Summary(1) The capillary bed of the central nervous system may be either reticular or composed of independent loops. When the organs are very thin, as the spinal cord of the lamprey, it may be absent, (2) When it is reticular the network is continuous throughout the brain and spinal cord and is supplied by many arteries but the spread of blood through the reticulum is not rapid enough for functional needs if one of the larger tributary vessels is obliterated, so that these apparently function, largely as end‐arteries despite the capillary continuity. When loops are present they are confined precisely to the central nervous organs and the arteries are structurally absolute end‐arteries. (3) The two types of capillary bed are so distributed through the vertebrate series (both occurring in every class except Aves and also in invertebrates) that it appears unquestionable that animals with one type must have originated from ancestors with the other type repeatedly. Which type is the more primitive is uncertain but the balance of evidence seems to point towards the loop type. (4) Ontogeny provides no evidence of relationship between the two types. (5) When a network occurs it usually lacks particular orientation but in some cases is disposed in special ways probably determined by the arrangement of the nerve fibres. The vessels tend to become mote tortuous higher in the phylogenetic series and there is a tendency for the flow of blood to be less centrifugal and more centripetal. (6) The looped or reticular character of the vessels seems to be imposed upon them by the brain tissue but is retained when once established so that looped vessels growing into a transplant from an animal with reticular capillaries remain non‐anastomosing loops. (7) Quantitatively there is an irregular trend towards increased vascular richness and greater differentiation in‐the supply of‐specific regions as the phylogenetic scale is ascended. The poorest supply recorded is in the granular layer of the cerebellum ofNecturus, which has 5 mm. of capillaries per cu. mm. of tissue, the richest is in the supraoptic nucleus of the monkey, with over 2600 mm. of capillaries in the same volume. Within mammals differentiation increases but the general richness of the supply seems to vary inversely with size. (8) In mammalian foetuses, as in lower vertebrates, the capillary bed is less rich and more uniform than in the adult, a rapid increase and differentiation taking place soon after birth coincidently with the development of functional activity. (9) In warm‐blooded animals the capillaries are narrower than in cold‐blooded animals of similar size and, within one group, the calibre of the vessels usually increases with the size of the animal. In the rat 1 cu.mm. of blood is exposed to four times as great an area of capillary wall as inNecturus.In hibernation the vessels dilate but do not change their total length. (10) The varying richness of the capillary bed must correspond with varying metabolic activity but the relationship is probably not simple and direct. The total length of the vessels may increase in response to prolonged excessive activity and, normally, all the vessels probably remain per
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