1294 SURFACE ANATOMY AND SURFACE MARKINGS 



border of the mastoid process at its root. A line from the upper end of the posterior 

 line to the point where the anterior intersects the line of the posterior ramus of the 

 lateral fissure indicates the position of the central sulcus. The precentral and 

 postcentral sulci are practically parallel to the central sulcus; they are situated 

 respectively about 15 mm. in front of, and behind, it. The superior frontal sulcus 

 can be mapped out by a line drawn from the junction of the upper and middle 

 thirds of the precentral sulcus, in a direction parallel with the longitudinal sulcus, 

 to a point midway between the middle line of the forehead and the temporal line, 



4 cm. above the supraorbital notch. The inferior frontal sulcus begins at the junc- 

 tion of the middle and lower thirds of the precentral sulcus, and follows the course 

 of the superior temporal line. 



The horizontal limb of the intraparietal sulcus begins from the junction of the 

 lower with the middle third of the postcentral sulcus and curves backward parallel 

 to the longitudinal fissure, midway between it and the parietal eminence; it then 

 curves downward to end midway between the lambda and the parietal eminence. 

 The external part of the parietooccipital fissure runs lateralward at right angles 

 to the longitudinal fissure for about 2.5 cm. from a point 5 mm. in front of the 

 lambda. If the line of the posterior ramus of the lateral cerebral fissure be 

 continued back to the longitudinal fissure, the last 2.5 cm. of it will indicate the 

 position of the parietooccipital fissure. 



The lateral ventricle may be circumscribed by a quadrilateral figure. The upper 

 limit is a horizontal line 5 cm. above the zygomatic arch; this defines the roof of 

 the ventricle. The lower limit is a horizontal line 1 cm. above the zygomatic arch ; 

 it indicates the level of the end of the inferior horn. Two vertical lines, one through 

 the junction of the anterior and middle thirds of the zygomatic arch, and the other 



5 cm. behind the tip of the mastoid process, indicate the extent of the anterior 

 horn in front and the posterior horn behind. 



Vessels. The line of the anterior division of the middle meningeal artery is 

 equidistant from the frontozygomatic suture and the zygomatic arch; it is obtained 

 by joining up the following points: (1) 2.5 cm., (2) 4 cm., and (3) 5 cm. from 

 these two landmarks. The posterior division can be reached 2.5 cm. above the 

 auricular point. 



The position of the transverse sinus is obtained by taking two lines: the first 

 from the inion to a point 2.5 cm. behind the auricular point; the second from the 

 anterior end of the first to the tip of the mastoid process. The second line corre- 

 sponds roughly to the line of reflection of the skin of the auricula behind, and its 

 upper two-thirds represents the sigmoid part of the sinus. The first part of the 

 sinus has a slight upward convexity, and its highest point is about 4 cm. behind 

 and 1 cm. above the level of the auricular point. The width of the sinus is 

 about 1 cm. 



The Face. Air Sinuses (Fig. 1199). The frontal and maxillary sinuses vary 

 so greatly in form and size that their surface markings must be regarded as only 

 roughly approximate. To mark out the position of the frontal sinus three points 

 are taken: (1) the nasion, (2) a point in the middle line 3 cm. above the nasion, 

 (3) a point at the junction of the lateral and intermediate thirds of the supraorbital 

 margin. By joining these a triangular field is described which overlies the greater 

 part of the sinus. The outline of the maxillary sinus is irregularly quadrilateral 

 and is obtained by joining up the following points: (1) the lacrimal tubercle, (2) 

 a point on the zygomatic bone at the level of the inferior and lateral margins of the 

 orbit, (3) and (4) points on the alveolar process above the last molar and the second 

 premolar teeth respectively. 



External Maxillary Artery. The course of this artery on the face may be indicated 

 by a line starting from the lower border of the mandible at the anterior margin 

 of the Masseter, and running at first forward and upward to a point 1 cm. lateral 



