SUMMARY. 247 



parts. In man particularly, the lower portion of the oesophagus is surrounded by 

 branches of the par vagum ; the two trunks are thus differently combined, but it is 

 probable that the nerves are at the same time made capable of being excited by the 

 passage of the food along this canal, preparatory to their active functions in the 

 stomach. In invertebrate animals the oesophagus is generally surrounded by the 

 beginning of the nervous system, which in the leech during the act of sucking is 

 agitated and stimulated, the full stomach also distends and excites the contiguous 

 nerves. The large artery passing between the divisions of the nervous cord as in the 

 lobster, and through the ring as in the crab, have a similar influence. 



The functions of the nervous system in the higher classes of animals are also 

 promoted in a considerable degree by the activity of the sanguineous system, and in 

 some parts for special purposes by peculiar arrangements of the vessels. The rete 

 niirabile shows that a modification of the supply is necessary for the brain in different 

 animals, a forcible current is, however, continued through this structure, for on dividing 

 the carotid arteries in the calf, the blood sent by the vertebral, returns, pulsating 

 through the upper divided ends. Whether the arteries reach the brain from the 

 carotid by continuous branches, or through the rete mirabile, they are ramified in the 

 pia mater and the cellular tissue containing the cineritious and medullary matter. A 

 greater proportion circulates in the cineritious matter ; but some vessels containing 

 red blood are found in the medullary; it is, however, sustained principally by the 

 colourless parts. The spinal cord appears to require a similar circulation. The 

 bloodvessels of the nerves ramify on the neurilema, and throughout their course are 

 moderately supplied with blood from neighbouring branches, but receive more during 

 their excitement, and it is most probable that they are generally sustained by the 

 colourless parts. Particular arrangements of the vessels are necessary for completing 

 the functions of some nerves, as in the cavernous form of the veins of the nose, the 

 choroid coat of the eye, and in others in various parts of the body. 



Ganglia are supplied with blood by neighbouring arteries ; in their healthy state 

 they do not require mucn, but are supported chiefly by the colourless parts ; it is 

 probable that, wnen tney are excited, numerous vessels containing red globules become 



