BRAIN AND NERVE 99 



of the two cerebral hemisplieres which have coalesced by their 

 internal faces. 



The reader must not omit to note the two little bodies that 

 are attached to the roof and floor of the third ventricle. That 

 one attached to the roof is called the pineal body, and it was here 

 that Descartes placed the seat of the soul. It is really a " ves- 

 tigial organ " which has acquired a new function. Early in the 

 history of the primitive vertebrates there were either one or 

 more " cyclopean " eyes, and even in some of the lizards such 

 eyes exist, though they are never functional organs of vision. 

 In modern mammals the pineal body has become a ductless 

 gland — that is, an organ that forms some substance which is 

 discharged into the blood-stream. What the substance is we 

 do not know, but it is said that it exercises an inhibitory in- 

 fluence, restraining precocity of growth and a too early maturity 

 of the reproductive organs. 



The organ on the floor of the third ventricle is called the 

 pituitary body, and it also is a vestigial structure consisting of 

 two parts, one of which appears to have formed the primitive 

 vertebrate mouth. The pituitary body is a ductless gland 

 secreting some substance into the blood which inhibits or controls 

 the growth of the skeleton, particularly the bones of the face. 

 Removal of the gland is always a fatal operation, and disease or 

 hypertrophy produce curious exaggerations of growth, some of 

 which, it has been noted, recall in bizarre fashion the charac- 

 teristics of the extinct Neanderthal human race. 



Connections within the Central Nervous System. 



The white tracts in the spinal cord, in the peduncles of the 

 cerebellum, and in the parts called the cerebral peduncles or 

 crura in Fig. 27 are the great paths along which nervous im- 

 pulses travel within the central nervous system. It is known 

 that there are, in the white matter of the cord, two main cate- 

 gories of nerve fibres: (1) such as convey impulses to the brain — 

 ascending tracts; and (2) those that transmit impulses from the 

 brain to the grey matter of the cord — these are the descending 

 tracts. Starting, then, with the white matter of the cord, we 

 may next consider the main paths along which impulses travel 

 in the central nervous system. 



The Sensory Tracts. — First we take the ascending paths, 

 those along which impulses originating as the results of stimula- 

 tion of the receptor organs (see p. 106) reach the lower brain. 



