LESSON 70.1 NERVOUS SYSTEM. 235 



1062. John Hunter left a preparation in his Museum of Asterias 

 papposa (a many-rayed Star-fish) ; a white, delicate ring is seen sur- 

 rounding the oral (mouth) aperture, from which a series of white 

 chords are traced into the several rays ; this has been assumed, by his 

 successors, to be the nervous system Hunter's notes, in relation to 

 his preparations, having been destroyed. 



1063. Subsequently, Tiedemann published a valuable monograph 

 on the Echinodermata (prickly-skinned animals, in which class the 

 Star-fishes are included). He gave a description, accompanied by 

 beautiful figures, of the anatomy of the common five-rayed Star-fish 

 Asterias rubens and amongst the tissues figured was the nervous sys- 

 tem, a copy of which is appended (Fig. 361). 



Here a white ring is distinctly seen sur- 

 rounding the oral aperture, and distributing 

 three branches to each of the five rays. 



1064. He also described the vascular sys- 

 tem in the following remarkable words : " The 

 vessels which absorb the chyle from the di- 

 gestive sac, terminate after a series of reticu- 

 late anastomoses, in a circular trunk, which Nervous system of Asterias. 

 likewise receives branches from the radiated cceca. The venous cir- 

 cle communicates by means of a dilated tube, regarded as a rudimen- 

 tal form of heart, with an arterial circle surrounding the mouth, 

 from which branches diverge to the rays, and other parts of the body." 



1065. Authority is not wanting to confirm the fact that the vascu- 

 lar system, so accurately described by Tiedemann, is the same structure 

 which himself and others have mistaken for a nervous system. 



1066. In the uninjected state of the animal, the vessels are filled 

 with white, coagulated blood, and in roundness, fulness, color, and 

 general appearance, accurately simulate nerves. 



1067. From all that is known of the necessities of a nervous 

 system, it appears to be quite improbable that any such condition 

 exists in the animal kingdom as that alleged to belong to the Kadiata. 



1068. It is extremely difficult to understand a nervous system, 

 consisting only of a series of simple chords unprovided with a sin- 

 gle nervous centre, or ganglion. 



1069. Now a ganglion is a knot of nervous matter ; a brain of a 

 kind ; a centre of reinforcement ; and the probability is that no ner- 

 vous system exists, without, at least, one ganglion. If this be true, 

 the nervous system in the Radiata has yet to be discovered. 



Nerves possess distinct functions : thus, a nerve of sensation has 



