MAMMALIA— PHYSIOLOGY 469 



loosely connected with the arachnoid by strands of connective tissue 

 forming a spongy network but there is considerable space between 

 the two called the sub-arachnoid space. This space contains a fluid 

 called the cerebro-spinal fluid which differs from ordinary lymph (i) 

 in its small per cent of proteins, (2) in the absence of cells and (3) 

 in the absence of a fibrin ferment. 



Cerebro-spinal fluid is colorless and alkaline, having a specific 

 gravity of between 1,006 and 1,008. It consists mostly of water. 

 The solids include a trace of protein, chiefly globulin; if the mem- 

 branes of the brain or cord are inflamed, there is much more protein 

 present. A small amount of carbohydrate as glucose and the salts 

 found in blood plasma occurs, with occasionally a few leucocytes and 

 a large quantity of CO2. It contains no antitoxins, opsonins, nor 

 alexins which are present in blood plasma and in tissue fluid. 



Cerebrwn. — Much of the cerebral cortex is occupied by large tri- 

 angular cells called pyramidal cells, the largest of which is one-six 

 hundredth of an inch. These cells are agents in the psychic activity 

 of the cortex.^ Cajal called them psychic cells. The fibers of white 

 matter of the cerebral hemisphere consist of projection fibers and 

 association fibers. The projection fibers are those which project the 

 impulses of the outside world upon the sensorium, or the reverse. 

 These are cortico-afferent^ leading from lower levels to the cortex, or 

 cortico-efferent, leading from the cortex to lower levels. The associa- 

 tion fibers which correlate cortical areas are of two types, the arcuate 

 which correlate areas of the same hemisphere, and the commissural 

 which correlate areas of the opposite hemisphere. The most impor- 

 tant commissures in the mammalian brain are the corpus callosiim 

 and the anterior^ middle and posterior commissures. The fibers of 

 the cortex myelinate late, none receiving the medullary sheath in the 

 human brain until the ninth fetal month. (See page 479.) 



Cerebellum. — The cerebellum, important in coordination, con- 

 sists of an outer clear gray molecularlayer with an inner reddish gray 

 granular layer. At the junction of these two layers are the large 

 flask-shaped cells called the Purkinje cells. It is by contact and not 

 by continuity of structure that nervous impulses are transmitted. 

 The system of the cerebellum shows not only this but shows that 

 different parts of the same cell may be supplied with terminal fibers 

 from quite different sources. 



8 "Learning without thought is labor lost; thought without learning is perilous." 

 Confucius. 



