466 



PRINCIPLES OF GENERAL PHYSIOLOGY 



anterior end, and a segmented nerve cord passing along the ventral middle line 

 and extending to the posterior end. The integument contains many sense cells, 

 each giving rise to a single nerve fibre, which enters a ganglion of the ventral 

 nerve chain and divides therein into numerous branches, forming the so-called 

 neuropile. Whether this neuropile is actually a network, that is, whether there 

 is actual physical continuity between the branches of different cells, is difficult to 

 make out. But it seems that we have here a definite system of " neurones," 

 which play so large a part in the higher nervous systems. 



The name "neurone" was given by Waldeyer (1891, p. 1352) to the 

 elementary unit of which the higher nervous systems are built. A motor neurone 

 consists of a cell body, with a nucleus, a nerve fibre conveying excitation from 

 the cell body outwards, and which may be long or short, together with a number 

 of branched processes or " dendrites," receiving impulses from outside the cell. 

 The nerve fibre process usually ends in a branched form either on the cell of an 

 effector organ of some kind, muscle, gland cell, etc., or on another neurone. In 

 the sensory neurone, the long fibre usually receives the impression from the 

 exterior and conveys it to other neurones by the dendrites. The peculiarity of 

 a neurone, as compared with other cells, is the possession, in the larger animals, of 



great length ; it 

 may have its cell 

 body with nucleus 

 in the brain at the 

 anterior end of an 

 animal of several 

 feet in length, 

 while the termina- 

 tion of its out- 

 going process, the 

 axis cylinder pro- 

 cess or axone, may 

 be on a nerve cell 

 at the distant end 

 of the spinal cord. 

 There is no actual 

 continuity of pro- 

 toplasm where 

 neurone joins 

 neurone, merely 

 contact, thus 



differing from a nerve network, and giving rise to very important properties of the 

 higher nervous systems. We shall return to the elementary properties of the 

 neurone presently. 



In the earthworm, then, we have a primary sensory neurone, with its cell body 

 in the skin, and its nerve process ending in ramifications in the neuropile of the 

 segmental ganglia. In these ganglia, we find also large nerve cells, whose thick 

 axones pass out as motor fibres to the muscles of the body wall. These motor 

 neurones also are possessed of dendrites, which contribute to the formation of the 

 neuropile and form connections, either of direct continuity or merely of contact, 

 with the endings of the sensory neurones. This is the simplest possible reflex arc, 

 sensory impressions giving rise to a motor response. 



In the central nervous system of the earthworm, we find also association 

 neurones ; these have processes serving to connect neurones within one ganglion, or 

 from one ganglion to another, but rarely extend over more than two segments. 

 These neurones do not pass out of the central nervous system, and it may be noted 

 here that the complexity of this system, with rise in the scale of evolution, depends 

 on the number and length of these association neurones ; so that we reach finally 

 the cerebral cortex of the higher apes and man, which consists entirely of this 

 kind of neurones, having no direct connection with the exterior. It is not 

 difficult to understand why specially sensitive and elaborate receptor organs should 



FIG. 141. DIAGRAM TO ILLUSTRATE THE EARLY STAGES IN THE 

 DIFFERENTIATION OF THE NEURO-MUSCULAR MECHANISM. 



A, Epithelial stage. 



B, Differentiated muscle cell at stage of sponge. 



C, Partially differentiated nerve cell in proximity to the fully differentiated muscle cell. 



D, Nerve and muscle cell of Coelenterate stage. 



(See text, page 465.) 



(Parker, 1911, p. 222.) 



