33° On the Cellular and Extra-Cellular [June, 
without the co-operation of cellular elements, this would be 
“ extra-cellular transportation.” 
That the difference between these two forms of the con- 
veyance of nutriment is not merely theoretical, but involves 
an essential difference in our conception of the process of 
nutrition, will appear from the following simple considera- 
tion : — Nutrient matter dissolved in blood or lymph, and 
consequently circulating in an extra-cellular state, must 
naturally follow the existing currents of liquids, in as far as 
the laws of diffusion and filtration permit. If they have 
been taken up by the blood, they are conveyed by it to all 
the organs in one and the same degree of concentration. 
Along with the transuding phasma they permeate every 
tissue, whether it requires them or not, and arrive finally in 
the organs of secretion. Whether they are held back at 
these points of exit, or find a passage and are thus lost to 
the organism, depends only on the arrangement of the 
secreting apparatus and on its condition for the time being. 
Very different must be the behaviour of nutrient matter 
which enters into the circulation in combination with cells. 
Here there is no question of their uniform distribution in 
the liquids of the body. The cells containing nutriment are 
small, itinerant magazines, in which relatively large propor- 
tions of nutrient matter are heaped up in a single point. 
They are not at all subje<51 to the laws of diffusion, and obey 
the existing liquid currents only within narrow limits. If 
they have no inherent motive power the coats of the vessels 
present to them an impassable limit ; but if they have an 
automatic motion, and are capable of travelling, they are, 
when outside the current of the blood, to a great degree 
independent of the air-current. In either case there is the 
possibility of supplying the organs with nutriment exactly 
as required. To fulfil this condition we need merely, in the 
former case, suppose that the circulating cells charged with 
nutriment give up their cargo to the surrounding tissues 
through the capillary lining to such an extent as it is re- 
quired. Tissues which for the time being do not require a 
supply are simply passed over in the distribution. Such an 
arrangement, we know, prevails in the supply of oxygen to 
the tissues. 
In the second case, if we admit that the nutrition-carrying 
cells are capable of travelling, the affair is still simpler. 
The cells can arrive diredtly at the place required, and can 
be utilised either with their entire mass or only with a part 
of their cargo. This arrangement, even more than the first 
case, permits the direction of large quantities of nutriment 
