MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 355 



If we imagine that a fourth spherical cap unites itself with the system of the 

 three preceding ones, we can conceive two different arrangements of the assemblage 

 besides that in which the fourth cap should so place itself as to be united with but 

 one of the others. One of these arrangements would contain four partitions unit- 

 ing by a single edge, and the other would contain five uniting by two edges. To 

 simplify the question and the graphic constructions, I will suppose the four caps to 

 be equal in diameter, in which case all the partitions will evidently be plane. 

 Then, it may be conceived, in the first place, that the four caps unite in such a 

 way that their centres shall be placed like the four middle points of the sides of a 

 square, which will give the system whose base is represented by Fig. 26, where there 

 are four partitions terminating at the same edge under right angles ; this system is 

 evidently one of equilibrium, since everything in it is symmetrical. It may be 

 conceived, in the second place, that three caps being first united the fourth unites 

 itself with two of them; in this arrangement, the four centres will be at the middle 

 points of the sides of a lozenge, and we shall have the system whose base is repre- 

 sented in Fig. 27, where there are five partitions. This system is also, by reason 

 of its symmetry, evidently a system of equilibrium ; but here not more than three 

 partitions terminate at the same edge, forming between them angles of 120°. Now 

 if we attempt to realize on the glass plate the first of these systems, Fig. 26, we 

 shall either not succeed, or, if produced at all, its duration will be inappreciable, 



Fig. 26. Fig. 27. 



and it passes rapidly into the second. The second system, Fig. 27, is obtained 

 directly without difficulty and persists. Hence we may conclude that in the former 

 system the equilibrium is unstable, and it thus becomes probable that four partitions 

 terminating at the same edge cannot coexist. 



Any change of volume or form produced by compression or distortion is called 

 a " strain," and their treatment is entirely a kinematical question, until we come to 

 regard them as produced in physical bodies and consider their cause. The system 

 of forces which is said to produce a strain is called a " stress." But just as we 

 study velocity as a preparation for the discussion of the effects of force in a free 

 body, so we study strains as a preparation for the discussion of the effects of stress. 

 Every action between two bodies is a stress. When we pull one end of a string the 

 other end of which is fixed, we produce a tension, when we push one end of a fixed 

 jod we produce pressure, and this merely amounts to saying that there is stress 



