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 Amphioxus. In the Marsipo- 
branchii and Elasmobranchii it is cartilaginous. In other 
fishes it is cartilage overlaid with bone. In Amphibians 
and Keptiles, 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 Crocodile and the almost 
vertical profile of civilized Man. A straight line drawn 
