PATHOLOGY reveals the indwelling power of the tissues, and especially the vessel-making power. Placental Analogy of Placental new Formations. The first adaptations for develop- the placenta are not in the pre-existing vessels, but in the pre- meuts. existing tissues around. The elon gated and almost fibre -like cells become more plump, they join to form cylinders of nucleated proto plasm, the adjoining cylinders open out to form meshes between them, and all this takes place in the intervals between the vessels and their capillaries (fig. 1). The cells of the tissue return to that embryonic state which preceded ]J the formation of blood-vessels, supplying their own juices, as it were, and opening out so as to form plasmatic canals in their FIO. .a, uterine tissue at early stage midst. In the placental rudiment of decidua ; b, c, the same at later it is a mucus-like albuminous fluid stages. that they mostly yield, but there is some evidence that they also yield blood-corpuscles. Meanwhile, the same process of enlarge ment has been taking place in the cells immediately surrounding the blood-vessels ; and at a later stage it is ^ the perivascular cells that keep up this acti vity (fig. 2). The phase of development in which the cells supply their own juices, retaining them in meshes of the tissue, is succeeded by a new formation of ves sels, a more perman ent provision. Certain tracts of cells are told Fl - 2 -~ From deeper part of placenta (guinea-pig), off to form the walls of ^$8 "^ eU-growth m and around the blood-vessels, the chan nel of the vessel being the space between two such adjoining tracts (fig. 3). These selected cylinders of cells become the new and enlarged system of blood-vessels, adequate to the requirements of the part. In this placental process the original capillaries play a FIG. 3. New formation of vessels in placenta (guinea-pig). very subordinate part ; the thin cell-plates that form their walls are far outrun in the hyperplastic race by the cells of the tissues around, and it is the latter which furnish the materials for the new vessels. That which distinguishes the placental new formation is the enormous thickness of the walls of the new vessels and their terminal capillary loops. It remains to consider whether this pla cental new formation of vascular tissue the only instance of the kind in the ordinary course of adult life offers any help to the understanding of granulation-tissue. Tendon- Tendon in a Granulating Stump. It is at once evident that the granula- tissues of a stump after amputation have a very unequal value for tions. formative purposes, and probably all of them a lower value than the uterine tissue, which is at no time far removed from embryonic characters. This inequality is seen in the order in which granula tions appear first on the vascular layer of the skin, on the ends of muscle, and on the marrow of bone, and last on the ends of tendon. The attempt of a severed tendon to cover itself with a cap of granulations is somewhat feeble, and its slowness gives us an opportunity of marking points of detail. Tendon consists of wavy bundles of fibres in close order, and in full-grown animals its cellular elements are reduced to small dimensions. They are thin plates folded round the bundles, presenting in the face view the appearance in a, fig. 4, and in the side view the appearance in b, fig. 4. In the granulating end of a tendon the appearance is that of c, fig. 4 ; the thin plates have become solid or cubical, and where they have increased in number at the free end of the tendon they have lost their orderly arrangement ; they have, in fact, become granulation-cells. The tendon has drawn upon its reserve of cells and placed them at the disposal of the reparative process. All the $/ nip^pi & Kvirif l iW mm wiSf* m&&w &L> FIO. 4. a, tendon-bundle covered by cell-plates, detached plate beneath (highly magnified ; after Ranvier) ; b, ordinary appearance of normal tendon in section, the plates being seen in profile as linear thickenings ; c, tendon from a gran ulating stump of the leg, the cell-plates have become cubical. other tissues of the part have already done the same, some much earlier and more extensively than others. Wherever capillaries are most numerous there the cellular activity is greatest, the cells nearest to the wall of the capillary becoming more plump or more embryonic. The cellular material for the purposes of repair is supplied first around the severed vessels (according to some it is even supplied from within the vessels in the form of colourless blood- corpuscles) of the highly vascular muscle, of the marrow of bone, and of the subcutaneous tissue, and ultimately even by the ends of the tendons. In the placental process the formative materials had been furnished much more evenly over the whole area. Blood-vessels of Repair. The next step is towards the nutrition Blood- of the formative cells. Whether their nutrition is for a time plas- vessels! matic (as in fig. 1, from the placental growth) does not appear ; granula- about the third day the formative tissue begins to be furnished tion. with numerous blood vessels. Their for mation is very diffi cult to observe in young granulations ; . in older granulation- tissue they have the appearance drawn in fig. 5, a series of parallel tubes mak ing straight for the surface, ramifying on the same, joining by numerous loops near the surface, and of unequal calibre throughout their course, being widest on or nearthcsurface. These vessels are dif ferent in several respects from the vessels in a vascular area of the normal organism of corresponding extent, unless it be in the decidua uterina. They are not branching arteriolcs ending in a fine capillary network, but they are of somewhat uniform and exceedingly simple structure throughout, and their calibre is often greater at the distal than at the proximal end. AVe have next to consider how these vessels have originated. The youngest granulations that can be prepared for examination consist of a uniform mass of cells, mostly round, and of somewhat wide vascular channels separated from the mass of cells by thin walls of more elongated cells (fig. 6). The most probable analogy for these new and wide vessels is not the embryo nor the tadpole s tail, but the placenta ; that is to say, certain of the cells along pre determined lines agminate to form the opposite sides of a tube, be coming adapted in shape to that end (fig. 8). According to IHllroth, there is hardly ever in granulations an extension of the pre-existing capillaries by outgrowth of branching cells from their walls such FIO. ,. Blood-vessels in the surface-layer of chronic granulations.
