844 THE APFABATUS OF THE SENSES. 



extremity of the Eustachian tube ; it passes downwards and backwards, and 

 terminates by a tendon which is reflected outwards, in front of the fenestra 

 ovalis, to be inserted into the neck of the malleus. 



b. Muscle of the stapes (stapedius). — Lodged in an excavation in the 

 inner wall of the tympanum, near the fenestra vestibuli, on the course of 

 the aquseductus Pallopii, this muscle is remarkable for its brevity, its 

 relatively considerable thickness, and its conical shape. It terminates by a 

 small tendon in front of the head of the stapes. In the Horse, Ox, and 

 Sheep, a small bony nucleus is found in the tendon (Fig. 395, o). 



(The tensor tympani retracts the bones of the ear inwards. In con- 

 tracting it draws the handle of the malleus towards the cavity of the 

 tympanum, and this brings the membrane with it ; consequently, the con- 

 vexity of the latter is increased and its tension is augmented. In addition, 

 ■while the handle of the malleus is carried inwards, its head is turned 

 outwards by a pivoting motion, and this pulls the body of the incus also, 

 the long process of which is raised and inclined inwards, pushing the os 

 orbioulare and stapes towards the fenestra ovalis. The base of the latter 

 bone being in contact with the fluid in the vestibule, this is stirred ; so that 

 this muscle likewise acts indirectly in producing the undulations in this 

 fluid. The muscles of the tympanum are classed as tensors and laxators. 

 It is well to know that all are tensors and none of them act as relaxors ; 

 relaxation of the membrane occurring when the muscles are not in action.) 



5. Hie Mucous Membrane of the Tympanum. 



Very fine and vascular, this membrane covers all the angularities of the 

 middle ear, is reflected on the chain of bones, and continued into the 

 mastoid cells. It is continuous with that lining the Eustachian tube, and 

 thenceforward should be considered as a prolongation of the tegumentary 

 membrane spread over the walls of the pharyngeal vestibule. It is covered 

 by a simple pavement epithelium. 



6. Eustachian Tube. 



The Eustachian tube is a fibro-cartilagiuous canal that communicates 

 between the cavity of the middle ear and the pharynx. 



Extending in a straight line beneath the base of the cranium, from the 

 tympanic case to the upper and lateral part of the pharyngeal cavity, this canal 

 is also named the guttural duct of the tympanum ; it is nearly four inches long 

 in Solipeds, " is flattened on both sides, and bordered by the stylo-pharyn- 

 geus muscle. Its upper or tympanic orifice is narrow ; the inferior, guttural, 

 or pharyngeal orifice, situated near and behind the guttural openings of the 

 nasal cavities, is wide, and represents a great slit extending obliquely down- 

 wards and outwards; the contiguous borders of this aperture are sustained 

 by a cartilaginous plate — a kind of pavilion formed by the expansion of the 

 tissue constituting the base of the tube. 



" In its length, the guttural duct is cleft inferiorly, and by this long 

 aperture the mucous membrane escapes and descends to form the large sac 

 peculiar to monodaotyles, known as the guttural pouch. 



7. Guttural Pouches. 



" The mucous membrane lining the Eustachian tube is continuous,forward, 

 with that of the pharynx; above and behind, it is prolonged into the tym- 

 panic cavity, which it lines. Below, it is dilated and forms the guttural pouch. 



