The ear,intracranial region and cranial nerves

The organ of hearing and balance is divided into an external ear, conveying sound waves to the tympanic membrane; a middle ear relaying the membrane's vibrations to the internal ear; and the cochlea of the internal ear, which translates these vibrations into nerve impulses. The semicircular canals, the organ of balance, are also in the inner ear. The external ear consists of the auricle (pinny) and the external acoustic meats. The auricle is formed of an irregularly shaped piece of cartilage covered by firmly adherent skin. It has a dependent globule used for earrings and an anterior Travus which projects and overlaps the opening of the menus. The external acoustic meat us (SEAM) passes almost horizontally from the Travus to the tympanic membrane in a slightly anterior direction. It is about 4 cm long. Its cartilaginous lateral third is continuous with the cartilage of the auricle; the bony medial two-thirds are formed mainly by the temporal bone. The meat us is lined by skin containing many wax secreting (coterminous) glands. It is innervated by the horticulturalist nerve anterior and the vague nerve posterior. The translucent tympanic membrane (eardrum)  separating the external and middle ears is oval in shape and lies obliquely, with its outer (lateral) surface facing down-wards and forwards. The handle of the mallets is attached to its medial surface and produces a small elevation on the drum, which can be seen with an periscope as the dumbo. 
 
 The middle car (tympanic cavity;  lies within the temporal bone. It is about 15 mm high, 15 mm long, and its medial and lateral walls curve inwards, being about 2 nun apart in their middle and 6 mm apart at the roof. The cavity, lined in part by cilia ted columnar epithelium, contains three small bones, mallets, incs and spates  the auditory vesicles. The lateral wall is formed mainly by the tympanic membrane. Above and behind the membrane is an upward extension of the cavity, the tympanic recess, which contains part of the mallets and incs. The medial wall has a central bulge, the promontory, overlying the base of the cochlea (see below). Above and behind it is the fenestration vestibule (oval window) and below it the fenestration cochleae (round window). Both these formalin lead to the inner ear. The fenestration vestibule is closed by the footplate of the states and the fenestration cochleae by a fibrous membrane. The anterior wall has two openings, the upper for tensor tympani muscle, which attaches to the handle of the mallets, the lower for the auditory (Eustachian) tube, which communicates with the pharynx. High on the posterior wall is the audits, the opening into the mastoid ant rum. Below it formalin transmit the stampedes muscle to the spates and the chords tympani. The facial nerve grooves the medial wall and then descends the medial and part of the posterior wall in a bony canal.

GASEOUS EXCHANGE IN A MAMMAL

Structure of the respiratory system

The respiratory surface of a mammal consists of many air sacs called alveolar inside a pair of lungs. The lungs are situated next to the heart in the thoracic cavity and arc connected to the atmosphere by tubes. Air passes into the lungs through these tubes. Twelve pairs of bony ribs surround and protect the lungs and heart. intercontinental muscles are attached to the ribs, and a large diaphragm separates the thorax from the abdomen.

These are involved in the ventilation mechanism, as described in section. Air enters the body through two nostrils, each of which possesses a border of large hairs which trap particles in the air and filter them out of the system. While passing through the nasal passages the air is warmed and moistened and its dour detected. Air passes from the nasal passages, through the pharynx and into the trachea. This is a tube which lies in front of the esophagus and extends into the thoracic cavity. The wall of the tube is strengthened and held open by horizontally arranged C-shaped bands of cartilage. The open section of the C is next to the esophagus. The cartilage prevents collapse of the tube during inspiration (breathing in). The cartilage can be seen in a section of the trachea. At its lower end the trachea splits into two bronchi. Within the lungs each bronchus subdivides many times into 

much smaller tubes called bronchi oles. These in turn branch into finer and finer tubes, ending with the alveolar ducts which lead into sacs called alveolar sacs. Into each alveolar sac opens a group of alveolar. A summary of these structures and their main features is provided in. The walls of most of the respiratory passage are lined with cilia ted epithelial cells and goblet cells, which secrete mucus. Mucus traps any particles, such as dust and bacteria, that have managed to pass through the hairs of the nostrils. The beating of the cilia then carries the trapped particles to the back of the buccaneer cavity where the mucus is swallowed. Note that it is not the cilia which trap the particles don't confuse them with the hairs in the nose. Mucus also moistens the incoming air.