**2.1 The frontal sinus**

The frontal sinus is an air-filled space which occupies the two cortical layers of the frontal bone, the dorsal part of the skull, medial and dorsal to the orbital cavity. It superimposes both the cranial and nasal cavities. The sinus divides into left and right sinuses using a complete median bony interfrontal septum.

### **Figure 1.**

*Topography of frontal and maxillary sinuses in the donkey skull. 1. Nasoincisive notch. 2. Medial angle of the eye. 3. Supraorbital foramen. 4. Caudal border of supraorbital process. 5. Zygomatic arch. 6. Midline of skull. 7. Facial crest. 8. The end of facial crest. 9. Frontal bone. 10. Nasal bone. 11. Orbit. 12. Frontal sinus. 13. Frontal septum between right and left frontal sinuses. 14. Lateral compartment of caudal maxillary sinus. 15. Frontomaxillary opening. 16. Rostral maxillary sinus. 17. Medial compartment of caudal maxillary sinus. 18. Maxillary septum between rostral and caudal maxillary sinuses. 19. Infraorbital foramen. 20. Infraorbital canal. 21. Nasomaxillary opening. 22. Conchomaxillary opening. 23. Maxillary bone. 24. Dotted line indicated the caudal approach line of conchofrontal sinus. 25. Dotted line indicated the rostral approach line of conchofrontal sinus. 26. 2nd maxillary molar tooth. 27. 3rd maxillary molar tooth. 28. Site of approach of caudal maxillary sinus. 29. Site of approach of rostral maxillary sinus. 30. Dorsal Conchal sinus. 31. The lateral segment connected the lateral extent of the caudal and rostral lines. 32. Bulla of ventral nasal Conchal sinus (opened). 33. Bony plate. A, B) 5 year old donkeys, C&D) 12 year old donkeys. Reproduced with permission [13] journal of veterinary anatomy.*

## *Anatomical Guide to the Paranasal Sinuses of Domestic Animals DOI: http://dx.doi.org/10.5772/intechopen.106157*

In the horse and donkey (**Figures 1**-**3**), the frontal sinus lies close to the dorsal nasal concha that forms the conchofrontal sinus in horse and donkey (**Figure 1**). The interior of the sinus cavity is incompletely divided by several bony spicules [11]. There is a convexity which exists on the floor of the frontal sinus due to the presence of the underlying ethmoidal labyrinth (**Figures 1** and **3**). It communicates directly with the caudal maxillary sinus by a large oval frontomaxillary opening, there is no frontomaxillary opening in any other domestic animal except in equines [1]. This opening is at the level of the caudal third of second molar tooth and extends caudally by 2–3 cm to the last molar tooth on the lateral floor of the conchofrontal sinus (**Figure 1**, see label 15) [11, 13].

In cattle and buffalo, the frontal sinus extends rostrally to the level of the middle of the third molar tooth at the level of rostral half of orbital rim, which is represented by the nasofrontal suture in the macerated skull. The caudal boundary of the sinus is the nuchal line of occipital bone. The sinus extends laterally to the medial boundary of the orbital rim and temporal line. In turn the cranial cavity bulges into the central

#### **Figure 2.**

*Sagittal sections of the donkey head. 1. Frontal sinus. 2. Dorsal Conchal sinus. 3. Ventral Conchal sinus. 4. Middle Conchal sinus. 5. Sphenopalatine sinus. 6. Ethmoidal turbinate. 7. Bulla of the dorsal nasal concha. 8. Bulla of the ventral nasal concha. 9. Septum of ventral nasal concha. 10. Septum of dorsal nasal concha. 11. Straight fold. 12. Alar fold. 13. Basal fold. 14. Dorsal nasal meatus. 15. Middle nasal meatus. 16. Ventral nasal meatus. 17. Hard palate. 18. Soft palate. 19. Frontal bone. 20. Nasal bone. 21. Pharynx. 22. Frontomaxillary opening. 23. Infraorbital nerve. 24. Maxillary bone. 25. Bulla of ventral nasal Conchal sinus. 26. Conchomaxillary opening. 27. Nasomaxillary opening. 28. Lateral part of caudal maxillary sinus. 28*′*. Medial part of caudal maxillary sinus. 29. Rostral maxillary sinus. 30. Maxillary septum. 31. Infraorbital canal. 32. Nasolacrimal duct. 33. Sphenopalatine opening. 34. Frontal septum. 35. Opening of middle nasal Conchal sinus into caudal maxillary sinus. 36.1st maxillary premolar tooth; 37. Second maxillary premolar. 38. 3rd maxillary premolar. 39. 1st maxillary molar tooth. 40. 2nd maxillary molar tooth. 41. 3rd maxillary molar tooth. 42. Facial crest. Reproduced with permission [13] journal of veterinary anatomy.*

**Figure 3.**

*Sinus anatomy in the donkey. (A) Lateral CT scan shows the levels of the section and paranasal sinus. (B) CT at the level of the second cheek tooth maxillary sinus and frontal sinus. (C) Cross section at head) showing the frontal sinus (F). Maxillary sinus (M). Sphenopalatine sinus (SP). Adapted with permission [11].*

part of the frontal sinus. The sinus cavity is subdivided into several compartments; these vary in size and differ in position between differing animals and also from left and right sinus [4]. The frontal sinus cavity is divided by an oblique transverse septum creating the rostral and caudal sinuses. The rostral sinus is subdivided into several small lateral, medial and intermediate compartments by the presence of two irregular oblique longitudinal and transverse septa. The rostral sinus compartments all communicate separately with the ethmoidal meatus and rostrally with the dorsal nasal sinus via a nasofrontal opening [4]. One part of the dorsal nasal concha projects caudally between the medial and intermediate rostral sinuses, in addition the lateral rostral sinus is separated by a thin septum from the lacrimal sinus. The caudal sinus is subdivided into the large caudolateral and small rostromedial sinuses by an incomplete oblique transverse septum, the two latter are also able to communicate with each other. An oblique frontal transverse septum divides the caudolateral sinus into two sub-compartments, of which the caudolateral part is separated from nuchal diverticulum by an oblique transverse septum. The caudolateral sinus cavity has three diverticula: the nuchal, cornual and postorbital diverticula. The nuchal diverticulum is more extensive due to the well-developed parietal bone, and is itself subdivided into four sub-compartments. The cornual diverticulum extends into the cornual process of the frontal bone, and is subdivided by a septum [2]. The postorbital diverticulum is located medially, caudal to the orbital cavity, and dorsal to the cranial cavity, and occupies the space between the orbital cavity and rostral frontal small compartments. The relatively short supraorbital canal passes through the lateral border of the caudal frontal sinus through an apparently bony septum. The caudal frontal sinus opens up in the ethmoidal meatus (**Figures 4**-**6**).

In the camel, each frontal sinus is subdivided by bony plates into six large compartments; two caudal, two lateral and two rostral, surrounding eight small compartments that communicate with the nasal fundus through small openings. The supraorbital canal is traverse to the large caudolateral compartment where the supraorbital foramen is present (**Figure 7**) [6].

In sheep and goats, each frontal sinus is subdivided into interconnected lateral and medial chambers, differing in size, by many bony plates. The medial chamber has only one elongated chamber, which contains no bony plates, communicating only with the middle part of the lateral chamber. Meanwhile the lateral chamber has three portions rostral, middle and caudal, and three diverticula nuchal, cornual and orbital [14]. The

#### **Figure 4.**

*Topography of the frontal and maxillary sinuses of the buffalo head (a& B). 1. Supraorbital foramen. 2. Medial angle of the eye. 3. Cornual process. 4. Infraorbital foramen. A. Medial limit of frontal sinus (Medline of the head). B. Lateral limit of frontal sinus. C. Rostral limit of frontal sinus. D. Caudal limit of frontal sinus. E. Rostral limit of maxillary sinus. F. Dorsal limit of maxillary sinus. G. Ventral limit of maxillary sinus. H. Caudal limit of maxillary sinus.*

#### **Figure 5.**

*Frontal surface of sheep (a) and buffalo (B) skulls showing the frontal sinuses. 1. Median interfrontal septum. 2. Parietal bone. 3. Nasal bone. 4. Cornual process of the frontal bone. 5. Postorbital diverticulum of the frontal sinus. 6. Cornual diverticulum of the frontal sinus. 7. Nuchal diverticulum of the frontal sinus. 8. Cornual septum. 9. Orbital cavity.*

dorsal part of the lateral chamber communicates with the dorsal conchal sinus, the middle part communicates with the medial chamber and its caudal part communicates with the three frontal diverticula. The cornual diverticulum is the largest diverticula and is itself divided into a small dorsal part and a large ventral part by an oblique transverse septum [14]. The orbital diverticulum is surrounded by the orbital cavity dorsally and caudally, however, the nuchal diverticulum is the smallest one. The supraorbital canal meanwhile is a short canal, located at the longitudinal interfrontal septum between the lateral chamber and orbital diverticulum of each frontal sinus (**Figure 5**).

In the dog and cat, the paired frontal sinuses have open communication with the nasal cavity. The rostral frontal sinus lies between the median osseous septum of the frontal sinus and the orbit. The lateral frontal sinus is the largest of the frontal sinuses and extends into the zygomatic process of the frontal bone. The medial frontal sinus is very small and lies between the other two, occasionally it is absent.

#### **Figure 6.**

*Lateral CT scan shows the levels of the section and paranasal sinus in the buffalo. F. frontal sinus. C.D. Cornual diverticulum of frontal sinus. P. Palatine sinus. M. Maxillary sinus. L. Lacrimal sinus.*

**Figure 7.** *Frontal surface of the camel skull showing the frontal sinuses.*
