Cells and Tissues. 



13 



ramify over it or its processes (Pig. 15, 3). There is yet a third kind 

 of ganglion cell connected with neither epithelium nor muscle, but 

 only with other ganglion cells (Fig. 15, l) . 



It maybe noticed here that many gland cells, like muscle cells, only 

 become active when they receive a stimulus from a ganglion cell ; in glands, 

 therefore, terminal branches of ganghon processes are present in great numbers. 



MW JJ MM m 



Fig. 15. Various ganglion cells, etc. 1 Ganglion cell, with dendritic, but without the 

 longer prouesses. 2 Motor ganglion cell connected to a muscle cell m by a nerve fibre n. 

 3 Two ganglion cells connected with one another. 4 Sensory ganglion cell, with its nerve 

 fibre branching in the epithelium e. 5 Sensory ganglion cell g, with two long processes ; 

 b the terminal tuft of the process. 6 Sense cells s ; b the terminal tuft of an efferent 

 nerve fibre ; a ganglion cell below. — Orig. 



The long processes of ganglion cells are called nerve 

 fibres, and according to their connection with muscle or with sense 

 cells, they are called motor or sensory: the short processes 

 are termed dendrites. Sometimes nerve fibres originate not in 

 ganglion, but in sensory, cells (Fig. 15, 6) : the epithelial cells 

 are, in this case, usually tall, narrow cells, with a tuft or fringe of 

 cilia at the free end ; they pass, on the inner side, into long delicate 

 processes, which are intimately connected with ganglion cells, for 

 the end breaks up into a fine anastomosis, which is closely apposed 

 to a ganglion cell or its processes. Such epithelial cells are called 

 sensory cells, and the prolongations, sensory nerve fibres. 



The nerve fibres are frequently enveloped in sheaths of speoiaUy developed 

 connective tissue. Most of those in Vertebrates have a double sheath, a strongly 

 refringent fatty medulla within, and an outer neurilemma or sheath 

 of Schwann. Some vertebrate nei-ve fibres are covered vsdth a neurilemma 

 only. 



II. Organs. 



Although the animal body forms a connected whole, yet in most 

 Metazoa a varying number of more or less independent organs may 

 be distinguished, each composed of one or more of the tissues already 



