Page:Proceedings of the Royal Society of London Vol 4.djvu/336

Rh ginous, and which, on account of its being subject to compression and concussion, does not contain any.

Among the second class of extra- vascular tissues, the cornea is first treated of ; and its structure is described as being very lax, and as containing corpuscles only in a small quantity. The opinions in favour of its vascularity are combated ; and it is shown that the blood-vessels which converge to its attached margin, and which are the principal source of the fluid that nourishes it, are large and nu- merous, and that at the circumference of this tissue the arteries, without any diminution of their calibre, return in their course, and become continuous with the veins. A second set of vessels, devoted to the nutrition of the cornea, is also described ; they extend to a short distance over the surface of the tissue, but do not penetrate into its substance.

The crystalline lens is described as being composed of corpuscles, of which the radiating fibres are constituted. The arteria centrahs retinse is described as ramifying over the posterior surface of the capsule, where it forms large branches ; these pass round the cir- cumference of the lens, and reach its anterior surface, at the peri- phery of which they become straight : the arteries terminate in loops frequently dilated, and become continuous with the veins. With respect to the vascularity of the vitreous humour, the author states that although many anatomists have, in general terms, represented the arteria centrahs retinse as giving off, in its course through this organ, minute branches into its substance, stiU those who have paid especial attention to the subject, have not been able to find such vessels. He believes that the nutrition of this structure is accom- plished by the fluid brought to its surface by the ciliary processes of the choroid, which fluid is diffiised with facility through its entire substance by means of the corpuscles of which its membrane is com- posed, assisted by the semifluid character of the humour.

The third class of extra-vascular tissues comprehends the epider- moid appendages. The* author describes them all as composed of corpuscles, which are round and soft where they are in contact with the vascular chorion, compressed and flattened where they are farther removed from it. He points out, in the substance of the hoof of the Horse, the existence of fine canals, which he supposes to conduct fluid through its mass ; and he states that the perspiratory ducts of the human subject possess a structure analogous to the spiral vessels of plants. The author describes each of the tissues of this class at length, and shows that the various modifications presented by the vascular system with which each is in contact, have the sole object of enabling a large quantity of blood to approach and circulate slowly around them. He also points out, in connexion with this subject, the remarkable vital properties which are possessed by these non-vascular tissues.

In concluding this paper, the author states that his object has been to establish as a law in animal physiology, that tissues are capable of being nourished, and of increasing in size, without the presence of blood-vessels within their substance. He shows the analogy