THE ORGANS OF SENSATION 171 



In other mammals . . . 5-2 to I 



birds 10-2 to i 



frogs, about . . . . 2 to I 



,, the haddock . . . . i to i 



As a further specific human characteristic, it may be men- 

 tioned that the brain contains much more cerebral fluid than 

 is present in other mammals ; also in the spinal cord which 

 in man only reaches to the last dorsal or first lumbar vertebra, 

 the pyramidal tracts are particularly well developed and quite 

 in contrast with the conditions found in all animals hitherto 

 examined, where the decussation is incomplete. 



The sympathetic nervous system, which in the Cephalo- 

 chordata is absent, appears in man not to differ from that of 

 the other Vertebrates, certainly not the Mammalia. The 

 course of the main trunk of the sympathetic, extending on 

 the visceral aspect of the vertebral column from the atlas to 

 the sacrum, the course of the plexus, with or without ganglia 

 arising from the sympathetic cord, and the connections with 

 the cerebro-spinal nerves present as a whole the same structure. 



Lastly there are the supra-renal bodies which from their 

 relation to the sympathetic system must be regarded as nerve 

 organs. In fishes and amphibia they are multiple organs, in 

 the higher Vertebrates only a single pair exist, and of recent 

 years they have come into considerable prominence from their 

 use in organotherapy, no important differences being discovered 

 between the glands of man and those of animals. 



IX. The Organs of Sensation. 



All sense organs are developed from the integument, and 

 have nerves connected with them. This is the common bond 

 which unites the whole animal kingdom from the highest 

 Vertebrates down to the Invertebrates, with the exception only 

 of the Protozoa {Hydrozoa, Lucernariidae and Anthozoa), which 

 possess no differentiated sense organs but receive impressions 

 indifferently over any part of their surface. The most primi- 

 tive sense is that of 



i. Touch. It has been assumed, with perhaps but little 



