44 The EncepJialon 



cerebellar of the basilar. The/^/j is supplied by small transverse 

 branches of the basilar, and the medulla by the anterior and posterior 

 spinal of the vertebrals, the anterior spinals becoming fused in their 

 descent into a slender median artery. 



THE ENCEPHALON 



The brain, which weighs 49^ oz. in the male and 44 oz. in the female, 

 is surrounded by a thin film of subarachnoid fluid, so that, floating on 

 a water-bed as it were, it may not be seriously shaken when one is 

 running or jumping. The violence may be so great as to fracture the 

 base of the skull, yet the layer of water saves the brain from con- 

 cussion, as is evinced by the man retaining perfect consciousness, 

 though he may die shortly after from other effects of the fracture. 



There are many other natural provisions against rough inter- 

 ference with the delicate structure of the brain, such as the ' give ' in 

 the joints of the foot, knee, hip, and pelvis ; the curves of the tibia, 

 femur, and spinal column ; the fibro-cartilages of knee and spine ; the 

 arrangement of cancellated tissue of the bones, and so on. But some- 

 times, and especially when the subject is taken unawares, and so is 

 unable to arrange his muscles and joints to break the shock, the brain 

 is violently shaken in its bone-case, and, for a time, thrown out of work- 

 ing order. In common parlance, the man is ' stunned' ; the surgical 

 equivalent for the condition being * concussion? 



The shock which causes concussion is apt to lacerate a meningeal 

 or cerebral vessel, the haemorrhage occurring either outside or inside 

 of the dura mater, the effect being compression of the brain, a much 

 more serious condition than simple concussion. The larger the vessel 

 torn, the more rapidly would the symptoms come on. 



A deep longitudinal fissure separates the two cerebral hemi- 

 spheres and lodges the falx cerebri ; in its depths may be seen the 

 fibres of the corpus callosum crossing the middle line. 



The lower limit of the cerebrum is approximately shown by drawing 

 a line from just above the supra-orbital ridge to the external auditory 

 meatus, and thence up to the external occipital protuberance. 



As the protuberance marks the level of the lateral sinus (p. 39), 

 and, with it, the attachment of the tentorium, the cerebellum is neces- 

 sarily below the hinder part of this line. 



The outer surface of the hemisphere is deeply cut by the fis- 

 sure of Sylvius, the beginning of which is at the front of the base of 

 the brain ; it lodges the lesser wing of the sphenoid, and the middle 

 cerebral artery, * the artery of cerebral haemorrhage ' (p. 42), winds 

 into it. The fissure quickly divides, one part running upwards for an 

 inch into the frontal lobe, the other extending backwards : these parts 

 are respectively the vertical and horizontal limbs of the fissure. The 

 horizontal limb cuts off the temporo-sphenoidal lobe below from the 





