in support of an assertion of Aristotle. 499 



horns resembles eyes, and Swammerdam has attempted to de- 

 lineate the different parts of the organs, Mr. Bauer has shewn 

 the two nerves in different states. The medulla spinalis forms a 

 larger mass than the brain, but equally made up of two distinct 

 parts. From the upper edge of this mass, there is an azygos 

 branch going directly upward to the muscles of the tongue, be- 

 yond which are the glands of the mouth, and the oesophagus cut 

 through. This nerve, so similar to the recurrent in the humaa 

 body, only differing in being single, justifies me in having given 

 the name of spinal marrow to the part that gives it off." 



Of the second figure, (PI. xvii. fig. 4.) we have this explanation : 

 *' The point of one of the large horns, magnified fifty diameters ; 

 to shew that the external point of its termination in no respect 

 resembles a cornea, but consists of five bundles of nervous fila- 

 ments, the terminations of the branches of the nerve." 



I need scarcely point out the exact agreement of this state- 

 ment of the nature of the extremities of the larger tentacula of 

 the Snail, with that of M. Gaspard on the same subject, who, after 

 recounting the experiments he instituted to ascertain the true 

 nature of those organs, observes, *' In a word, I find in these pre- 

 tended optical bodies, nothing more than the organs of an ex- 

 quisite sense of touch, with extreme sensibility to heat, dryness, 

 moisture, to the slightest shock, or the least agitation of the air; 

 and this arises from a large nerve zohich is expanded over the 

 extremitt/." 



As no further account of Mr. Bauer's figures is given in the 

 Philosophical Transactions, it may be useful to say a few words 

 in explanation of them. In fig. 3, one of the tentacula appears tp 

 be represented, with the black point on the extremity, having a 

 pellucid centre ; together with the nerve, and portions of the 

 epidermis and black rete mucosum beneath it. The nerve and 

 black point only of the other are shewn, the integuments having 

 been dissected away. Fig. 4, represents, magnified fifty diameters, 

 as above stated, the portion of rete mucosum, which, protruding 

 from beneath the epidermis, forms the black point, mistaken by 

 Swammerdam for nigrum pigmentum ; together with the nerve 



2 I 2 



