44 PSYCHOBIOLOGY 



body. After complete development these are axons of efferent neurons, 

 and dendrites of afferent neurons. 



Fig. 29. Cerebro-spinal axis, reduced to % diameter. (After Bougery.) A longi- 

 tudinal section through the median plane of the spinal column, and in part of the- 

 skull ; leaving in relief the brain, cord and spinal nerve-roots. Lobes of the cerebrum: 

 Pd, parietal ; F, frontal ; 0, occipital ; T, temporal [compare Fig. 65] ; C, cerebellum ;. 

 mo, medulla oblongata; Ci-Cviii, cervical nerves; Dl-Dxu, thoracic nerves (formerly- 

 known as dorsal nerves) ; Li-Lv, lumbar nerves; Sl-Sv, sacral nerves; Co, coccygeal 

 nerve ; ms, ms, upper and lower extremities of the spinal cord. 



THE NEURONS. 



The essential elements of the nervous system are the nerve cells with- 

 their prolongations. The nerve cell, exclusive of the prolongations, is 

 called the cell-body. The prolongations are called nerve=fibers. The 

 cell-body and its fibers together are known as the neuron. The nucleus. 



