196 



ANIMAL PHYSIOLOGY. 



details in different individuals, and even on the two sides of 

 the brain. They are absent in the lower orders of mammals, 

 become more abundant the higher 

 we ascend in the series, and reach 

 their greatest complexity in man. 

 The only plausible explanation of 

 the use of these convolutions is this, 

 that grey matter requiring a rich 

 vascular supply, and only minute 

 vessels being admissible within it, 

 increase of surface is a necessary 

 condition of increase of bulk, so as 

 to bring its parts sufficiently near 

 the pia mater, in which the arteries 

 for its nourishment divide. Nor is 

 this supposition contradicted by the 

 presence of convolutions in the brains 

 of small animals; for in them the 

 length of capillary travelled by the 

 blood in each circulation is likewise 

 small, and, therefore, the distance 

 to which the blood can penetrate 

 from the pia mater may be supposed 

 to be in proportion to the size of tho 

 animal. 



The hemispheres are in contact 

 with very nearly the whole extent of 

 the cranial wall above the ten- 

 torium. They are separated one 

 from the other above, behind, and in 

 front by a deep longitudinal fissure, 

 into which dips a process of dura 

 mater, called the falx cerebri, at- 

 tached behind along the middle line 

 of the tentorium, and in front to 

 the mesial part of the ethmoid bone. 

 At the bottom of the longitudinal 



TV TAT n TIT fissure they are united by a thick 



Fig. 101. GREY MATTER , J 1,11? 



OF THE CONVOLUTIONS: transverse commissure or bond of 



ideal vertical section. junction, the corpus callosum, which 



