i80 THE FUNCTIONAL RELATIONS OF THE 



limbless Keptiles ; and (b) those in which muscles on one side, and 

 especially of one limb, are called into activity alone — either in an 

 ordinary reflex or in a volitional manner. 



2. The great bulk of the movements of Fishes and of Ophidian 

 Eeptiles would belong to the former category, and as Broadbent* 

 first pointed out (in regard to Man) we have evidence to show that 

 movements of this class may be equally well evoked by a stimulus 

 passing from either side of the Brain to one of the halves of their 

 double but intimately combined Spinal Centres. This being so, it 

 would, perhaps, be a matter of comparatively little importance for 

 such creatures whether some particular leading sense organs, such 

 as the eyes, were respectively in structural connection through 

 their optic nerves, with the half of the brain on the same side 

 or with that of the opposite side. 



3. Fishes are the animals in which we first find a cross arrange- 

 ment of certain important sensory channels. Their Optic ISTerves 

 decussate in a very complete manner.f We do not know for cer- 

 tain, however, that any of their other sensory channels are similarly 

 disposed ; neither is there any evidence to prove that the fibres con-. 

 Btituting their motor channels decussate with one another. 



4. In Fishes, then, we have to do with what may be, and prob- 

 ably is, a mere partial initiation of the cross relation between the 

 Brain and the body ; and it seems conceivable that such a relation 

 may have been determined in some of the earliest Fishes, or at 

 least favoured, by two or three of the physical peculiarities of such 

 creatures. The elongation of the head of a Fish — a conformation 

 which is doubtless in intimate relation with the animal's life and 

 movements in an aquatic medium — together with the lateral posi- 

 tion of its eyes, may have had something to do with the fact of the 

 occurrence of a decussation of the budding optic tracts in some of 

 the early forms of Fishes. J 



* "Brit, and For. Med. Chir. Eeview," 1866. 



f Though according to Siebold an exception to this rule is to be 

 found in the case of Bdellostoma, one of the Myxinoid or lowest 

 class of Fishes. 



;|: Marshall (" Outlines of Physiology," vol. i. p. 602) endeavours 

 to account for this one primary decussation b}'- supposing it to 

 depend upon the lateral reversion of optic images occasioned by 

 the concave shape of the retina in Fishes. But his reasons seem 

 unsatisfactory, because with a similarly shaped retina no cross 



