43 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



a,pes, in which it varies from -i- to T \, by the sparrow 

 in which it is ^ T , and the titmouse in which it is j 1 ^ 

 (Bischoff). 



On the other hand, when we compare man with 

 his nearest zoological allies, we find that not only is 

 the capacity of his skull and the weight of his brain 

 greater, but that there is a notable increase in the 

 complexity of the secondary gyri of the surface of 

 his cerebral hemispheres, as compared with those 

 of the apes. 



The spinal cord differs from its anterior enlarge- 

 ment, the brain, in having the grey ganglionic mate- 

 rial placed internally to the white fibrous cords, which 

 act as the conductors of nervous stimuli, but, like it, 

 it is hollow internally, and the epithelium whicli 

 lines it is temporarily or permanently ciliated. 

 It is marked above and below by a median groove, 

 and, in all vertebrates, has paired nerves issuing from 

 it, each of which is connected with it by a superior 

 and an inferior root. It is cylindrical in all Verte- 

 brates except the Cyclostomata and Chimsera ; not 

 unfrequently it extends throughout the whole length 

 of the neural canal formed by the spinal column, but 

 in the sun-fish it is greatly shortened, so as to look 

 indeed like a mere appendage to the brain, and in the 

 anurous Amphibia, in Birds, and various Mammals 

 (among which are the hedgehog and man), the 

 terminal portion is filamentous, and is accompanied on 

 either side by a number of nerves, thereby giving rise 

 to the so-called cauda equiiia (horse's tail). 



SENSORY ORGANS. 



It has been already stated that all the organs 

 of sense have their primitive seat in that outer 

 layer of the body which, in the embryo, is called the 

 epiblast or ectoderm ; and we have already learnt 

 that the nervous system itself does, in most cases, 



