REMOVAL OF BRAIN. 25 



This external envelope of the brain consists of white fibrous and elastic 

 tissues so disposed as to give rise to two strata, viz., an external or en- 

 dosteal, and an internal or supporting. At certain spots those layers are 

 slightly separated, and form thereby the spaces or sinuses for the passage 

 of the venous blood. Moreover, the innermost layer sends processes be- 

 tween different parts of the brain, forming the falx, tentorium, etc. 



The /a/a: cerebri is the process of the dura mater, in shape like a sickle, 

 which dips in the middle line between the hemispheres of the large brain. 

 Its form and extent will be evident if the right half of the brain is gently 

 separated from it. Narrow and pointed in front, where it is attached to 

 the crista galli of the ethmoid bone, it widens posteriorly, and joins a 

 horizontal piece of the dura mater named the tentorium cerebelli. The 

 upper border is convex, and is fixed to the middle line of the skull as far 

 backwards as the occipital protuberance ; and the lower or free border, 

 concave, is turned towards the central part of the brain (corpus callosum), 

 with which it is in contact posteriorly. 



In this fold of the dura mater are contained the following sinuses : the 

 superior longitudinal along the convex border, the inferior longitudinal in 

 the hinder part of the lower edge, and the straight sinus at the line of 

 junction between it and the tentorium. 



The superior longitudinal sinus (fig. 3, b) extends from the ethmoid 

 bone to the occipital protuberance. Its position in the convex border of 

 the falx will be made manifest by the escape of blood through numerous 

 small veins, when pressure is made from before back with the finger along 

 the middle line of the brain. 



When the sinus is opened it is seen to be narrow in front, and to widen 

 behind, where it ends in a common point of union of certain sinuses (tor- 

 cular Herophili) at the centre of the occipital bone. Its cavity is trian- 

 gular in form, with the apex of the space turned to the falx ; and across it 

 are stretched small tendinous cords chordae Willisii near the openings 

 of some of the cerebral veins. Occasionally small glandulae Pacchioni are 

 present in the sinus. 



The sinus receives small veins from the substance and exterior of the 

 skull, and larger ones from the brain ; and the blood flows backwards in 

 it. The cerebral veins open chiefly at the posterior part of the brain, and 

 lie for some distance against the wall of the sinus before they perforate the 

 dura mater ; their course is directed from behind forwards, so that the. 

 current of the blood in them is evidently opposed to that in the sinus : this 

 disposition of the veins may be seen on the left side of the brain, where 

 the parts are undisturbed. 



Directions. Before the rest of the dura mater can be examined, the 

 brain must be taken from the head. To facilitate its removal, let the 

 head incline backwards, whilst the shoulders are raised on a block, so that 

 the brain may be separated somewhat from the base of the skull. For the 

 division of the cranial nerves a sharp scalpel will be necessary ; and the 

 nerves are to be cut longer on the one side than on the other. 



Removal of the brain. As a first step cut across the anterior part of 

 the falx cerebri, and the different cerebral veins entering the longitudinal 

 sinus ; raise and throw backwards the falx, but leave it uncut in the 

 middle line behind. Gently raise with the fingers the frontal lobes and 

 the olfactory bulbs of the large brain. Next cut through the internal car- 

 otid artery and the second and third nerves, which then appear; the large 

 second nerve is placed on the inner, and the round third nerve on the 



