go AN INTRODUCTION TO ZOOLOGY 



auriculo- ventricular junction. Both ganglia appear to be connected 

 with twigs from the cardiac branch of the vagus nerve, they are also 

 related to the cardiac plexus, and all combine to regulate the speed 

 and strength of the heart beat. 



As has been pointed out previously, a nerve fibre can convey 

 an impulse in one direction only, thus the messages pass along the 

 axon away from the cell and in the dendrons towards the cell. The 

 exact nature of nervous impulse is not known, but it is fairly closely 

 allied to an electric current, although its rate of propagation is rela- 

 tively very slow, being only about 125 metres per second. It has 

 been found by experiment, e.g. by cutting the roots of the spinal 

 nerves stimulating the cut ends and noting the results, that the 

 dorsal root is composed almost entirely of afferent or sensory fibres, 

 conducting messages towards the spinal cord, while the ventral root, 

 on the other hand, is efferent, carrying its messages to the periphery. 

 Efferent nerves are sometimes termed motor nerves, but in the strict 

 meaning of the term they are only motor if -they pass to muscles 

 and by stimulating them cause their contraction. If they go to a 

 gland their action is probably to cause an increase in glandular 

 activity, and so they may be termed excitatory, and those going to 

 the heart may cause it to beat more rapidly, i.e. are acceleratory or 

 produce the reverse effect when they are termed depressor or 

 inhibitory. 



Just as the anatomical unit of the entire nervous system is 

 the neuron, so the functional or physiological unit is the combination 

 of such neurons, termed a reflex arc. This in its simplest condition 

 may consist of two neurons. Take an example of a spinal reflex, 

 that is, one occurring in the spinal cord. It consists, in the first 

 place, of the sensory epithelium in which the nerve terminates in a 

 series of small sense organs. On the stimulation of these a message 

 passes up the dendron of the afferent nerve to a cell in the dorsal root 

 ganglion, and thence on by the axon into the spinal cord. Here the 

 terminal dendrites come into relation with the dendrons of an 

 efferent ganglion cell situated in the ventral horn of grey matter, and 

 so the message is handed on. It can then be sent out by the axon 

 of the afferent cell through the ventral root to the corresponding 

 muscle. In this way stimulation of the epithelium will bring about 

 the contraction of the muscle related to it without the intervention 

 of the brain. The whole structure is termed a reflex arc, and the 

 action a reflex action. The arc is not always, indeed but rarely, 

 so simple as this, for generally the terminal dendrite of the dorsal 

 root fibre come into relation with the dendrons of a ganglion cell 

 in the dorsal horn of grey matter, and this cell sends out an axon 

 whose dendrite is related to the dendrons of the ventral horn cell. 



