THE BRAIN. 



39 



The motor fibres of the internal capsule pass downward through the anterior 

 portion of the crus cerebri and pons into the medulla, at the lower part of which 

 the majority decussate and pass into the anterior columns of the cord as the pyram- 

 idal tracts. Thus, it is seen that destruction of any portion of the motor tract, 

 from the point of pyramidal decussation below, through the internal capsule to the 

 cortex above, will cause a paralysis on the opposite side of the body. 



CRANIOCEREBRAL TOPOGRAPHY. 



For the purpose of operating on the brain it is essential to know the bony land- 

 marks of the skull, the lower level of the brain, and the relation which the various 

 fissures and convolutions bear to the surface. 



The most important fissures are the longitudinal, Sylvian, Rolandic, zs\Aparieto- 

 occipital. If these can be properly located, the convolutions and subsidiary fissures 

 can be readily filled in. 



BONY LANDMARKS. 



Nasion. The nasofrontal suture in the median line. 



Glabella. The smooth spot in the median line on the frontal bone between 

 the superciliary ridges. It is about on a level with the upper edge of the orbit. 



Bregma 



Stephanion 



Sylvian point 

 Pterion 



Lambda 



Temporal ridge 



Glabella 



__ External angular process 

 Nasion 

 .Malar tubercle 



In 



Asterion 



FIG. 49. Landmarks of the skull. 



Bregma. The point in the midline where the sagittal and coronal sutures 

 cross. It corresponds with the anterior fontanelle in the infant. 



Lambda. The point of meeting of the sagittal and lambdoid sutures. It is 

 about 6 cm. (2^ in.) above the occipital protuberance. 



Inion. The external occipital protuberance. 



Pterion. This name was given by P. Broca to the point where the frontal, 

 parietal, and sphenoid bones meet in the region of the temple. It is about 2.5 cm. 

 ( i in. ) behind the angular process and should not be confounded with the Sylvian 

 point, which is 1.5 cm. (/6 in.) farther posterior, where the temporal, parietal, and 

 sphenoid bones meet. Horsley called this latter point the pterion. The region of 

 the pterion is the seat of the anterolateral fontanelle in the foetus. 



Asterion. This lies 2 cm. (-| in.) behind the base of the mastoid process, 

 where the parietal, occipital, and temporal bones meet. It is on the superior curved 

 line and in fetal life forms the posterolateral fontanelle. 



Temporal Ridge. This marks the upper attachment of the temporal fascia 

 and muscle. It begins at the external angular process of the frontal bone and ends 



