Nostrils to lungs
The nostrils are oval, longitudinally arranged and face dorsolaterally (Fig. 3.4). The nasal cavity extends from the nostrils to the choanae with the left and right halves being separated by the nasal septum. The nasal cavity is divided into ros- trodorsal, rostroventral and caudal compartments by nasal turbinate bones and a dorsally incomplete, transverse shelf of bone. The caudal compartment is com- plex, containing openings of both the lacrimal duct and the duct of the nasal gland (Fig. 3.4), and it communicates with the oral cavity and infraorbital sinus. The latter extends caudally around the dorsal, rostral and ventral parts of the eye. Its dorsocaudal limit lies just behind the medial canthus of the eye and its cau- doventral limit lies caudally to the lateral canthus of the eye. The caudal com- partment of each nasal cavity communicates with the oral cavity through a choana. The choanae are two oblique slits in the palate separated from each other by a low ridge of mucous membrane. Together the two choanae form a tri- angular opening in the palate with the apex directed rostrally.
The laryngeal skeleton consists of two cricoid and two arytenoid cartilages. The cricoid cartilages lie ventrally, and together form a ring. The mucous
membrane surrounding the larynx and laryngeal opening (glottis) lack papillae. The arytenoid cartilages lie dorsally, their lateral borders are serrated and the medial borders are smooth. The mucous membranes that overlie the medial bor- ders are thickened, forming two longitudinal ridges which, when opposed by mus- cle action, close off the laryngeal cavity. The laryngeal opening lies caudally to the choanae and the respiratory pathway cannot be excluded from the buccal cavity. The tip of the tongue lies rostrally to the choanae and can therefore exclude off the nasal respiratory pathway from the buccal cavity.
The trachea extends from the larynx to the syrinx, is oval in transverse sec- tion and formed by approximately 200 complete, cartilaginous (MacAlister, 1864) or bony (Duerden, 1912a) rings. It lies ventrally to the cervical vertebrae. In the cranial cervical region the oesophagus lies dorsally to the trachea but when more caudally, to its right. The trachea enters the thorax and divides into the two primary bronchi.
The syrinx of the ostrich is simple and consists of the last tracheal rings and the proximal rings of the primary bronchi. Vibrating membranes are present in the walls of the bronchi (Duerden, 1912a). Forbes (1881) compared the syrinx of the various ratites. The bony rings of the bronchi are incomplete and extend into the lungs for a short distance only (Duerden, 1912a). King and McLelland (1984) give more details of the bronchi and their divisions.
The lungs lie in the dorsal third of the thorax, between the second and sev- enth vertebral ribs (Figs 2.7 and 2.8). They are closely bounded dorsally by the
Fig. 2.7. Thoraco-abdominal viscera, left view. 1, Trachea; 2, thymus;
3, sternotracheal muscle; 4, brachial plexus; 5, left subclavian artery; 6, pulmonary trunk; 7, left cranial vena cava; 8, heart; 9, sternum; 10, left lobe of the liver; 11, gizzard; 12, proventriculus; 13, left caecum; 14, duodenum; 15, ilium; 16, rectum; 17, antetrochanter; 18, acetabulum; 19, capital ligament of the femur; 20, cranial lobe of the left kidney; 21, left lung; 22, thyroid gland.
thoracic vertebrae and the heads of the ribs, such that the dorsal and dorsolateral surfaces of the lungs are deeply grooved. Laterally they are limited by the verte- bral ribs as far ventrally as the uncinate processes and ventrally by the horizontal septum. The primary bronchus, pulmonary artery and pulmonary veins enter and leave the ventromedial (septal) surface of the lung. A series of openings along the ventral surface of the lung open into the air sacs. There is no pleural cavity. The ostrich lung is typically avian in structure. Additional details of the lung and its bronchial tree can be obtained from Schultze (1908), Duerden (1912a) and King and McLelland (1984).
Air sacs
There are ten air sacs in the ostrich (Fig. 2.9; Roche, 1880; Schultze, 1908; Bezuidenhout et al., 1998). Soley et al. (1998) describe the ultrastructural features of the epithelial linings of the air sacs which resemble those observed in other birds, although in the ostrich ciliated and non-ciliated cuboidal elements, together with globlet cells, make up a large component of the luminal epithelium. This is in contrast with the squamous cells observed in other birds.
The cervical air sacs communicate freely with each other, forming an unpaired structure which extends up the neck as far as the head, and is closely related to the cervical and thoracic vertebrae and the vertebral canal. This air sac is vulnerable to injury at the base of the neck.
The paired lateral clavicular air sacs lie ventrolaterally in the cranial thoracic region, with sternal and axillary diverticula. The axillary diverticulum does not enter the humerus. The left and right medial clavicular air sacs fuse with each other, forming a centrally situated air sac. It has diverticula between the large blood vessels above and cranially to the heart, as well as diverticula that extend caudally along the dorsal and ventral surfaces of the oesophagus. The diverticu- lum along the ventral surface of the oesophagus communicates with a large gas- tric diverticulum. The latter extends along the caudal surface of the proventriculus, curves ventrally around its distal border and into the gizzard.
The paired cranial thoracic air sacs are small and lie ventrally to the lungs, against the ribs between the horizontal and oblique septa. They are separated from the clavicular and caudal thoracic air sacs by transverse septa. The paired caudal thoracic air sacs are larger than the cranial air sacs and occupy a similar region, but more caudally.
The right abdominal air sac is relatively small and lies to the right of the mesentery, dorsally to the liver. The left abdominal air sac is large and lies to the left of the mesentery. Both abdominal air sacs have femoral and perirenal diver- ticula. The femoral diverticulum enters the proximal end of the femur, invades the hip joint and forms an extensive intramuscular diverticulum. The latter diverticulum extends distally along the medial aspect of the limb to the level of the stifle (Fig. 2.9). The perirenal diverticulum extends over and around the kid- neys and extends almost to the caudal limit of the abdomen.