MAN CONSIDERED PHYSIOLOGICALLY. 665 
is more urine passed and more urea excreted in proportion to 
the weight. 
The lymphatic system, as a whole, is more pronounced in 
youth. Certain glands, the functions of which are not well 
understood, for which reason we have thought it well to pass 
them over entirely, are at their highest development during 
infantile life, as the thymus and thyroid. These atrophy as 
puberty approaches, especially the thymus gland. 
The prominence of the lymphatic system harmonizes with 
what we know of the functions of the colorless corpuscles of 
the blood in the work of building up tissues. They may be re- 
garded as remnants of embryonic life, undifferentiated cells 
awaiting their opportunity to develop, though we do not, of 
course, mean to affirm that in the blood and elsewhere they 
have no other functions; in fact, it has been shown that in the 
alimentary tract they are porters of digestive products (fat, 
etc.); and they also likely play an important part as scavengers 
and as guardians of the nobler cells against micro-organisms, 
etc. 
Dentition—The change in the metabolic powers of the ani- 
mal is foreshadowed by the gradual appearance of teeth for the 
preparation of a more solid food to meet the altered wants of 
the economy. 
The first appearance of teeth is in the upper jaw, the two 
central incisors, soon to be followed by the corresponding ones 
of the lower jaw. This is at about the seventh or eighth 
month, to be succeeded by the lateral incisors a couple of 
months later; the first molars about the end of the first year of 
life; the canines (eye-teeth) half a year later; and the whole of 
the temporary set before the second year is completed. 
The permanent teeth replace the milk-teeth very gradually, 
and are thus adapted to the growing jaws. The new dentition 
begins to appear about the sixth year, and may continue for 
six or eight years. The last molar (wisdom tooth) appears very 
late, between the seventeenth and the twenty-fifth year. It is 
noteworthy that this tooth seems to be more and more delayed, 
and often never appears at all, which may be said of some 
others, especially the lateral incisors; so that it looks as if, as 
civilization progressed, the jaw were becoming smaller and the 
teeth suffering atrophy. Both the teeth and the hair are epi- 
dermic structures, and their defective growth at the present 
time in so many individuals raises suggestive questions. The 
face of civilized man seems also to be getting smaller relatively 
