摘要:

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

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00446.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY

摘要:

Summary(i) An injection of neurohypophysial extracts into frogs kept in water produces a temporary increase of body weight. This rise of weight (the ‘water‐balance effect’ or ‘Brunn reaction’) has beēn shown to be due to an increase of the body water. (2) The water‐balance effect has been clearly demonstrated in various species of amphibians only. It does not occur in teleost fishes or in mammals. Its occurrence in reptiles is doubtful and requires reinvestigation. It has not been investigated in cyclostomes or birds. (3) Evidence concerning the mechanism of the amphibian water‐balance effect may be summarized as follows, (a) The evidence for an increased water uptake through the skin following an injection of neurohypophysial extract appears satisfactory. However, further analysis of this extrarenal effect seems desirable. (b) Evidence for a renal inhibitory effect is suggestive but not unequivocal, (c) Evidence for a direct effect on the tissue imbibition is lacking. (4) The specificity of the neurohypophysial water‐balance effect with particular reference to the effect of other endocrine glands has been discussed: certain steroid hormones have been reported to produce slight increases of body weight in amphibians, but the mechanism of these increases has not been investigated. (5) Data referring to the influence of other hormones on the water‐balance effect of neurohypophysial extracts have been considered: it is pointed out (a) that in most of the experiments concerned the endocrine preparations were not “homologous, and (b) that the majority of the experiments were of very short duration. However, it has been shown that treatment with thyroxin for several days enhances the neurohypophysial water‐balance effect in winter frogs. (6) Subtotal hypophysectomy has been observed to produce polyuria in frogs and toads kept in water; the body weight of hypophysectomized animals is not significantly affected. Hypophysectomized frogs kept in dry surroundings do not seem to lose water more readily than normal controls. However, the probably unavoidable hypothalamic lesion and the removal of the adenohypophysis in the course of the operation obscures the issue. (7) There are not sufficient data to permit the formation of a theory regarding the physiological significance of the neurohypophysial water‐balance effect in amphibians. However, the fact that frogs in dry surroundings are less liable to lose water when injected with the oxytocic fraction of mammalian posterior pituitary extracts may be considered as supporting the suggestion that the response to the neurohypophysial water‐balance principle is an adaptation to an amphibious life. The absence of a water‐balance effect in wholly aquatic animals (fishes) and wholly terrestrial animals (mammals) may be noted in this connexion. (8) The principle producing a temporary increase of body water in amphibians (water‐balance principle) has been found in the neurohypophysis of all vertebrate classes investigated. The content of water‐balance principle has been quantitatively determined in the pituitary glands of elasmobranchs, marine and freshwater teleosts, amphibians, reptiles, birds and mammals. Cyclostomes have not been examined. (9) The water‐balance principle has been shown to be not identical with the mammalian posterior pituitary antidiuretic hormone and to be mainly contained in the oxytocic fraction. Its identity wit

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00447.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY

摘要:

SummaryThe enzymatic methylation or demethylation of naturally occurring compounds represents significant metabolic processes which have gained additional interest since the discovery of a mechanism by means of which methyl groups can be transferred from one compound to another.The capacity to transfer methyl groups is confined to a fewN‐and S‐methylated compounds; methyl groups linked to C atoms, however, have not been found to be transferable but are known in some cases to be oxidized. OtherN‐methylated compounds, such as sarcosine, the methylated ammonium bases, the α‐ and γ‐betaines of amino‐acids other than glycine, are unable to take part in the methyl exchange in either direction, but are oxida‐tively demethylated by the organism or excreted unchanged. Choline, methionine, and to a lesser degree betaine, play a major part in transmethylation; these compounds are needed for the maintenance of growth and for the prevention of fatty infiltration of the liver, kidney haemorrhage, liver cirrhosis, or perosis. Since choline can methylate homocysteine to methionine in the organism and conversely methionine and betaine are able to supply methyl groups for the methylation of ethanol‐amine to choline, a system of continuous interchange of methyl groups is established and a ‘pool’ of labile methyl groups is formed. This ‘pool’ also acts as a source of methyl supply for other compounds such as creatine, methylnicotinamide, anserine and possibly others, but these latter compounds have not been shown to act as ‘methyl donors’. The role of glycine and of methylated purines as methylating agents is controversial.The liver is the main site of methylation, though kidney and muscle tissue were found to catalyse methylation in a few cases. The chemical mechanism of methylation and demethylation has not yet been clarified. Oxidation prior to demethylation is believed to take place in the case of methionine. A methyl synthesis from unmethylated sources is possible but not probable; as a consequence methylated compounds with labile methyl groups must be regarded as essential dietary factors.The physiological significance of the numerous naturally occurringN‐methylated compounds is further enhanced by their pharmacological effects and by their relationship to hormones. The latter manifests itself in the preponderant occurrence of many methylated compounds at the sites of hormonal activities connected with reproduction, and conversely by the alleged effect of some hormones on methylating activities. Thus it is not unreasonable to picture an influence of methylating reactions on the regulatory activities of hormones leading in pathological cases to a derangement of the hormonal balance and possibly to abnormal growth conditions such as malignancy.I wish to express my gratitude to Dr E. M. Crook, Kothamsted Experimental Station, for reading the manusc

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00448.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY