The Nervous System. 291 



the extensive ganglion connections by which it is brought 

 into communication with the cranial and spinal nerves. 



Gaskell has shown that these communications may be 

 classified as follows : (1) The long chain on either side of 

 the vertebra*, termed by him the ' vertebral ' or ' lateral 

 ganglia *; (2) in connection with these and with each other 

 are the large nervous plexuses of the chest and abdomen, 

 such as the mesenteric, solar, etc., and these he terms the 

 ' collateral ganglia '; (3) ganglia situated in the substance 

 of organs, known as ' terminal ganglia,' and which are in 

 connection with the collateral ganglia (Fig. 24). 



Each spinal nerve is connected by fibres proceeding from 

 the superior and inferior roots, with the ganglia on the 

 vertebral portion of the sympathetic system. By this 

 remarkable chain of communication, the entire nervous 

 system is able to work as one harmonious whole. 



One of the most important functions of the sympa- 

 thetic, is the influence exercised over the bloodvessels by 

 means of the vaso-motor system of nerves. 



Vaso-motor nerves are of two kinds : 1. Vaso-constrictor. 

 2. Yaso- dilator. Both fibres arise in the brain or spinal 

 cord, but possess well-marked differences. 



The vaso-constrictor arise in the middle region of the 

 cord, viz., from the second dorsal to the second lumbar ; 

 running out with the inferior spinal nerve they reach, by 

 means of the ramus communicans, the gangliated cord of 

 the sympathetic (running beneath the arches of the ribs) ; 

 from here some pass forward to the head and neck, others 

 to the fore-leg, others to the thoracic and abdominal viscera, 

 and, lastly, others to the hind-leg ; they reach their various 

 destinations either directly through the sympathetic system, 

 or through recurrent branches of the sympathetic (grey 

 ramus communicans), which join the cerebro-spinal nerves 

 of the fore and hind legs and trunk. It is to be noted that 

 the constrictor fibres, after passing through the ganglia 

 found on the cord of the sympathetic, have, from being 

 medullated, now become non-medullated fibres. 



It is believed that the vaso-constrictor fibres are con- 



19—2 



