76 MANUAL OF MLEME-NTARY ZOOLOGY 



The sympathetic system possesses a long nerve-cord on 

 each side of the body below the backbone and 

 Syatem. hBtl ° alongside the aorta and systemic arch. It is 

 connected by a ramus communicans with each 

 spinal nerve. At the junction of each ramus communicans 

 the sympathetic cord swells into a ganglion. Between the 

 first two ganglia it is double, joining thus a loop, the 

 annulus of Vieussens or ansa subdavia, through which passes 

 the subclavian artery. In front the longitudinal cord enters 

 the skull with the ninth and tenth nerves, is connected 

 with the tenth, and ends in the Gasserian ganglion. 

 From the sympathetic ganglia small nerves are given off 

 to those of the opposite cord and to the blood vessels and 

 viscera. These nerves break up and rejoin to form net- 

 works ox plexuses. 



If any nerve be traced outward from the central nervous 

 system, it is found, after dividing into finer 

 Functions of and finer branches, to end by entering some 

 Qenerai. organ. Here the fine fibres of which every 



nerve is composed (see p. 94) end by coming 

 into connection with the cells of various tissues. Effer- 

 ent fibres {i.e. those derived from the ventral root) join 

 muscular tissue, which the impulses they conduct will 

 cause to contract, or glandular tissue, which their impulses 

 will cause to secrete. Afferent fibres (i.e. those which pass 

 through the dorsal root or one of the sensory cranial nerves) 

 come into relation with cells of various kinds which are 

 especially irritable by some kind of stimulus (as those of the 

 lining of the eye by light), and their function is to conduct 

 to the central nervous system impulses set up by these 

 stimuli. Thus we may sum up the arrangement of the 

 nervous system by saying that it consists of a central mass 

 and a series of afferent and efferent paths along which 

 impulses pass to and from it. 



It will be clear from the arrangement of the nervous 

 system which we have just described that it is 

 Physiology of a complicated apparatus for conveying messages 

 system. between the different parts of the body through 



the intervention of a central' exchange. In it 

 conductivity is highly developed, as irritability is in the 

 sense organs ; its arrangement, however, is such that im- 



