■50 PHYSIOLOGY AXD MORPHOLOGY OF ANIMALS. 



cerebrospinal system may be treated under three heads 

 of (i) the brain, (2) the spinal cord, and (3) the nerves. 

 The brain and spinal cord are centers, the nerves are 

 conductors. The first two contain gray matter as well 

 as white, the last white matter alone. The first two are 

 generators of nerve energy, the fibers of the third are 

 transmitters only. The former may be likened to bat- 

 tery cells, the latter to conducting wires. 



SECTION I. 

 Brain of Man. 



We can give only such general description as is neces- 

 sary to explain physiology. 



Skull. — The brain is inclosed in a bony box consist- 

 ing of many pieces fitted together by sutures with inter- 

 locking teeth. The growth of the skull to accommo- 

 date the growing brain takes place along these sutures. 

 The sutures finally consolidate and the brain can grow 

 no more. The age of consolidation is later in the 

 higher races, and is probably also later in educated 

 men.* 



Membranes. — Take off the skull, and beneath we 

 see the brain still enveloped by its membranes. These 

 are (1) the dura mater, a thick, strong, fibrous membrane. 

 This invests the brain and dips in and separates all the 

 great divisions of the brain, but not the convolutions. 

 It carries also the large blood vessels of the brain. 

 Beneath and more closely investing the brain, passing 

 between not only the larger but also the smaller di- 

 visions, and even dipping down between the convolu- 

 tions of the surface and carrying the smaller blood ves- 

 sels which penetrate the substance of the brain itself, there 



* Galton, Nature, vol. xxxviii, p. 14, 188S, 



