!< THK NKRVnl s SYSTEM AM) ITS COXSKKYAT1OX 



these two in contact and conceal the inidbrain from view. 

 Incidentally it may be noted that in the rabbit the dorsal 

 aspect of the inidbrain is scored by a cross-fissure, so that 

 instead of the two lobes found in the frog there i> a four- 

 fold arrangement. On this account the region in question 

 is known as the c<iri>nrn t/u<i</ri<// mi/ni. 



A change in our point of view from dorsal to lateral 1 

 will make it easier to grasp the result of the progressive 

 development of the cerebrum and the cerebellum. The 

 figure will show how the inidbrain is covered in the process 

 and will suggest at the same time how the cerebrum rolls 

 forward over the olfactory nerves, and how the cerebellum. 

 extending backward, overhangs more and more the 

 cavity of the fourth ventricle. In cats and dogs a stage 

 is reached at which no glimpse of the inidbrain can be 

 had without forcibly separating the overlying parts. 



To understand the ordering of the various divisions in 

 the brain of man it is first of all necessary to appreciate 

 this preponderance of the cerebrum and the cerebellum 

 over the remaining portions. But there is another con- 

 dition which enters in to complicate the interpretation. 

 This is a bending of the axis upon which the brain may be 

 conceived to be built up. In the frog a line drawn from 

 the nose through the brain and back to the end of the 

 spinal cord will be nearly straight. In the rabbit it is still 

 true that the organization can be referred to an axis which 

 is substantially straight. It is very plain that the attempt 

 to project a corresponding line from the human nose 

 through the brain and down 1 he full length of the cord 

 will yield a curve suggestive of a fish-hook (Fig. 12). 



This bending of the axis is developed in connection 

 with the carriage of the head which is appropriate to the 

 erect position. The lower animals generally travel along 

 the line of their spinal columns. Man moves in a line 

 perpendicular to his spine; that is to say, he walks and 

 ga/es hori/ontally while his vertebral axis is kept vertical. 

 The nervou- -v-leni must be accommodated to this state 



IJ, upper drawing. 



