TRANSACTIONS OF SECTION D. 779 



plied by a distinct and separate nerve in the first place. By the development and 

 evolution of tlie simple embryonic dorsal and ventral strata of muscles, formed of 

 mesoblastic tissue derived from the original somites, the comjilicated systems of 

 muscles are formed which exist in the adult. 



These changes taking place in the myotomes it follows that similar alterations 

 will occur in the primitive nerves which supply them. The myotomes undergo 

 fusion, elongation, contraction, and a complex muscular system results. The nerves 

 distributed to the original parts of this complex system undergo similar changes ; 

 adjacent dorsal and ventral divisions become fused, and give rise to a compound 

 nerve, from which jyari passu with the development of individual adult muscles 

 branches are given off to supply them. It follows, therefore, that as one muscle 

 may be formed from some only of the myotomes implicated in the limb, the par- 

 ticular nerve for this muscle may arise only from a certain number of the nerves of 

 origin. On the other hand, one muscle may represent the whole breadth of the 

 surface of the primitive limb ; in such a case the nerve supplying it will derive 

 fibres from all the nerves entering the limb-plexus. 



From these considerations it is concluded that, as this Jirst step in the forma- 

 tion of a limb-plexus, the division of the nerves of origin into dorsal and ventral 

 trunks is a primary process in the development of the limb ; so the secojid step, the 

 interconnection of these adjacent dorsal and ventral divisions, is a secondary process 

 in the same direction. 



The limb-plexuses, that is, the formation of the nerves of distribution from 

 certain nerves of origin by the division and union of the latter, are entirely the 

 efi'ect of the mode of development of the limb itself. They possess no physio- 

 logical significance. There is no reason to believe that a nerve-fibre divides in its 

 course. Until the nerve breaks up into its terminal filaments there is no evidence 

 to show that it does not exist as a simple and individual fibre from its origin in 

 the spinal cord. In other words, it is not known that an axis cylinder divides 

 dichotomously in the limb-plexuses, so as to connect a single cell in the spinal cord 

 with two distinct and separate muscles, or parts of the same muscle. This being 

 the case, the existence of these plexuses is not explained by a vague reference to 

 ' the co-ordiuation of muscidar action.' 



The conclusion I would submit is that they are an integral part of the process 

 of evolution and development of the limb. They result in a convenience of nature 

 in the adult condition. They are due to the changes which produce that condi- 

 tion, concomitant with other processes, all tending to the conversion of the simple 

 into the more complex. 



5. The Normal Phenomena of Entoptic Vision distinguished from those 

 produced hy Mechanical Causes. By Beateice Lindsay, Oirton 

 College, Cambridge. 



Experiments on the subject of entoptical vision, extending over a period of 

 three j'ears, have led me to believe that the dilficulty which has hitherto been 

 found in explaining the formation of entoptic images of retinal structure has been 

 largely due to want of clear distinction between those images which are formed in 

 accordance with the laws of optics, and those which are due to the specific energy 

 of tbe optic nerve, exhibited when mechanical stimuli affect its peripheral 

 branches. I have therefore framed the following classification of the entoptical 

 images of parts of the retina, that may be obtained by various experiments. Those 

 images in the list which are marked with an asterisk are, I believe, now for the 

 first time described in their relation to the actual structure of tbe retina. 



I. Entoptical images obtained by normal sight ; distinguished by their vitreous 

 lustre, semi-transparency, colour as of natural tissues, translucent or red, and absence 

 under complete darkness. 



1. Purkinje's figures. 



2. Images of blood-corpuscles. These, the existence of which has been dis- 

 puted, are apparent throughout the visual field, the theory of the non-vascularity 



