36 THE ELEMENTS OF STRUCTURE. 



The minute structure of muscle has been studied with extraordinary- 

 enthusiasm for many years, but the result is not quite certain. Accord- 

 ing to many, there is within the muscle-fibre, as within many another 

 cell, an intricate network with a more fluid stuff in its meshes. If this be 

 true, it is likely enough that the strands of the network are the most 

 directly important elements in contraction. 



But most attention has been bestowed on the transverse markings, in 

 regard to which there are many theories. I believe in the simplest, 

 that of Dr. Berry Haycraft according to whose results the markings are 

 optical effects due to the shape of the fibre. He maintains that the fibre 

 is made up of many uniform ampuUated or beaded fibrils. The 

 ' ' stripes " do not mark the position of alternating layers of different 

 structure, they mark the shape of the fibre. Dr. Haycraft has recently 

 found a conclusive corroboration of his reasonable theory, in the fact that 

 muscle-fibres pressed upon a collodion film leave a stamp upon its 

 surface, on which the cross-striping can be clearly seen and photo- 

 graphed. 



(d) Nervous Tissue. 



Origin. — Starting again with the Amoeba, we recognise that it is 

 sensitive, and that a stimulus can pass from one part of the cell to 

 another. 



In some Coelenterates it is jjiossible that some of the external cells 

 combine contractile, nervous, and even other functions. Under this 

 impression many call them " neuro-muscular. " 



But in Hydra, there are special nervous or sensitive cells, whose basal 

 prolongations are connected with the contractile roots already described. 

 Here there is a neuro-muscular apparatus of the simplest kind. The 

 nerve-cells seem to receive impressions from without, and transmit 

 them as stimuli to the contractile elements. 



In sea-anemones {Actinia), and some other Coelenterates, there is an 

 interesting complication, withal very simple. There are superficial 

 sensory cells, connected with subjacent nerve- or ganglion-cells, from 

 which fibres pass to the contractile elements. 



In higher animals the sensory cells are integrated into sense-organs, 

 the ganglionic cells into ganglia, while the dehcate fibres which form 

 the connections between sensory cells and ganglionic cells, and between 

 the latter and muscles, are represented by well-developed nerves. 



Sensitive cells analogous to sense-organs of higher animals. 



Basal prolongations ,, sensory or afferent nerve- fibres. 



Ganglion-cells ,, brain or ganglia. 



Fibrous prolongations „ motor or efferent nerve-fibres. 



Muscular cells ,, muscles. 



So far as we know, nervous tissue always arises from the outer or 

 ectodermic layer of the embryo, as we would expect from the fact that it 

 is the layer which, in the course of history, has been most directly 

 subjected to external influences. How nervous structures precisely arise 



