TISSUES 43 



In some Coelentera a few of the external cells seem to combine 

 contractile and nervous functions. Therefore they are sometimes called 

 " neuro-muscular." 



But in Hydra there are superficial sensory cells, whose basal pro- 

 longations are connected either directly with contractile cells, or with 

 deeper ganglion-cells, some of which give off motor processes to the 

 contractile cells. 



In sea-anemones and some other Coelentera there is a more sharply 

 defined division of labour. Superficial sensory cells are connected 

 with subjacent nerve- or ganglion-cells, from which fibres pass to the 

 contractile elements. 



In higher animals the sensory cells are mostly integrated into sense 

 organs, the ganglionic cells into ganglia, while the delicate fibres 

 which form the connections between sensory cells and ganglionic cells, 

 and between the latter and muscles, are represented by well-developed 

 nerves. 



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 

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

 subjected to external stimulus. 



Let us consider first the ganglionic cells which receive stimuli and 

 shunt them, which regulate the whole life of the organism, and are the 

 physical conditions of "spontaneous" activity and intelligence. They 

 are of very varied shape, but consist always of a cell-body which give? 

 off one or more processes. One of these processes is long, -branches 

 very sparingly, and is known as the axis-cylinder. There are usually 

 present other processes which ramify like the branches of a tree and 

 are called dendrites. The cell-body contains a nucleus, distinct 

 granules, and a network of fine fibrils. The nervous system is built up 

 of such "neurones." In the ganglia they are supported and held 

 apart by much-branched neuroglia cells. 



In all but a few of the simplest Metazoa, the nerve fibres (axis- 

 cylinders) are surrounded by a sheath called the neurilemma, said to be 

 formed by adjacent connective tissue. Several nerve fibres may com- 

 bine to form a nerve, but each still remains ensheathed in its netiri- 

 lemma while fibrous sheaths bind the nerve fibres together. In Verte- 

 brate animals each nerve fibre usually has in addition a medullary 

 sheath. But even in the higher Vertebrates, " non-medullated " or 

 simply contoured nerve fibres are found in the sympathetic and olfactory 

 nerves, and this simpler type alone occurs in hag, lamprey, and 

 lancelet, as well as in all the Invertebrates with distinct nerves. 



A nerve fibre contains numerous fibrils like those seen within a 

 ganglion cell. These are regarded by some as the essential elements in 

 conducting stimuli, while others maintain that the essential part is the 

 less compact, sometimes well-nigh fluid stuff between the fibrils, or that 

 the fibrils are but the walls of tubes within which the essentially nervous 

 stuff lies. 



The nerve fibres arise as prolongations of the ganglion cells, 

 which extend themselves in the embryo like Amoebae sending put 

 pseudopodia. < 



