SOS THE NER VE SYSTEM 



the developmental, structural, and functional unit of the nerve system. It is in 

 reality a single cell presenting unusual structural modifications. It comprises not 

 only the nerve-cell body with its numerous protoplasmic processes or dendrites, but 

 also the axone, which may vary in length from a fraction of a millimetre to fully 

 half a man's stature; so that, despite the delicacy of the axone, its bulk may be 

 almost two hundred times greater than that of the cell body from which it pro- 

 ceeds. The long axones serve to make a connection with a peripheral or distant 

 nerve cell, muscle cell, or gland cell, while the shorter axones of certain neurones 

 divide into terminal branches in the immediate vicinity of its cell body, presumably 

 to come into relation with other nerve cells in the same or adjacent groups. 



Neurones, being devoted to the maintenance of functions manifested by various 

 phenomena of nerve force, are differentiated in their polarity, both structurally 

 and dynamically. Receptive neurones are so arranged as to receive afferent nerve 

 impulses from other tissues; emissive or excitor neurones give out efferent nerve 

 impulses. The former are generally termed sensor neurones, the latter motor 

 (excitomotor) neurones if connected with muscle, excito glandular if connected 

 with gland cells. Were the nerve system made up solely of such initial and ter- 

 minal neurones, the apparatus would be merely a system of reflex arcs. Such it 

 is in low forms of animal life which, by their very organization, and because of 

 the close juxtaposition of their sensor and motor elements, are compelled to 

 react to stimuli from without. In higher forms, with more profoundly differ- 

 entiated nerve systems, the sensor impression must pass through an interposed 

 medium which is capable of either transmitting the molecular change in the form 

 of an excitomotor impulse or, on the other hand, is capable of reducing or check- 

 ing the impulse. In other w r ords, reaction is not imperative; there is a freedom 

 of choice exercised by intermediate neurones endowed with inhibitory function. 

 The simple arc, composed of an afferent sensor neurone and an efferent motor 

 neurone, would act independently of all other arcs were it not for the interposition 

 of this intermediate neurone and of other association neurones which, by their 

 relations tow r ard similar arc elements, produce harmony of action. The basis, 

 then, of the nerve system is a series of neurones, with projecting and association 

 processes, coordinated for the purpose of performing specific actions manifested 

 either by motion, by trophic changes, or by the apperception of stimuli of a 

 chemical, mechanical (tactile and auditory), thermal, or photic nature. When 

 we consider the profoundly complex manifestations of nerve phenomena in the 

 mental and physical life of man it is not surprising to learn that his nerve system 

 is made up of an immense multitude of aggregations of neurones. 



Fundamental Facts Regarding the Development of the Nerve System. 

 The nerve system is formed by a remarkable metamorphosis of the ectodermic 

 layer of the developing ovum. Along the mid-dorsal line of the embryonic mass 

 ,a thickening of the ectoderm forms a well-defined layer of cells, the neural plate. 

 The proliferative process passes rapidly from the cephalic toward the caudal 

 end, and as development advances it is seen that the most intense growth energy 

 takes place at the cephalic end, indicative of the higher functional potentiality 

 of what is to become the brain. The neural plate undergoes a trough-like for- 

 mation as its edges become elevated cephalad and laterally to form the neural 

 groove (Fig. 577). The edges become more and more elevated and bend toward 

 the median line until the margins of the groove coalesce to form a tube, the neural 

 tube, which sinks into the subjacent mesodermal tissues. The fusion of the mar- 

 gins of the neural plate occurs first in the cervical region and rapidly continues 

 both cephalad and caudad. The cephalic portion, destined to become the brain, 

 expands and gro\vs considerably, while the caudal portion elongates to form the 

 spinal cord. 



Eventually the neural tube, as it sinks into -the subjacent mesodermal tissue, 



