Fig. 25. Mastoid Process of Child, opened. 

 Fig. 26. Mastoid Process in Adult, opened. 



Fig. 25. Tlie various layers of the Mastoid region in a child, aged two vears, 

 liave been exposed, and the Mastoid Process cliiselled open. Air-cells, red. — 

 In Fig. 26 the Mastoid region of an adult has been more extensively dissected 

 but only that portion of the Mastoid Process containing air-cells has been 

 opened by chisel. The periphery of the Mastoid Process is indicated by a dotted 

 line. By removal of a portion of the Parotid Gland the Facial Nerve has been 

 exhibited as it emerges from the Stylomastoid Foramen. 



The Antrum is well marked in the New-Born though the Mastoid Process 

 is scarcely discernible; its posterior and external portion becoming formed during 

 the first years of life; it grows downwards as the formation of air-cells slowly 

 progresses. Even in the adult there are not necessarily any air-cells at the tip 

 of the Mastoid Process. The Facial Nerve after emerging from the Stylomastoid 

 Foramen runs forward at a right angle in the infant, at an obtuse angle in 

 the adult. 



The groove of the Lateral Sinus is shallow in the child, deep in the adult. 

 The iVntrum and Mastoid Cells are easily accessible for operative purposes from 

 the outer surface of the Mastoid Process. Subcutaneously in the angle between 

 the Pinna and the Skull the Posterior Auricular Artery takes its course. The 

 Periosteum is intimately connected with the tendinous fibres of origin of the 

 Sternocleidomastoid Muscle which gradually become lost in the Temporal Fascia. 

 About 'Vath inch behind the Suprameatal Spine is situated the Antrum at a depth 

 of V.r,th of an inch from the surface. Below this are the Mastoid Cells. 



The structures in relation with the Antrum are of great importance. 



The thin Tegmen Tympani alone separates the Antrum from the Cranial 

 Cavity, so that search for an Epidural or Temporal Abscess is easy after perfor- 

 ation of the Tegmen Tympani. If projected on to the surface the floor of the 

 middle Fossa of the Skull lies in the region of the attachment of the pinna either 

 above or on the level of the Temporal line. Posteriorly and internally is the 

 Lateral Sinus which should be avoided when the air-cells are opened. The position 

 of the Lateral Sinus varies, it may lie in a shallow groove on the Mastoid Process, 

 or in a deep furrow in both Mastoid and Petrous portions. 



According to Bezold the most marked outward curve of the Sinus is 

 V5 inch behind the Suprameatal Spine. At this point the bone is usually 0.3 inch 

 thick (o.i to 0.7). The Facial Nerve may be injured as it lies below the External 

 Semicircular Canal on the inner wall of the Tympanic Cavity close to the opening 

 itito the Antrum (Aditus ad Antrum). The wall of the Facial Canal is ver}' thin 

 so that by a careless use of the chisel this nerve may be divided. 



Lower down the Mastoid Cells are in relation with the Facial Canal ; this 

 portion has been laid free in both figures. 



