A manual of veterinary physiology

Front matter

..

lillllP'^

CORNELL UNIVERSITY.

Preface To The Third Edition

Circumstances beyond my control have delayed the revision of this manual. During the twelve years which have elapsed since the last edition was published considerable additions have been made to our knowledge of physiology. This has necessitated the manual being practically rewritten ; only the chapters on the Senses, Locomotion, and the Foot stand nearly as they were ; the others have been partly or wholly rewritten.

This book is essentially a veterinary, and not a comparative, physiology. It treats of physiology not only from its theoretical aspect, but from the point of view of clinical utility. The requirements of the student and practitioner have consequently not been lost sight of, and every opportunity has been taken in the text to point out the clinical application of physiological facts. To several chapters a special pathological appendix is added, in order to enforce the lesson that pathology is only physiology out of health. In the chapter on the Nervous System the appendix has been omitted, not because the pathological side is wanting in interest, but for the reason that it is at present so defective in exactitude.

As in previous editions, the horse has been taken as the type. Though he offers so many physiological peculiarities and differences from other animals, still his physiology among quadrupeds must always be of the first importance, and of the greatest practical interest.

By the process of elimination and compression, room has been found for much more material than existed in the previous editions, without adding unduly to the bulk of the book. The digestive system, owing to its extraordinary importance in herbivora, is dealt with very fully.

My cordial thanks are due to Professor Sherrington, F.R.S., who has again very kindly read the Nervous

vii viii PEEFACE TO THE THIRD EDITION

System, and contributed some new matter on the ' Scratch Reflex ' and ' Stepping ' in the dog.

My friend Dr. Sheridan Lea, F.R.S., has taken a deep interest in the production of this edition. As an old teacher of physiology he was able to advise me of those points which most students find some difficulty in grasping, and he has rendered the text of these portions clearer and more accurate by his careful revision and additions. He has kindly read all the proofs, amplified the chapter on the Muscular System, and brought the final chemical chapter up to date. I am glad of this opportunity of thanking him for his criticism and invaluable assistance.

Mr. Goodall, F.R.C.V.S., Christchurch, and Mr. Leeney, M.R.C.V.S., Hove, have both been good enough to supply me with information for the chapter on Generation and Development, based on their special experience. For the information contained in the footnote on p. 608 I am indebted to Mr. Leach, F.R.C.V.S., Newmarket.

As a rule no references have been made in the text to published works and papers, excepting where such appeared desirable. The literature of the subject is immense, but I must not omit to mention the help I have obtained from the Manuals and Text-books on Physiology published by Professors Halliburton, Howell (of Baltimore), Starling, Stewart (of Chicago), and from Dr. Leonard Hill's ' Recent Advances in Physiology and Bio-Chemistry.'

The number of figures in the text has been considerably increased, some of them being new and original. I am indebted to Professor Stewart for permission to use many of those illustrating his 'Manual of Physiology,' to Professor Cossar Ewart, F.R.S., for the figures illustrating the early embryos of the horse, and to Messrs. Macmillan for the use of certain figures in Foster's ' Text-book of Physiology ' and Huxley's ' Lessons in Elementary Physiology.' My thanks are also due to Messrs. Stahel of Wiirzburg, for permission to publish Fig. 159 from Dr. Michel's paper 'Zur Kenntnis der Giirber'schen Serum-Albumin-Krystalle.'

F. S.

London, August, 1907.

Preface To The Second Edition

I greatly regret the considerable delay which has occurred in the production of this edition, but it has been unavoidable ; the work has been rewritten in order to admit of its scope being enlarged, and this has taken longer than I anticipated.

In the revision of the sheets I have received great assistance from Professors M'Kendrick, Halliburton, Haycraft, Sherrington, and Dr. Waller. Professor Sherrington revised the whole of the Nervous System and supplied Figure 58. Professor Mettam, of the Eoyal Veterinary College, Edinburgh, kindly wrote the chapter dealing with the Development of the Ovum, while to Professor Macqueen, of the London Veterinary College, I am indebted for many useful suggestions and valuable criticism.

To all these gentlemen I offer my cordial thanks ; their corrections, suggestions, and criticisms have been of the greatest help, and cannot fail to enhance the value of the book.

As in the first edition, I have avoided dealing with histology, excepting where such was necessary to the clear understanding of the subject under consideration.

After due deliberation, I determined not to introduce for the present the metrical system of weights and measures.

The number of illustrations has been doubled, and for electrotypes of blocks I am indebted to Professor Foster, of Cambridge ; Professor M'Kendrick, of Glasgow ; Professor Hamilton, of Aberdeen ; and Dr. Waller, of London.

The rewriting of this edition has unfortunately necessitated an increase in the size of the book.

Woolwich,

September, 1895.

Preface To The First Edition

My object throughout this manual haB been to condense the information as much as possible, for which purpose I have omitted all special reference to the physiology of the dog, and have not touched upon the histology of the tissues, or methods of physiological inquiry.

The reasons for these omissions are obvious : special canine physiology is of subordinate interest to the profession, and our information about this animal is so complete, that when required no difficulty is experienced in obtaining it from human text-books. The histology of the tissues is already before the profession, and methods of physiological inquiry are only needed for laboratory work, for which purpose this book is not intended.

In the description of the physiology of the various organs and tissues the horse is necessarily taken as the type, but the ox, sheep, and pig are dealt with wherever their special physiology requires it. •

It was my original intention to publish nothing until I had gone over the field of equine physiology, but I found after several years of work, that the information I had collected was a mere drop in the ocean, for inquiries of this kind are necessarily slow, and as there appeared no reasonable prospect of covering within the space of one life the ground I had mapped out, I was advised that only good could result from placing on record what little we know of veterinary physiology.

I have, therefore, ventured, I know well how imperfectly, to state the broad facts of the science, so as to render them of use to the student and practitioner. The work does not pretend to be anything more than a stepping-stone to the study of physiology ; for those requiring more detailed information, reference must be made to the various text-books of human and comparative physiology which are available.

xii PEEFACE TO THE FIEST EDITION

Incomplete as the work is, it would have been still more so but for the assistance I have received from my friend Dr. Sheridan Lea, F.E.S., of Caius College, Cambridge, who, at great personal inconvenience, has kindly read and revised nearly all the sheets as they passed through the press. In saying this, and expressing to him my very great indebtedness, I in no way wish to shift the responsibility for error or inaccuracy which may exist, but I feel that whatever merit the book possesses is entirely due to him.

I have to thank Professor Michael Foster, F.E.S., for the loan of many of the woodcuts which illustrate this manual, and elsewhere I have acknowledged how much I owe to his encouragement.

To my friend and colleague, Assistant-Professor Butler, A.V.D., my best thanks are due for assistance in revising the proofs, and in the preparation of the index ; to Mr. W. Hunting, F.B.C.V.S., for suggestions on the chapter dealing with Locomotion ; and to Professor M'Fadyean for the loan of two woodcuts illustrating the chapter on the Foot.

To facilitate the study of locomotion, I have had the plates so arranged as to face as nearly as possible the letterpress describing the movements.

I have laid under contribution Colin's invaluable ' Traite" de Physiologic compared des Animaux ' ; Ellenberger's 'Physiologie der Haussaugethiere ' ; Foster's, M'Kendrick's, and Landois and Stirling's Text-books of Physiology; Gamgee's translation of ' Hermann's Physiology ' ; the same author's ' Physiological Chemistry of the Animal Body '; Halliburton's ' Text-book of Chemical Physiology and Pathology ' ; Bunge's ' Physiological and Pathological Chemistry ' ; Meade Smith's ' Physiology of the Domestic Animals,' and others mentioned in the text. With reference to Dr. Meade Smith's work, I regret to find that on page 105 I have inadvertently given the title as ' A Text- book of Comparative Physiology.'

I have endeavoured to acknowledge all sources of information, though it is possible that in drawing from such a wide area I may have omitted in places to do so.

Army Veterinary School, Aldershot, August, 1892.

Contents

CHAPTER PAGE

I. THE BLOOD ------ 1

II. THE HEART- - - - - - 28

Chapter I //

The special functions of the blood are to nourish all the tissues of the body, and thus aid in their growth and repair ; to furnish material for the purpose of the body secretions, to supply the organism with oxygen, without which life is impossible, and finally to convey from the tissues the products of their activity. To enable all this to be carried out the blood is constantly in circulation, is rapidly renewed, is instantaneously puxi^ed in the lungs and, by means of certain channels, is placed directly in communication with the nourishing fluid absorbed from the intestines, by which it is constantly repaired.

Physical Characters. — Blood is a red, opaque, rather viscous fluid, the tint of which depends upon whether it is drawn from an artery or a vein ; in the former it is of a bright scarlet colour, whilst in the latter it is of a purplish red. The colour is due to a pigment called haemoglobin contained in the red corpuscles. Whether the colour is scarlet, as in blood from an artery, or purplish, as from a vein, depends on the difference in the amount of oxygen with which the haemoglobin is combined.

The reaction of blood is alkaline; as the process of coagulation occurs this alkalinity diminishes. The alkaline reaction is due to the phosphate and bicarbonate of soda found in the fluid; the decreasing alkalinity observed on standing is probably due to the formation of an acid. The alkalinity of the blood is reduced by muscular work, owing to the production of an acid by the muscles.

The o^pjir of blood is believed to be due to a volatile body of the fatty acid series. The blood of the cat and dog has a peculiar and decidedly disagreeable smell ; this is not observed in the blood of the horse and ox, though it is said that the odour of butyric acid can always be obtained from the blood of the latter by heating it with sulphuric acid. The taste of blood is saltish, due to the amount of sodium chloride it contains.

The specific gravity varies in different animals : in the horse, ox, and pig, 1060 ; sheep, 1050-1058 ; dog, 1050 (Colin). According to Hoppe-Seyler the specific gravity of the liquor sanguinis of the horse is 1027 to 1028, and the specific gravity of the cells 1105. This considerable difference between the specific gravity of the cells and the liquor sanguinis in the horse, accounts for the rapid manner in which the cells sink in horses' blood when drawn from the body, producing during the process of clotting the so-called ' buffy coat.'

The composition of the blood is almost absolutely uniform so far as the presence of various substances is concerned ; the amount of these substances, however, varies in animals of different classes. The source from which the blood is taken also affects its composition/; the blood from an artery does not represent exactly that found in a vein.

Blood consists of :

1. A fluid part, Liquor sanguinis or Plasma, containing in solution

proteids, extractives, mineral matter and gases, the latter in a state of loose chemical combination.

2. Corpuscles. a. Red corpuscles.

/3. White corpuscles. 7. Platelets.

The liquor Sanguinis, or Plasma, forms about 66 per cent, of the total blood ; it is an albuminous fluid containing a small and variable amount of a yellow colouring matter of

TliJU KLiVVV 3

a fatty nature. It holds in solution three proteids — viz., fibrinogen, serum globuhn (paraglobufin)," and seram albumin. Eecent researches have shown~that what has always been regarded as serum globulin consists in reality of two proteids, to which distinctive names have been given.

It is a simple matter to separate these proteids from plasma, as they are differently acted upon by neutral saltg. For example, fibrinogen is precipitated by balf saturation with common salt, serum globulin is precipitated by saturation with magnesium sulphate, serum albumin may be wholly precipitated by ammonium sulphate.

During the life of the blood the liquor sanguinis is termed the plasma, but after it has been shed from the body and coagulation has occurred, the liquid residue is called serum. Serum is, therefore, plasma which is modified as the result of coagulation, and as this latter process is attended by the production of fibrin, we may say that serum is plasma minus the fibrin-forming elements. Perhaps the nearest approach to pure plasma is the fluid found in the pericardium and abdominal cavity.

The fluid effused into the pleural cavity during pleurisy is plasma to start with, but if the fibrin in it becomes thrown down (forming the so-called false membranes), the remaining fluid is serum which is no longer capable of clotting.

The Proteids of Serum are serum globulin, serum albumin, and a ferment produced as the result of coagulation. As fibrinogen is used up in the process of coagulation it is not found in serum, but a proteid known as fibrino-globulin appears, though in small quantities. Fibrino-globulin is produced from fibrinogen during the process of fibrin formation. In the following table a comparison is made between the proteids of the plasma and serum :

Proteids of the Plasma. Proteids of the Serum.

Fibrinogen. Serum globulin.

Serum globulin. Serum albumin.

Serum albumin. Fibrin ferment (nucleo-

proteid).

Fibrino-globulin. 1—2

It has been shown that the proportion in which serum globulin and serum albumin exist in the blood varies in different animals. In the horse and _ox the globulins are in exgess of the alburn-ins ; in man and the rabbit this is reversed. Analyses show that the amount of total proteids is rather more uniform than is that of the different albumins of which they are composed, as may be seen from the following table, which represents the weight of proteid (grammes in 100 c.c.) of blood plasma of different animals :

Total

Serum

Para-

Proteids.

Albumin.

globulin.

Fibrinogen.

Dog -

- 6-03

3-17

2-26

0-60

- 7'29

3-83

3-00

0-46

- 8'04

2-80

4-79

0-45

Pig -

- 8'05

4-42

2-98

0-65

Fibrinogen is the precursor of fibrin, a substance of which we shall have more to say when dealing with coagulation ; it is found in blood plasma, but not in the serum, since it is converted into fibrin during the process of clotting ; it also" exists in the fluids exuded into the cavity of the chest, pericardium, etc.

Corpuscles. — Blood examined under the microscope is found to consist of an enormous number of bodies termed corpuscles floating in the liquor sanguinis. These corpuscles are of two kinds, rgd and white ; the former are the more numerous, the latter are the larger.

The Red Corpuscles constitute 33 per cent, or one-third of the total blood. Viewed under the microscope, they are found to be biconcave jdiscs, circular in shape, and possessing no nucleus (Plate I.) ; they are soft, flexible, elastic bodies, capable of having their shape""readily alterecTby pr§saure, which enables them to pass along the~finest capillaries. The colour of a single corpuscle is yellow, but when heaped together, they are red, and thus "give the colour to the blood.

In all mammals excepting the Camel tribe the red cells are circular ; in all verieht&tes below mammals they are biconyex, oval, and nucleated. The corpuscles vary in size in different animals, as may be seen in the diagram (Fig. 1). When a drop of blood is shed, the red cells at first move quite freely each over the other. In a short time they tend apparently to become sticky, and when this state is reached they have a tendency to lie in long rows, with their flat surfaces in close contact, resembling the appearance of a pile of pennies. This condition is not marked in horses' blood.

A red blood-cell is composed of a spongy stroma, holding in its meshes the red colouring matter. The stroma or

ELeflllcmf

■O09i-mm

M an

•0077

■0 055

■0 050

■0041

- - - Musk-deer

■ 0025

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Fig. 1. — Diagram showing Eelative Size of Red Corpuscles of Various Animals (Stewart).

framework of the corpuscle consists chiefly of nucleoalbumm, and also contains lecithin, cholesterin, and salts ; the r^ed colouring matter consists of an albuminous. crysEalline substance, haemoglobin, which forms no less than 90 to 94 per cent, of the total solid matter of the dried corpuscle.

The number of corpuscles in the blood is determined approximately either by the method of Gowers or Malagsez. The principle on which these methods are based is the same — a known quantity of blood is diluted with a known bulk of artificial serunTand thoroughly mixed ; of this a small drop is placed in a counting-chamber, which is ruled into squares, and examined under the microscope. The blood cells occupying the squares are counted, as may readily be done, and the mean of them taken. In the horse the mean number of red blood corpuscles per cubic millimetre is 7,212,500, and in the ox 5,073,000. Taking the amount of blood in the horse as 66 lbs. (50 pints or 29 litres), this gives 204,113,750,000,000 as the approximate number of red cells in the body (Bllenberger).* It is evident that a loss of water from the blood means a larger relative proportion of red cells present, while an excess of water by diluting the blood would show a loss of red cells ; thus the number of the red cells is increased by sweating, by the excretion of water from the bowels and kidneys, and by starvation, while it is diminished by pregnancy and copious draughts of water. But apart from these conditions, it is undoubted that an actual increase or decrease in the number of red cells may occur, this numerical variation being especially marked in some diseases. The shape of the red cell is affected by the amount of fluid in the plasma — if the latter be artificially concentrated water diffuses from the corpuscle to the plasma, and in consequence it shrinks and becomes wrinkled. If the plasma be diluted the red cells swell. A *9 per cent, solution of sodium chloride causes the corpuscles neither to shrink nor swell ; this strength is known as ' physiological salt solution,' and may be employed for the purpose of transfusion.

Each red cell offers a certain absorbing surface for oxygen, which, if calculated on the total number of corpuscles, is something enormous, being equal for the horse to a square having a side of 180 yards. The opacity of blood is due to the red cells reflecting light as the result of their peculiar shape; if the cells be destroyed either by freezing and thawing the blood alternately, or by the passage through it of electric shocks, or by the addition of certain agents such as chloroform, ether, bjle salts, water, tannic or b^ric acids, etc., the haemoglobin becomes liberated from the broken-up cell and stains the naturally yellow plasma of a red colour. Further, the destruction of the corpuscles leads to the blood becoming transparent or, as it is termed. * laky.'

The greater part of the red cell consists, as already stated, of hasmoglobin, a substance possessing a remarkable affinity for oxygen ; this it obtains at the lungs and leaves * ' Physiologie der Haussaugethiere.' Digitized by Microsoft® behind it in the tissues. The haemoglobin of the red cells, therefore, exists in two states, one in which it is charged with oxygen called oxy-haemoglobin, and the other in which it has lost its oxygen and is known as reduced haemoglobin. The process of oxidation in the lungs and reduction in the tissues is constantly occurring at every cycle of the circulation, with the ultimate result that the red blood disc gets worn out and dies. In this condition it is cast off from the system, being got rid of through the medium of the liver, and also, probably, destroyed in the spleen and elsewhere. When the red cells die their hsemoglobin is set free, and decomposed into an iron-free residue from which, probably, all the pigments of the body are formed, especially those of the bile.

The production of red cells is a matter of extreme rapidity, as may be witnessed,, for example, after haemorrhage ; the seat of their formation is in the red marrow of bones, where they are formed from certain nucleated cells ; there are several varieties of cells in the red marrow, and it is not quite definitely settled which of these furnish the red blood cells. All other seats of formation are doubtful, though it should be mentioned that the formation of red cells from blood platelets in the blood stream has been put forward as of possible occurrence. In the embryo the future red cells for a certain period are nucleated and contain no hsemoglobin, but these are gradually replaced by non-nucleated haemoglobin-holding corpuscles before birth. It is interesting to observe that both in the embryo and in the adult the red cells are derived from a nucleated precursor.

Blood Platelets are small bodies one-quarter the size of a red cell, which have been observed in the circulating blood, but can al$o be seen immediately after the blood is shed. They have been supposed by some to be the precursors of the red cells, but this point is not as yet settled. We have mentioned that the red colouring substance haemoglobin is retained in the pores of the stroma of the red cells, and with this we must now deal.

Haemoglobin is a most remarkable substance. It is a proteid, distinguished from the majority of the other members of its class by the comparative ease with which it may be obtained in a crystalline form, whilst, on the other hand, its behaviour in a dialyser is not that of a crystalloid but of a colloid. It is one of the most complex substances in organic chemistry, containing C, H, 0, N, S, and Fe, and its molecule is an enormous one, the molecular weight being quoted at 13,000 to 14,000. Crystals of haemoglobin when seen in bulk are of a dark-red or bluish-red colour ; they are extremely soluble in water, the solution being dichroic — viz., green by reflected and bluish-red by transmitted light. Hsemoglobin is remarkable as being the most important proximate constituent of the body containing iron, the amount being about "4 per cent. The source of the iron is not settled, but there is an organic iron-containing substance in food known as licematogen, belonging to the nucleo-albumin group, which possibly furnishes it. Its formation from inorganic iron is probably of doubtful occurrence.

The total amount of haemoglobin in a horse's body is about 8'8 lbs. (4 kilogrammes), and the amount of iron contained in this is about 257 grains (17 grammes). This calculation is based on the assumption that the amount of blood in the body is 66 lbs.

In the dried red blood cells haemoglobin exists in the proportion of 90 to 94 per cent., in the corpuscle under normal conditions it represents 32 per cent, of its weight, while in the total blood of the horse it forms 13-15 per cent., in the ox 9"96 per cent., sheep 1034 per cent., pig 12-7 per cent., and dog 9"77 per cent. (Ellenberger).* The younger the animal the less haemoglobin ; males have more than females, and castrated animals more than entires (G. Miiller).t

Haemoglobin has a remarkable affinity for oxygen, and

the ordinary laws relating to the absorption of gases by

fluids and solids do not apply— as we shall see later when

* ' Physiologie der Haussaugethiere.' -(■ Ibid.

dealing with Eespiration — to the absorption of oxygen by haemoglobin. According to Bohr, haemoglobin can absorb carbon dioxide, which combines with the globulin portion of the molecule.

We have mentioned that when haemoglobin is charged with oxygen it is spoken of as oxy-haemoglobin ; when it has" discharged its oxygen, which it is capable of doing with considerable facility, it is described as reduced haemoglobin, or simply as haemoglobin. As oxy-haemoglobin it is charged with oxygen in the capillaries of the lungs, brought back to the heart and distributed all over the body ; in the tissues it gives up its oxygen, and as partially reduced haemoglobin is brought back by the veins to the heart for distribution to the lungs, where it renews its oxidized condition. Haemoglobin is never completely reduced in the body excepting in the last stage of asphyxia.

Oxy - haemoglobin crystallises in

some animals, horse, cat, dog, and

guinea-pig, with facility; in others,

ox, sheep, and pig, with difficulty.

The crystals are generally rhombic

plates and prisms, but the form

differs according to the animal FlG- 2.— Crystals op

(Fig. 2). Beduced haemoglobin can ^^6^0™'

only be crystallised with great difficulty rel; c, Guinea-

, , „ n pig (Stewart).

in an atmosphere free from oxygen. v '

When examined by the spectroscope the two haemoglobins produce quite distinctive spectra, by which they may be readily recognised. To put the matter broadly, oxy-haemoglobin gives two well-marked dark absorption bands or shadows in the green portion of the spectrum, one band being wide, the other narrow ; while reduced haemoglobin gives one wide single band in nearly the same position (Fig. 3). Oxy-haemoglobin may readily be reduced to haemoglobin by the addition of Stokes's Fluid (an alkaline solution of ferrous tartrate).

Oxygen and haemoglobin are so lightly bound together that they are readily separated ; oxygen is given off if the blood be placed in a vacuum or boiled, or if it be brought into contact with indifferent gases such as nitrogen and hydrogen ; it is the facility with which haemoglobin parts with it's oxygen which enables the tissues to obtain it.

Hemoglobin forms certain compounds with oxygen, carbon monoxide, and nitric oxide :

With oxygen it forms oxy-haemoglobin and methssmoglobin. „ carbon monoxide it forms CO haemoglobin. ,, nitric oxide „ NO „

BCD B *

a

Oxy-hfe-

moglobir

G

Haemoglobin (reduced

E

1: ,_

J

FIG. 3.— Blood Spectka (Waller).

Oxy-haemoglobin we have dealt with ; the others, in a work of this kind, can only receive a short notice at our hands, though the subject is full of interest.

Methaemoglobin is produced by allowing blood to be exposed to the air until it becomes brown in colour and acid in reaction ; or it may be prepared by the action of acids or alkalies on oxy-b.£emoglobin. This substance separates from its oxygen with difficulty, and gives a three-banded spectrum. Methasmoglobin does not occur normally in the body, but may be found in the urine whenever a sudden breaking down of red corpuscles occurs, as, for example, in the so-called azoturia of the horse.

Carboxy-hsemoglobin. — In this compound the oxygen is replaced by carbon monoxide, which forms a stable compound with the hemoglobin and is not displaced on breathing oxygen ; hence the rapidly fatal results of this form of poisoning. The blood of people who have died from CO poisoning is of a cherry-red colour, and yields the spectrum of CO-hsemoglobin — viz., two bands very much like those of oxy-hsemoglobin, though somewhat darker and situated slightly nearer to the violet end of the spectrum. These two bands are unaltered by Stokes's Fluid. Nitric oxide haemoglobin in many respects resembles CO-hsemoglobin.

Hsemoglobin is easily decomposed either by boiling or the addition of alkalies, acids, or acid salts ; in either case it splits up into a substance containing the iron, known as hsematin, and a proteid substance or substances termed globin. Hsematin in the dry state strongly resembles iodine in appearance ; it has a metallic lustre, a blue-black colour, is not crystallisable, and yields, when pulverised, a dark brown powder which contains 8'82 per cent, of iron. Hsematin is a remarkably stable substance, and the colouring matter presents a distinctive spectrum both in an acid and alkaline solution. Alkaline solutions of hsematin can take up and give off oxygen like hsemoglobin. When hsematin is treated with glacial acetic acid and common salt it yields hsemin, which, when examined microscopically, is found to consist of prismatic crystals, dark, or nearly black in colour. Hsemin crystals may be readily produced by warming the dried blood with a drop of glacial acetic acid on a slide ; this is used as a microscopical test, but it is said that from the blood of the ox and pig hsemin can only be obtained in very irregular crystalline masses.

When reduced hsemoglobin is decomposed by acids or alkalies, oxygen being carefully excluded, it yields hsemochromogen, a substance presenting a definite spectrum and thus a ready means of detecting old blood-stains. Haematoporphyrin is obtained by the action of strong sulphuric acid on hsematin, which thereby loses its iron ; hsematoporphyrin is really hsematin from which the iron has been removed ; it is isomeric with bilirubin. L