THE NERVOUS SYSTEM 425 



spinal and sympathetic ganglia. Others lurin, according to some 

 authorities, enveloping cells, such as the capsule cells around the 

 ganglion cell bodies and the neurilemma cells around the peripheral 

 nerve fibres. These latter would thus correspond to the neuroglia 

 cells of the central nervous system. The majority of the 

 neuroblasts of the cerebro-spinal ganglia develop two processes: 

 peripheral processes {aferent nerve fibres) to various structures {re- 

 ceptors) which receive stimuli; and central processes, forming the 

 afferent roots and passing into the central nervous system where they 

 usually bifurcate and form longitudinal ascending and descending 

 arms. These cells are at first bipolar, later the cell body withdraws 

 from the two processes and thus assumes the adult unipolar con- 

 dition (see page 433). Other processes may also appear. Many 

 of the neuroblasts of the sympathetic ganglia develop dendrites and 

 axones while others form branching cells in which the two kinds of 

 processes cannot be easily distinguished. Sympathetic cells are 

 also derived from cells which migrate from the neural tube along the 

 efferent root (p. 424). 



The cerebro-spinal ganglionic neurones, together with some sym- 

 pathetic neurones, certain neurones in the olfactory mucous mem- 

 brane, retina, and possibly midbrain roof, constitute the aferent per- 

 ipheral neurones of the entire nervous system. Most of the sympa- 

 thetic neurones are efferent. The peripheral processes of the afferent 

 peripheral neurones, the axones of the efferent peripheral neurones 

 after they have emerged from the central nervous system, and the 

 axones of the sympathetic ganglion cells, together with all their 

 sheaths and connective-tissue investments, form the peripheral 

 nerves. The bodies of the afferent peripheral neurones and sym- 

 pathetic neurones form groups known as ganglia. The peripheral 

 neurones are arranged segmentally, as shown by the series of ganglia 

 and nerves. 



The sympathetic neurones and those cerebro-spinal neurones 

 which innervate sympathetic ganglia, viscera, glands, blood-vessels, 

 and smooth and heart muscle are peripheral visceral or splanchnic 

 neurones. Those cranial nerve neurones which innervate the striated 

 voluntary muscles of jaw, ear, face, pharynx, and larynx (branchio- 

 motor) are usually also classed as splanchnic. The remainder of 

 the cerebro-spinal peripheral neurones are peripheral 5c wa/zc neurones. 



From the foregoing it will be seen that all the neurones of the 

 nervous system fall into two categories: I. Peripheral neurones, a 



