502 ELEMENTARY PHYSIOLOGY less. 



the bulb is exposed. Unlike the anterior surface, which 

 is conspicuously convex (see Fig. 160, M) the posterior 

 surface is marked by a shallow elongated diamond-shaped 

 depression, forming the cavity of the fourth ventricle. 

 This cavity arises from the gradual divergence of the 

 posterior white columns of the spinal cord, while the 

 depth of the posterior fissure is at the same time dimin- 

 ished, so that the central canal of the spinal cord 

 approaches the floor of the fourth ventricle, and actually 

 opens into the lower end of the cavity (Fig. 161) ; this 

 lower end of the ventricle is known as the calamus 

 scriptorius, from its fancied resemblance in shape to 

 the nib of a pen. The narrowed upper end of the fourth 

 ventricle is continued forwards under the cerebellum. 



Having thus made out so much of the arrangement of the 

 brain as may be seen by mere external inspection, we may 

 now proceed to examine its internal structure. For this 

 purpose the most instructive method is to cut a vertical, 

 longitudinal section through the brain from front to back, 

 passing through the middle line, and thus dividing it into 

 two similar and symmetrical halves. When the cut 

 surface of the right half of the brain, as exposed by this 

 section, is examined, the following further structural 

 details may be made out, and are shown in Fig. 161. 



The corpus callosum is seen cut across at cc. cc. cc. Above 

 this, and extending forwards and backwards, is the flattened 

 exposed surface of the right cerebral hemisphere, which 

 forms one side of the median fissure between the hemis- 

 pheres. The upper end of the spinal cord, Sp.c, passes 

 into the bulb B, in front of which the transverse fibres 



in front of that line, between the /f and /// nerves on either side, are 

 seen the crura cerebri. The two round bodies in the angle between tlie 

 diverging crura are the so-called corpora albicanlia, and in front of them 

 is P, the pituitary body. This rests on the chiasnia, or junction, of the 

 optic nerves ; the continuation of each nerve is seen swee])ing round the 

 crura cerebri on either side. Immediately in front, between the separ- 

 ated frontal lobes of the cerebral hemispheres, is seen the coipvs 

 callomm, CC. The fisstire of Sylvius, about on a level with 7 on the loft 

 and // on the right side, marks the division between frontal and 

 temporal lobes. 



