NUTRITION. 
scarcely be doubted, therefore, that these are 
supplied directly from the blood. Dr. Barry 
and Mr. Addison have spoken with much con- 
fidence of the metamorphosis of the white cor- 
puscles of the blood into epithelium-cells ; but 
that this idea is totally inadmissible is proved 
by the existence of a continuous stratum of 
basement-membrane, between the capillary net- 
work and the epidermic or epithelial layer. It 
is not impossible, however, and perhaps may 
be considered probable, that the cell-germs 
contained in this basement-membrane, from 
which the cells on its external surface appear to 
take their origin, may be the offspring of the 
white corpuscles of the blood, which thus sup- 
plies both the plastic materials and the germs 
of the constantly-forming new crops of epithe- 
lial cells. There is no other tissue in the body, 
after all its organs had attained their full deve- 
lopement, which can be regarded as taking its 
_ origin from the blood in the same degree ; but 
__ it may be questioned whether in the formation 
of new parts, either during the developement of 
the embryo, or in the reparation of injuries, 
the office of the blood is not of a similar du- 
_ plex character. Thus when plastic lymph is 
thrown out, between the two surfaces of a 
wound, the first process, as already mentioned, 
is its fibrillation; but at the same time a deve- 
 lopement of cells takes place in it, which cells 
may possibly undergo a subsequent metamor- 
osis into the various forms of tissue which 
newly-formed part afterwards contains, 
precisely as in the first developement of the em- 
ryonic structure. Such a view, at least, would 
seem probable in regard to the capillary vessels, 
which seem to be formed at least as much by 
the inherent powers of the coagulum, as by the 
extension of the vessels from the subjacent 
urface. 
‘These views are thrown out as hints, rather 
than as settled ideas. It would be premature, 
in the present state of our knowledge, to at- 
_ tempt to decide questions of ssuch importance 
ithout much further examination; and we 
an only attain a balance of probabilities by 
nterpreting the insufficient results of observa- 
‘tion by the aid of the best analogies we can 
 d nd. The whole subject has made immense 
| progress during the few years which have 
elapsed since the commencement of the pre- 
sent work ; but here, as elsewhere, retardations 
have occurred through hasty generalization and 
dogmatic assumptions ; and much patient, well- 
directed, sagacious observation will he needed 
‘to unravel the many intricate questions that yet 
Temain to be solved. 
Varying activity of the nutritive processes. 
—Without any change in the character of the 
Nutritive processes which we have been de- 
seribing, there may be considerable variations 
in their degree of activity ; and this, either as 
regards the entire organism or individual parts, 
though most commonly the latter. These va- 
Tiations may be so considerable as to constitute 
; though there are some which take 
jplace as part of the regular series of physiolo- 
gical phenomena. Thus the nutritive processes 
hould have a degree of activity more than suffi- 
751 
cient to supply the waste of the body during 
the whole period of infancy, childhood, and 
adolescence, until, in fact, its full dimensions 
are attained ; whilst, on the other hand, they 
are usually less rapid than the disintegrating 
processes in old age, so that the bulk of the 
body diminishes. Now as the waste of the 
body, so far from being more rapid in old age 
than in childhood, is much less so, it follows 
that the difference in the activity of the nutri- 
tive processes in these two states must be very 
considerable; and this is manifested, not only 
in the greater demand for food which exists in 
the child (relatively to the bulk of its body), 
but also in the greater quickness and facility 
with which injuries are repaired. Local va- 
tiations may also occur as part of the regular 
train of vital actions in the adult; thus we 
perceive an enormous increase in the amount 
of tissue contained in the uterus and mammary 
glands during pregnancy, and a decrease in 
the bulk of the thymus gland after the first 
year of infancy.’ Now in these cases we see 
that increased nutrition is invariably connected 
with increased functional activity, and dimi- 
nished nutrition with diminished functional 
activity : and this we shall tind to be the con- 
stant rule in regard also to those variations 
which must be considered as abnormal. 
Increased nutrition, or hypertrophy, is 
never known to affect the whole body to a de- 
gree sufficient to constitute disease. It cannot 
be produced as a consequence of the ingestion 
of an undue supply of food, for this does not 
increase the formative activity of the tissues, 
but merely renders the blood richer in nutritive 
materials, a part of which the excreting organs 
are called on to be continually removing, with- 
out its being rendered subservient to the wants 
of the body; whilst another part may be em- 
ployed in the nutrition of one particular tissue, 
the adipose, which has a tendency to increase 
with the superfluity of non-azotized food, pro- 
vided that the requisite amount of cellular tissue 
be generated to hold the fatty matter. But 
examples of hypertrophy of particular tissues 
or organs are very common. Thus any parti- 
cular set of muscles which is subjected to fre- 
quent and energetic use acquires a great in- 
crease in bulk, as we see in the arms of a black- 
smith or waterman, the legs of an opera-dancer, .. 
&e. The hypertrophy of these muscles is a 
consequence of their increased functional acti- 
vity, which being produced by an exertion of 
the will, and unaccompanied with any inju- 
rious effects on the system, can scarcely be re- 
garded as morbid. But there are many in- 
stances in which the involuntary muscles ac- 
quire a greatly-increased strength, in conse- 
quence of an obstruction to their action which 
results from disease. Thus we see the right 
ventricle of the heart become hypertrophied 
(and dilated at the same time) where chronic 
pulmonary disease produces a difficulty in the 
propulsion of the blood through the vessels of 
the lungs ; the muscular fibres of the bladder 
become enormously hypertrophied, when stric- 
ture, diseased prostate, or other causes pro- 
duce a demand for increased expulsive force 
