rought into relation with the
life of the rest of the cells which compose the organism. Fourthly,
the investigation of the phenomena of life in general, on the
assumption that the physical and chemical processes which take place
in the living body are of the same order as those which take place out
of it; and that whatever energy is exerted in producing such phenomena
is derived from the common stock of energy in the universe. In the
fifth place, modern physiology investigates the relation between
physical and psychical phenomena, on the assumption that molecular
changes in definite portions of nervous matter stand in the relation
of necessary antecedents to definite mental states and operations. The
work which has been done in each of the directions here indicated is
vast, and the accumulation of solid knowledge, which has been
effected, is correspondingly great. For the first time in the history
of science, physiologists are now in the position to say that they
have arrived at clear and distinct, though by no means complete,
conceptions of the manner in which the great functions of
assimilation, respiration, secretion, distribution of nutriment,
removal of waste products, motion, sensation, and reproduction are
performed; while the operation of the nervous system, as a regulative
apparatus, which influences the origination and the transmission of
manifestations of activity, either within itself or in other organs,
has been largely elucidated.

[Sidenote: Practical value of physiological discovery.]

I have pointed out, in an earlier part of this chapter, that the
history of all branches of science proves that they must attain a
considerable stage of development before they yield practical
'fruits;' and this is eminently true of physiology. It is only within
the present epoch, that physiology and chemistry have reached the
point at which they could offer a scientific foundation to
agriculture; and it is only within the present epoch, that zoology and
physiology have yielded any very great aid to pathology and hygiene.
But within that time, they have already rendered highly important
services by the exploration of the phenomena of parasitism. Not only
have the history of the animal parasites, such as the tapeworms and
the trichina, which infest men and animals, with deadly results, been
cleared up by means of experimental investigations, and efficient
modes of prevention deduced from the data so obtained; but the
ter