DE. E. KLEIN ON THE SMALLPOX OF SHEEP. 
237 
proliferation of the deepest layer of cells of the rete Malpighii can be traced into the 
depth at every place with great certainty (see fig. 6). In consequence of the extremely 
active proliferation of the deep stratum of the rete Malpighii at the centre of the pock 
into the corium, the epidermis as a whole is thicker in the centre than at the periphery, 
notwithstanding that a great part of it has been converted into a horny mass, which 
occupies less space than it did before its transformation. 
It is further necessary to state that the conversion of the middle layers of the original 
rete Malpighii into a horny stratum gradually extends outward, i. e. towards the peri- 
phery of the pock, and that the proliferation of the deep stratum is the more active the 
more rapidly the horny stratum increases in breadth and thickness. 
These observations enable us to see how it happens that the central part of the pock 
becomes depressed and pale as regards the peripheral portion, — depressed, because a 
great number of layers of the original rete Malpighii have become converted into layers 
of horny scales, while the deep stratum of the rete Malpighii grows very actively 
into the corium ; and pale, because the central portion is covered with a thick horny 
dry membrane, i. e. the layers of the rete Malpighii, which have changed in the above- 
named manner. Consequently, as has been already hinted, the appearance of the 
depressed pale centre of most of the examples of sheep-pox has nothing whatever to do 
either with glands or hair-follicles, or with the spreading of the pustules towards the 
periphery of the pock, for all those changes take place before there is a trace of the 
formation of the pustules. It is to be noted that the formation of the horny stratum 
as described above is not constant ; for in some pocks it does not occur until after the 
appearance of the pustules, the superficial layers of the rete becoming gradually trans- 
formed into a horny substance, spreading from the stratum corneum towards the depth. 
The changes of the other parts of the skin are these : — The whole corium and the 
whole subcutaneous tissue in the peripheral portion of the pock shows infiltration with 
lymph-corpuscles ; this infiltration is especially marked in the corium around the glands 
and in the deep subcutaneous tissue. From the peripheral portion the infiltration 
extends into the corresponding strata of the central parts, but is here very much slighter. 
The older the pock the more intense is the infiltration of the peripheral part; hence, 
although the infiltration of the papillary tissue extends a little way in the surrounding 
zone of normal skin, there is a sharp line of demarcation corresponding to the edge of 
the pock. In the centre the papillary tissue becomes the more infiltrated the older 
the pock. 
As regards the distribution of the lymph-cells, it is very easy to notice, on those places 
where the infiltration is not too intense, that most of the lymph-corpuscles are situated 
around the blood-vessels, and extend from here into the interfascicular channels towards 
the lymphatic vessels, many of which contain a greater or smaller number of them. 
The superficial stratum of the subcutaneous tissue is especially interesting in prepa- 
rations stained with heematoxylin. Here it is seen that the interfascicular lymph-spaces 
are very much dilated ; and one can follow the lymph-cells from around the blood- 
MDCCCLXXV. 2 K 
