TISSUES. 43 



that it is diffusely sensitive, and that a stimulus can pass from one part 

 of the cell to another. 



In some Ccelentera 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 special nervous cells, whose basal prolonga- 

 tions are connected with the contractile roots already described. This 

 is a neuro-muscular apparatus of the simplest kind. The nerve cells 

 probably receive impressions from without, and transmit them as stimuli 

 to the contractile elements. 



In sea-anemones and some other Ccelentera 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 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. The 

 simplest are prolonged at one pole into an outgrowth which branches 

 into an afferent and efferent nerve fibre. Most, however, give off 

 outgrowths from two poles or on all sides. Internally they consist in 

 great part of a network or coil of fine fibrils, amid which lies the usual 

 cell kernel or nucleus. Ganglionic cells, aggregated to form ganglia, 

 generally lie embedded in a fibrous cellular substance called neuroglia, 

 usually regarded as an ensheathing and supporting material. 



In all but a few of the simplest Metazoa, the nerve fibres are sur- 

 rounded by a sheath called the neurilemma, said to be formed by 

 adjacent connective tissue. Several nerve fibres may combine to form 

 a nerve, but each still remains ensheathed in its neurilemma. In 

 Vertebrate animals each nerve fibre usually consists of an internal "axis 

 cylinder/'' the important part, and an external unessential 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. Furthermore, 

 nerves are usually surrounded by an enveloping nucleated layer called 

 Schwann's sheath, or else by neuroglia. 



A nerve fibre consists of 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. 



According to some authorities, the nerve fibres arise as extensive pro- 



