Book - Buchanan's Manual of Anatomy including Embryology 1
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Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.
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Chapter I Terminology and Relative Positions
The word anatomy, used in the strict sense, implies the knowledge of what can be studied by the process of dissection. Morphology is the name given to the more scientific aspect of anatomical research —the reason why, and the method by which, the various anatomical structures have reached their present conditions. In modern times the term “anatomy” is frequently used in a wide general sense, covering both descriptive and morphological anatomy. Comparative anatomy, the structure of animals other than man, is an essential foundation for human morphology. Human embryology, the study of the development of the body, is an extension of direct anatomical investigation on the one hand, and on the other hand, particularly when the study of human and other types is carried on together, it is a second fundamental support for the superstructure of morphology. Histology, the examination and study by the microscope of the minute structure of tissues and organs, is microscopical anatomy, anatomical research pushed to the farthest visual limits of practical value.
Anatomy, the plain descriptive structure of the body, is a subject of primary importance to the medical man. If he wishes to understand what has happened—either from injury or disease—to produce some abnormal physical condition, he must first know the normal state; this knowledge is necessary for scientific diagnosis, and frequently for scientific treatment. Hence it comes that the well-equipped physician or surgeon possesses a good working knowledge of the anatomy of the body as well as of the functions of its parts, some information on morphology to give him a deeper understanding of the matter, and a sufficient acquaintance with histology and embryonic development to enable him to grasp more fully some of the pathological problems which he has to consider. It may be said here, for the benefit of those who are as yet students, with little or no clinical experience, that there are practically no items of anatomical information which may not be of value—to-morrow, if not to-day—if they are known; their value cannot be apparent if they are not known. The more detailed the knowledge possessed by the practitioner, the more useful, and the more frequently useful, he will find it, and the firmer will be the standpoint from which he views the clinical problems before him.
The anatomy of the body must be studied on the actual human frame itself. It may be pointed out in this connection that dissection and investigation of the dead body ought to be amplified by study of the living body where this is practicable. Textbooks on anatomy must be looked on as mere explanatory guides to actual anatomy, as summaries and co-ordinations of what is observed during this actual study, and as convenient references when occasion arises; they must not be considered to be anatomy itself.
The descriptions of anatomical findings, whether in a book or as the record of some investigation, must rest on certain fixed and recognized foundations. Unless this rule is respected the anatomical description, which ought to be absolutely exact and unmistakable, becomes chaotic and misleading. One of the first things that the student of anatomy must learn and understand is the proper use and meaning of terms of position; these have fixed and definite values, and should never be used in any other sense than those recognized by anatomists as correct.
Fig. i. Human Figure to illustrate the ‘ Formal Position ’ of Anatomy. The diagram on the right is a section through the trunk to show planes, etc. For description refer to text.
In the first place, the body, for purposes of description , is always supposed to be standing upright (Fig. i), with the arms by the side, the palms looking forward, and fingers straight, and the legs and feet close together and fully extended. This is the * formal position * of anatomy, and the description of any part or organ is always put into language applicable to this formal position, whatever may be the actual position of the body during dissection, or during the progress of the description. The body is not lying on its back, or in any other dissection position, when it is being spoken about, but is supposed to be standing up. This mental adoption of the formal position must be understood thoroughly by the student if the statements he hears and reads are to be intelligible to him.
The position of the body for descriptive purposes being fixed, it is now possible to employ fixed terms of relative position which will always have the same meaning, whatever may be the actual position of the body at the moment. These fixed terms deal with the three dimensions in space, and are therefore six in number; variants exist for many of the terms, but this does not, of course, increase the number of meanings implied. The terms are: Superior and inferior, anterior and posterior, medial and lateral. Their use and meaning can be illustrated and understood in this way. In Fig. i are shown two points, A and B; one of them is higher than the other when the body is in the formal position, so it is said to be above or superior to the other, which is inferior to it or below it. Again, it can be seen that A is not directly above B, but is nearer the middle line of the body, so that it is not only above, but is medial to B, which is lateral to it. It is very important to recognize that these terms refer to nearness or distance from the median plane of the body, and have nothing to do with the middle line of any of its parts, such as a limb; for example, of the two points C, D on the arm of the figure, C is medial to D because it is nearer the middle line of the body when in the formal position, whereas it is farther away than D from the middle of the limb. The transverse section of the trunk in Fig. i shows two points, E and F; of these, E is nearer the posterior surface of the body than F, so is said to be behind or posterior to it, and F is in front of or anterior to E. This is, of course, in addition to the medial and lateral relations they evidently possess also.
The fixed terms of relative position, then, refer to the formal position of the body, and their meanings can be shortly stated as follows:
Above : nearer to the top of the head.
Below : nearer to the soles of the feet.
Medial: nearer to the median plane of the body.
Lateral : farther from the median plane of the body.
Anterior : nearer to the front surface of the body.
Posterior : nearer to the posterior surface of the body.
Many synonymous terms are used in place of these, giving a variety of choice, but no difference in meaning; those in commonest use for medial and lateral are internal and external, and dorsal and ventral for posterior and anterior. Other descriptions have more limited applications; thus, cephalic and caudal are sometimes used to express nearness (on the trunk) to its upper or lower end, as the case may be; proximal and distal are employed in the limbs to signify a position nearer to, or farther from, the attachment to the trunk; volar or palmar is often used in place of anterior when dealing with the hand, and sometimes (though improperly) when speaking of the forearm; and plantar and dorsal, in the foot, imply nearer to the sole or the upper surface of the foot respectively.
It must be pointed out here that these fixed terms of position have a different signification when used in strict early embryological description. In this case the embryo is described in terms of comparative anatomy— i.e., it is considered as lying on its ventral surface on the ovum; its dorsal surface is now above, its head end is in front, with corresponding changes in the meanings of below and behind. Medial and lateral remain as before, referring to relation to the median longitudinal plane. This strict embryological usage is only adopted as a rule, so far as human description is concerned, during the earlier embryonic period of development; after the third month, when the embryo is known as a foetus, it is usual to find the terms used in the adult sense.
In addition to the fixed terms there are two descriptive words, superficial and deep, which are not fixed in their meaning with regard to the whole body, but vary according to the way in which any part is dissected, looked at, or described. When used with proper care, to avoid doubt or confusion as to the meaning implied, they are terms of great descriptive value, and can frequently replace with advantage a more cumbrous employment of fixed terms. We can, for example, speak with much more convenience and brevity of the subcutaneous tissue as being deep to the skin than we would experience if we were to attempt to describe its relation to the skin all over the body by fixed terms. Speaking generally, the words superficial and deep a PPty fhy order in which things would be met with in the ordinary course of dissection from the nearest surface, but if there is any possibility of doubt as to what might be inferred from their use, the meaning should be defined clearly before proceeding further. It must be clearly understood that these terms, although in every case necessarily corresponding with some terms of fixed descriptive value, do not in each case necessarily correspond with the same terms, and they must never be used as if they possessed a fixed value of the same sort.
There can be no hesitation in repeating the assertion that the student must understand and become accustomed to the proper use these expressions of relative situation, and their application to the body m the formal position; otherwise he will find written descrip- tion apparently confused or even untrue, and he himself will not be able to give a clear and comprehensive account of any part he may wish o escribe. He must get rid of the tendency to looseness in expression and meaning which is so commonly found in ordinary conversation, an ie must beware especially of thinking that any of the terms of fixed relation are synonymous with expressions of superficiality or depth.
There are many words and expressions commonly used in anatomical science, but otherwise unfamiliar. A large number of these have a definite topographical application, and will be considered in the appropriate places; but others have more general reference, and may be dealt with conveniently in this chapter.
Among the terms used to describe position or relation there are several which have not been noticed so far, and call for explanation.
Coronal and sagittal are terms referring to vertical planes in the body, transverse or antero-posterior respectively in disposition. In Fig. i two sagittal planes are indicated at S and T, and two coronal planes at X and Y. But a sagittal plane may be in any place so long as it passes directly from before backwards, and a coronal plane may be anywhere between the front and back walls so long as it is at right angles to the sagittal direction, and the planes illustrated are only some out of an innumerable number. It is evident that the median plane is only one of the series of sagittal planes.
Prone and supine are words occasionally used. The former, applied to the body as a whole, is practically the same as ' lying on its face,’ and the latter term implies its position ‘ on its back.’ The terms are most frequently used in speaking of the upper limb; the hand and forearm, when in the ' formal position,’ as in Fig. i, is said to be supinated, and it is pronated when turned over on to its front surface.
Preaxial and postaxial only apply to the limbs. The axis referred to is the axis of the limb. The lateral border of the arm is its preaxial border, the postaxial being its medial border. But in the lower limb the preaxial border begins at the lateral side of the upper end, crosses the thigh obliquely, and passes down the medial side of the leg and foot; the postaxial border passes down and out to the lateral side of the foot. These borders are only descriptively true in early development, when the limbs are plate-like, and project from the body with definite cranial and caudal borders. They come in to the side later, but in the case of the lower limb the plate is twisted inwards and ventrally, so that the original dorsal surface comes to look ventrally, and the cranial (preaxial) border is turned towards the middle line. The twist is completed when the legs are brought straight down.
Terms of General Application
Normal is a word which, when applied to some condition, implies that it is the condition found in the majority of cases; it is frequently extended to cover common, though not most frequent, occurrences.
Abnormal, strictly speaking, means that the condition to which the word is applied is not that usually found. The term is often used as if it were synonymous with ‘ pathological ’ or ‘ monstrous.’ Such a mental limitation of the meaning of the word must be avoided; all monstrous conditions are abnormal, but all abnormalities are not to be classed as monstrous. Nevertheless, it must be admitted that it is very hard to draw a line of distinction when dealing with the more extreme degrees of departure from the normal.
Typical is a word which signifies that the thing to which it refers is one possessing all the characteristics of the ‘ type ' to which it belongs. In practical use it implies almost the same thing as the use of the word ' normal/ but it is not quite the same; for a ‘ typical' example of some region, for instance, may not actually correspond with any particular or individual region known to the observer, but may be more like an average summing up of several known regions. Normality, on the other hand, is a word essentially applied to individual instances.
Atypical, then, merely implies some definite departure from the state recognized as typical.
In dealing with the relations of structures with one another, and with similar or related structures in other animals, etc., certain terms are in frequent use, and call for some explanation.
Structures are often said to be homologous. Homology expresses the relation between parts which own for their origin similar embryonic or evolutionary structures, as in the case of a man's arm and the wing of a bird or the foreleg of a dog. Homogeny is a word with practically the same meaning and use as homology , but has reference more to the evolutionary side of development; thus it can apparently be used to throw back the resemblances between structures further than actual embryonic observation would seem to justify. Homogeny must not be confounded with homogenesis , which is simply the name of the production of like from like, as in the case of one animal producing a similar animal; nor with homogeneity , the quality of being homogeneous , which is a word used to imply that the thing described has a uniform structure or substance. It may be pointed out here that probably nothing is absolutely uniform in its composition provided that sufficiently searching methods of examination are employed, and that ‘ homogeneousness ’ is therefore a term of only relative value.
Serial homology is the name given to the relation between parts which are developed from structures that may be described as units in a series of things essentially similar. Thus, each vertebra is ‘ serially homologous ’ with other vertebrae, and each hypothetical segment of the body is the serial homologue of any of the others.
Homodynamy expresses the relation between structures which owe their existence, or their form, to the influence of similar forces or the serving of similar functions. Thus the arm and leg are homodynamic or homodynamous, though they cannot be said to be homologous.
Terms used in dealing with the formation or evolution of the body include:
Atavistic. — This implies the reversion, in some structure, to some peculiarity of a more or less remote ancestor. It is used in various ways in description. Certain structures, as, for example, some epiphyses on bones, appear to be degenerated representatives of better formed and functional parts in other animal types, and are classed as atavistic, although they are parts of the normal skeleton. On the other hand, the ' reappearance ’ of a structure normally absent in the human body, though existing in other types, is referred rather vaguely to ‘ atavism ’; the implications of such use of the term must not, however, be taken too strictly.
Phytogeny is the development of the body considered from an evolutionary standpoint, and has to do with the connection between human formation and that of types in or near the line of descent.
Ontogeny is the formation of the individual apart from his evolution. The ontogenetic development of an individual is, in a very general way, a recapitulation of his phylogenetic development, but any phylogenetic suggestions gained from ontogenetic study must be submitted to the tests of extended comparative embryological search before they can stand.
Other terms of more particular application will be explained as occasion arises.