358 



MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



Fig. 31. 



to each other. Prom this we readily perceive that if three surfaces come in con- 

 tact the surfaces of separation meet in a line straight or curved. Let 0, Fig. 31, 

 be a point in this line, and let the plane of the paper be supposed to be normal to 



the line at the point 0. The three angles between the 

 tangent planes to the three surfaces of separation at the 

 point are completely determined by the tensions or pres- 

 sures of the three surfaces. For if in the triangle a b c the 

 side a b is taken so as to represent on a given scale the 

 tension of the surface of contact of a and b, and if the other 

 sides, b c and c a, are taken so as to represent on the same 

 scale the tensions of the surfaces between b and c and be- 

 tween c and a respectively, then the condition of equilibrium 

 at for the corresponding tensions R, P and Q is that the 

 angle ROP shall be the supplement of a b c, P Q of 

 b c a, and, therefore, Q R of c a b. Thus the angles at 

 which the surfaces of separation meet are the same at all parts of the line of con- 

 course. When three films of the same liquid meet, their tensions are equal, and, 

 therefore, they make angles of 120° with each other. 



Now whilst the conditions of growth of the cerebral surface are variously inter- 

 fered with by secondary or tertiary displacements, producing undulations and 

 tortuosities, the general action of the fundamental forces at work can be seen 

 displayed in the various types of fissuration found. For the surface tension of films 

 we must substitute the pressure forces produced by cerebral swellings aggregating 

 around certain centres, the peripheries of which meeting each other within a con- 

 fined space produce the various lines of fissuration. In brains where the complexity 

 of fissuration is not excessive we find marked regularity and constancy as regards 

 the length, depth and inclination of the different fissures, as the following measure- 

 ments will indicate. They are taken from the brains of four monkeys, two of the 

 same species, Macacus nemestrinus, compared together ; and two of different genera, 

 Macacus cynomolgus and Cercopithecus callitrichus. The measurements were 

 made on both hemispheres of the brain and it was found that they were alike. 

 These have been placed in columns for convenience of comparison. The brains 

 were hardened in chloride of zinc and then transferred to alcohol, the conditions 

 of preservation being as nearly similar as possible. 



