30 TOPOGRAPHIC AND APPLIED ANATOMY. 



the size of the head, behind the external angular process. The posterior branch is exposed by 

 making an opening on the same line where it is bisected by a vertical line from the posterior 

 border of the mastoid process. It must not be forgotten that a trephine opening which has 

 failed to exactly locate a given structure may be easily enlarged and thereby rendered effective 

 by the use of the rongeur. ED.] 



2. The dura mater serves the brain as a supporting and protective envelope. From its 

 inner layer it sends partitions or processes between certain of the brain divisions. These are: 

 (i) The falx cerebri (Figs. 8, 9, 12, and 16), which separates the cerebral hemispheres and extends 

 from the crista galli to the internal occipital protuberance and the upper surface of the tento- 

 rium cerebelli. (2) The tentorium cerebelli (see Fig. 17), which separates the occipital lobes of the 

 cerebrum from the upper surface of the cerebellum and is adherent to the superior margin of the 

 petrous portion of the temporal bone, continuing as far forward as the anterior and posterior 

 clinoid processes of the sphenoid bone. Its posterior attachment corresponds to the course of the 

 lateral sinus. If it is desired to remove the cerebellum together with the cerebrum, the insertion 

 of the tentorium at the petrous portion of the temporal bone must be previously divided, since the 

 cerebellum cannot pass through the opening which transmits the cerebral peduncles (see Fig. 17). 

 (3) The jalx cerebelli, a process of dura mater which, varying somewhat in its formation, runs along 

 the internal occipital crest corresponding to the fissure between the cerebellar hemispheres (see 

 Fig. 12). The sella turcica and the contained hypophysis cerebri are covered by a portion of 

 dura mater known as the diaphragma sella, which is perforated by the infundibulum. 

 This infundibulum is easily torn in removing the brain. 



The dura mater is of additional importance to the brain since it receives the return blood, 

 which is collected in the venous sinuses and conducted to the jugular foramen. The sinuses are 

 more or less rigid canals situated in the dura between its outer and inner layers. They are lined 

 by a prolongation of the lining membrane of the veins, are destitute of valves, and do not collapse 

 when injured. Such injuries easily occur with injuries to the cranium; but they are relatively 

 free from danger, since the hemorrhage is easily checked and cases are exceptional in which air 

 gains access to the vascular system from an opened sinus. A more dangerous injury is the com- 

 plete internal perforation of a sinus with effusion of blood into the subdural space and the symp- 

 toms of cerebral compression. In trephining, care is of course taken to avoid the sinuses. From 

 a practical standpoint the following three sinuses are the most important : 



1 . The superior longitudinal sinus, holding the most exposed position along the insertion of 

 the falx cerebri (see Figs. 8 and 12). At the internal occipital protuberance, the location of the 

 torcular Herophili, it unites with the lateral, the occipital, and the straight sinuses. [This sinus 

 increases rapidly in size from before backward. It receives veins from the scalp through the 

 parietal foramina, from the diploe, the dura mater, and the hemispheres of the cerebrum. Those 

 from the cerebrum run into the sinus from behind forward opposite to the direction of the blood- 

 current, piercing the sinus wall very obliquely (Deaver). Its position is indicated by a line 

 dra\vn over the median line of the vertex from the root of the nose to the external occipital 

 protuberance. ED.] 



2. The lateral sinus, the median portion of which is called the sigmoid sinus, is situated in 

 a furrow in the bone (see Figs. 9 and 22). This furrow is very often particularly deep upon the 



