308 COMPARATIVE ZOOLOGY. 



or both. Vertebrates are the only animals which breathe 

 through the mouth. 



The nervous system has two marked divisions : the 

 cerebro-spinal, presiding over the functions of animal life 

 (sensation and locomotion) ; and the sympathetic, which 

 partially controls the organic functions (digestion, respi- 

 ration, and circulation). In no case does the gullet pass 

 through the nervous system, as in Invertebrates, and the 

 mouth opens on the side opposite to the brain. Probably 

 none of the five senses are ever altogether absent. The 

 form of the brain is modified by the relative development 

 of the various lobes. In the lower Vertebrates, the cere- 

 bral hemispheres are small in certain Fishes they are 

 actually smaller than the optic lobes in the higher, they 

 nearly or quite overlap both olfactories and cerebellum. 

 The brain may be smooth, as in most of the cold-blooded 

 animals, or richly convoluted, as in Man. 



There is no skull in Arnphioxus. In the Marsipo- 

 branchii and Elasmobranchii it is cartilaginous. In other 

 fishes it is cartilage overlaid with bone. In Amphibians 

 and Reptiles, it is mingled bone and cartilage. In Birds 

 and Mammals, mainly or wholly bony. The human skull 

 contains fewer bones than the skull of most animals, ex- 

 cepting Birds. The skull of all Vertebrates is divisible 

 into two regions : the cranium, or brain-case, and the face. 

 The size of the cranial capacity, compared with the area 

 of the face, is generally the ratio of intelligence. In the 

 lower orders, the facial part is enormously predominant, 

 the eye-orbits are directed outward, and the occipital con- 

 dyles are nearly on a line with the axis of the body. In 

 the higher orders, the face becomes subordinate to the 

 cranium, the sensual to the mental, the eyes look forward, 

 and the condyles approach the base of the cranium. Com- 

 pare the " snouty " skull of the 4 Crocodile and the almost 

 vertical profile of civilized Man. A straight line drawn 



