ORGANIC FORM AND ARCHITECTURE 695 



sarcolemma sheath of the muscle fibre. When a motor fibre is fol- 

 lowed inwards towards the cell from which it grows out. there is a 

 disappearance of neurilemma, nodes, and medulla. 



In the core of a nerve fibre there are indications of fibrils. These 

 are regarded by some as the essential elements in the transmission 

 of impulses, while others maintain that the essential part is the less 

 compact, sometimes wellnigh fluid stuff between the fibrils, or that 

 the fibrils are but the walls of tubes withm which the essentially 

 nervous substance lies. These are details awaiting still more pene- 

 trating analysis. But the important point in an outline survey like 

 ours is to understand that the principle of the nervous system is 

 simple — consisting of interhnked sensory, associative, and motor 

 neurons, which integrate the whole body. 



In outlining the evolution of nervous tissue, we may begin with 

 undifferentiated external ectoderm, whose cells are in part contractile 

 and in part sensitive. In sponges there are no distinguishable nerve 

 cells, but in some types, as we have mentioned, there are slowly con- 

 tractile cells which respond to external stimulus, and may therefore 

 be called neuromuscular. 



In Hydra a specialised superficial sensory cell may send a fibre 

 to the adjacent contractile root of a muscle cell; or it may send a 

 fibre to one of the insunk ganglion-cells, whence the stimulus passes 

 to a muscle root. 



In sea-anemones and medusae the stimulation of a tentacle may 

 be followed by movement of part of the body at some distance. This 

 implies superficial sensory cells with fibres passing to a deeper net- 

 work of ganglion cells ; and these may pass on the impulse to muscle 

 cells directly, or to other ganglion cells which affect more distant 

 muscle cells. 



Earthworms may be taken to illustrate a further step; for they 

 show a centralised nervous system in the form of a ventral nerve- 

 cord. A sensory neuron in the skin sends a fibre into the nerve- 

 cord, where it divides into an anterior and a posterior branch. One 

 of these branches communicates with those of an associative neuron, 

 whence the impulse is transmitted to a motor neuron, and so by 

 an efferent fibre to the muscles of the body-wall. But in some cases 

 in the earthworm the second link in the chain is omitted, and the 

 impulse passes directly from sensory fibre to motor neuron, as is 

 occasionally true even in Vertebrates, For an indication of further 

 steps we must refer back to the physiological section, but attention 

 must be called to the anatomical fact that in ordinary Vertebrates 

 the cell-bodies of the sensory neurons, except in the case of those 

 concerned with smell, have sunk into the spinal ganglia beside the 

 spinal cord. From these deeply insunk sensory neurons fibres pass 

 into the cord while others extend to nerve-endings on the surface, 

 which are often surrounded by a special group of epidermic cells. 



