lacrimal, and the malar. It is bounded internally by the maxilla, the inferior turbinal, and the lateral mass of the ethmoid bone. It extends backward to a transverse plane in front of the root of the supraorbital process, and its anterior limit is indicated approximately by a line drawn from the anterior end of the facial crest to the infraorbital foramen. Its upper boundary corresponds to a line drawn backward from the supraorbital foramen parallel to the facial crest. The floor is formed by the alveolar part of the maxilla; it is very irregular and is crossed by bony plates running in various directions. The last three cheek teeth project up into the cavity to an extent which varies with age; they are covered by a thin plate of bone. The cavity is divided into anterior and posterior parts by an oblique =septum=. The outer margin of the septum is commonly about one and a half to two inches (ca. 3.5 to 5 cm.) from the anterior end of the facial crest; from here it is directed inward, backward, and upward. The upper part of the septum (formed by the posterior end of the inferior turbinal bone) is very delicate and usually cribriform.

The position of the septum is quite variable. It is not rare to find it further forward, and in some cases it is further back than is stated above. In the recent state, i. e., when covered by the mucous membrane on both surfaces, it is nearly always complete, but in very exceptional cases there is an opening of variable size in the upper part.

The =anterior compartment=, often called the =inferior maxillary sinus=, is partially divided by the infraorbital canal into an external maxillary part and an internal smaller turbinal part. The latter communicates with the middle meatus by a very narrow slit situated at its highest part. The =posterior compartment=, often called the =superior maxillary sinus=, is also crossed by the infraorbital canal, internal to which it opens freely into the sphenopalatine sinus. It communicates dorsally with the frontal sinus through the large oval =fronto-maxillary opening=, situated at the level of the osseous lacrimal canal and the corresponding part of the inner wall of the orbit; the orifice is commonly about one and a half to two inches (ca. 4 to 5 cm.) long and an inch or more (2 to 3 cm.) wide. Just in front of this, and covered by a thin plate, is the narrow =naso-maxillary fissure= (Aditus naso-maxillaris), by which the sinus opens into the posterior part of the middle meatus.

The foregoing statements refer to the arrangement in the average adult animal. In the foal the cavity (with the exception of its turbinal part) is largely occupied by the developing teeth. In horses five to six years of age the maxillary part of the sinus is still filled up to a large degree by the embedded parts of the teeth. As the teeth are extruded to compensate the wear, more and more of the cavity becomes free, until in old age only the short roots project up in the floor, covered by a layer of bone. Other facts in this connection will be given in the description of the teeth. In exceptional cases the posterior part of the inferior turbinal is smaller than usual and leaves a considerable interval, through which the maxillary sinus communicates with the nasal cavity.

FIG. 39.—SKULL OF HORSE, LATERAL VIEW WITHOUT MANDIBLE. THE SINUSES ARE OPENED UP.

a, Posterior part, b and c, anterior part of frontal sinus; d, roof of superior meatus; e, lateral mass of ethmoid bone; f, f′, naso-lacrimal duct, exposed in its posterior part; g, h, posterior and anterior compartments of maxillary sinus (also designated as superior and inferior maxillary sinuses); i, septum between g and h; k, lower limit of upper thin and partly membranous portion of septum; l, infraorbital canal; m, turbinal part of maxillary sinus; n, bullous prominence of inferior turbinal; o, orbit; p, infraorbital foramen; q, continuation of infraorbital canal to premaxilla; r, limit of maxillary sinus. (After Ellenberger, in Leisering’s Atlas.) ]

The =frontal sinus= (Sinus concho-frontalis) consists of frontal and turbinal parts. The frontal part is bounded chiefly by the two plates of the frontal bone, but its floor is formed in part by the lateral mass of the ethmoid. It extends forward to a plane through the anterior margins of the orbits, backward to one through the temporal condyles, and outward into the root of the supraorbital process. It is separated from the sinus of the opposite side by a complete =septum=. It is partially subdivided by a number of bony plates. The turbinal part is situated in the posterior part of the superior turbinal bone, roofed in by the nasal and lacrimal bones. It extends forward to a transverse plane about half-way between the anterior margin of the orbit and the end of the facial crest. Behind it is in free communication with the frontal part over the lateral mass of the ethmoid. It is separated from the nasal cavity by the thin turbinal plate. The frontal and maxillary sinuses communicate through the large opening described above.

The =sphenopalatine sinus= (Sinus sphenopalatinus) consists of two parts which communicate under the lateral mass of the ethmoid. The sphenoidal (posterior) part is excavated in the body of the presphenoid. The palatine (anterior) part is between the two plates of the perpendicular part of the palate bone, under the lateral mass of the ethmoid; it communicates freely with the maxillary sinus. The septum between the right and left sinuses is not usually median in the sphenoidal part.

In about one-third of the cases (according to Paulli) the sphenoidal and palatine parts are separated by a transverse septum, and the sphenoidal part then communicates only with the lower ethmoidal meatuses.

The term =ethmoidal sinus= is often applied to the cavity of the largest ethmoturbinal. It communicates with the maxillary sinus.

FIG. 40.—SKULL OF HORSE, DORSAL VIEW, WITH SINUSES EXPOSED BY REMOVAL OF THE OUTER PLATE OF BONE.

1, Frontal bone; 2, nasal bone; 3, lacrimal bone; 4, maxilla; a, posterior part of frontal sinus; a′, middle part of frontal sinus; b, anterior (turbinal) part of frontal sinus; c, lateral mass of ethmoid bone; d, roof of superior meatus; e, fronto-maxillary opening; f, naso-maxillary opening below plate which forms the anterior margin of e; g, h, posterior and anterior compartments of maxillary sinus—often called the superior and inferior maxillary sinuses; i, septum between g and h; k, orbit; l, point at which superior turbinal bone may be perforated to obtain drainage into nasal cavity. (After Ellenberger, in Leisering’s Atlas.) ]

THE BONES OF THE THORACIC LIMB THE SCAPULA