216 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Both sorts of nerve-fibre are connected centrally, directly or 

 indirectly, with ganglion-cells. Peripherally their axis-cylinders 

 may break up into fine plexuses, or those of medullated fibres 

 may terminate in various end-bodies, such as sense-cells, and, in the 

 case of striated muscle, end-brushes and end-plates (Fig. 90). 



Non-nervous structures are intimately connected with the nervous 

 system, especially connective tissue, which forms the investing membranes, 

 and binds the nerve-fibres into smaller and larger bundles, while the fibres 

 and cells of the central organs are imbedded in an excessively delicate 

 connective-tissue network, the neurogiia. The pineal gland and pituitary 

 body are both non-nervous. The former is a rudimentary eye. 



The nerve-fibres are physiologically divisible into afferent and 

 efferent, along which impulses respectively pass to and from the 

 central organs. The majority of the former are sensory, since 

 they are connected with end-organs, the stimulation of which 

 lead, in many cases, to a sensation, judging from analogy. A 

 large number of efferent fibres, since they supply muscles, are 

 known as motor. The brain and spinal cord are, in the first 

 place, centres for reflex actions i.e., those which are independent 

 of volition, and dependent on external stimuli. The apparatus 

 involved in such an action normally consists of (a) end-organs, 

 (b) an afferent nerve, (c) a nerve-centre, (d) an efferent nerve 

 terminating commonly in (e) muscular or glandular tissue. 

 The spinal cord alone, after the removal of the brain, enables 

 fairly complicated and purposeful movements of the body to be 

 effected, in answer to external stimuli. Thus, for example, 

 pinching a foot causes the corresponding leg to be drawn up, 

 and the placing on the skin of the back a bit of blotting-paper 

 dipped in acid leads to leg-movements directed to its removal. 

 The cord also serves as a channel by which afferent impulses can 

 travel to the brain, and efferent impulses from it. If the 

 cerebral hemispheres only are removed, still more complicated 

 reflex actions can be evoked by external stimuli, such, for 

 example, as swimming, croaking, and leaping. The medulla 

 oblongata, next to the hemispheres, is the most important part 

 of the brain. It regulates respiration, and has much to do 

 (through the sympathetic) with the alimentary canal and circu- 

 latory organs. It is also concerned with the co-ordination of 

 many muscular movements. 



Spontaneity entirely resides in the cerebral hemispheres. 

 When these are removed, movements only occur after the appli- 



