THE BRAIN 



723 



a definite order. Administration of tetanus toxin or strychnine 

 abolishes the co-ordinated nature of the response. The injection of 

 absinthe into the vein of an animal causes epileptic convulsions of 

 a universal nature, which stop when the carotid arteries are clamped, 

 to begin again when the blood carrying the poison flows through the 

 cortex. 



The Evidence from Structure. The structure of the cortex is 

 complicated, and varies in details in different parts. There are five 

 layers, or laminae : (1) The outer fibre 

 lamina. (2) The outer cell lamina. 

 (3) The middle cell lamina. (4) The 

 inner fibre lamina. (5) The inner cell 

 lamina. 



The outer fibre lamina consists 

 principally of medullated fibres from 

 the underlying layers. It contains also 

 cells which give rise to processes run- 

 ning parallel to the brain surface. 



The outer cell lamina consists of 

 pyramidal cells, the outermost being 

 small, the middle ones medium-sized, 

 and the most internal large in size. 

 Running among the outer cells is a 

 strand of fibres known as the outer 

 line of Baillarger. 



The middle cell lamina consists of a 

 layer of small stellar cells. 



The inner fibre lamina consists 

 chiefly of an inner strand of fibres 

 known as the inner line of Baillarger. 

 In this lamina are also large pyramidal 

 cells. These occur particularly in the 

 motor area of the brain, and are there 

 known as the cells of Betz. 



The inner cell lamina consists of 

 various types of cells Golgi, stellate, 

 pyramidal, spindle-shaped, ovoid, etc. 

 Characteristic are pyramidal cells, 

 with the axon passing towards the 

 surface of the brain the cells of 

 Martinotti. 



These layers vary in structure and 



in thickness in the different parts of the cortex. They also develop 

 in the embryo at different times. The inner cell lamina is the first 

 layer to be developed, and is well marked in the lower mammals. 

 This layer and the inner fibre layer is held to be concerned in the 

 lower instinctive and voluntary activities of the animal, such as 

 feeding, excretion. In man, these layers are developed during the 

 fourth month of foetal life. 



FIG. 440. SECTION OF CRUCIAL 

 SULCUS OF THE BRAIN OF A Dou 



STAINED BY CoLGl'S METHOD, 



SHOWING A LARGE PYRAMIDAL 

 CELL GIVING OFF A LARGE 

 BRANCHING APICAL DENDRON. 

 GEMMULES CAN BE SEEN ON THE 

 PROCESSES, x 80. (Mott.) 



