144 HUMAN PHYSIOLOGY. 



THE SENSE OF HEARING. 



The Organ of Hearing is situated in the petrous portion of the tem- 

 poral bone, and is divided into three portions, viz.: the external ear, the 

 middle ear and the internal ear. 



The External Ear consists of two portions, the pinna or auricle, and 

 the external auditory canal. The former, consisting of cartilage, which is 

 irregularly folded and covered by integument, is united to the side of the 

 head by ligaments and muscles ; the latter, partly cartilaginous and partly 

 bony, is about one and a quarter inches in length; it runs downward and 

 forward from the concha to the middle ear, and is lined by a reflection of 

 the general integument, in which is lodged a number of glands, which 

 secrete the cerumen. 



The function of the external ear is to collect the waves of sound coming 

 from all directions and to transmit them to the membrana tympani. 



The Middle Ear or Tympanum is an irregularly- shaped cavity, narrow 

 from side to side, but long in its vertical and antero-posterior diameters. 



It is separated from the external ear by the membrana tympani^ and 

 from the internal ear by a second membrana tympani ; it communicates 

 posteriorly with the mastoid cells, anteriorly with the pharynx, through the 

 Eustachian tube. It is lined by mucous membrane, and contains three 

 small bones, forming a connected chain running across its cavity. 



The Membrana tympani is a thin, delicate, translucent membrane, cir- 

 cular in shape and measuring about two-fifths of an inch in diameter ; it is 

 received into a delicate ring of bone, which in the adult becomes consoli- 

 dated with the temporal bone ; it is concave externally and situated 

 obliquely, inclining at an angle of 45 degrees. 



The membrane consists of three layers: the outer is formed by a reflec- 

 tion of the integument lining the external auditory canal ; the middle is 

 composed of fibrous tissue, and the internal of mucous membrane. 



The Function of the membrana tympani is to receive and transmit the 

 waves of sound to the chain of bones; it is capable of being made tense 

 and lax by the action of the tensor tympani and laxator tympani muscles, 

 so as to vibrate in unison with the waves of sound in the external auditory 

 meatus. When the membrane is relaxed, its vibrations have a greater 

 amplitude, and it appreciates sounds of a low pitch. When it is made 

 tense it vibrates less forcibly and appreciates sounds of a high pitch. 



The Chain of bones is formed by the malleus, incus and stapes, united 

 together by ligaments. The malletis consists of a head, neck and handle, 

 of which the latter is attached to the inner surface of the membrana tym- 



