Introduction
The study of vertebrate anatomy is an interesting and valid field of study for gaining insight into the structure and function of vertebrates. But why should this be important? Of the numerous reasons, we mention only a few.
• It provides us with knowledge of the structures of different organisms and the great variety of form among vertebrates.
• It allows us to examine how the form of these structures is related to their function and thus how morphology is suited to a particular mode of life.
• The characteristics or features of vertebrates preserve information on their ancestry: The features are modified and passed on through the course of generations, and we may use such knowledge to discover the genealogical relationships among vertebrates.
• Comparative anatomical studies help us to understand how the major transitions in vertebrate design might have occurred. Soft tissues do not fossilize, meaning that (with rare exceptions) only transformations of the hard parts of the vertebrate body are preserved in the fossil record. For other parts of the body, we must rely on a sequence of living forms. There are problems with this approach, but if we begin with a robust phylogenetic hypothesis and keep in mind that
the living members of some groups are highly derived, then we may be confident in this method as a reasonable approach for deducing the major steps in the evolution of different vertebrate groups.
We will consider all of these aspects in the following course on comparative vertebrate anatomy. Before beginning this study, however, there are several important terms that unambiguously describe position and direction. These indispensable terms greatly facilitate navigating through the complex three-dimensional structure of vertebrate bodies.
Directional Terminology and Planes of Section
As with all advanced fields of research, anatomical study requires the use of specialized terminology.
Such terminology includes not only special words for the anatomical structures themselves and concepts or processes (such as homology, for example) but also terms to designate unambiguously the orientation and direction of structures of the vertebrate body. These terms may at first seem superfluous, but that is because most people have never dealt with anatomy in a comprehensive and detailed manner. It is perfectly adequate in everyday life to say that the stomach is lower than the heart or the appendix is in the lower right part of the belly. But this is not anatomy. You will quickly come to realize the importance of the terms presented in this section, and you are urged to learn and become familiar with them.There are two main sets of terms. One is used in medicine and by some anthropologists, the other by comparative anatomists, paleontologists, and veterinarians. To compound the problem, various synonyms exist for some terms in each set. These circumstances may be cause for confusion, but we may simplify matters by adhering to one set of terms. As we are studying comparative anatomy, we will use the system commonly used for nonhuman vertebrates.
Unlike humans, the vast majority of vertebrates go through life with the long axis of the body oriented horizontally or parallel to the substrate. It is with reference to this position that the main directional terms are defined. Most of these terms are coupled; that is, there are two terms that describe opposite directions along a single axis. Refer to Figure 1 while reading through the
FIGURE 1 Directional terms and main planes or sections through the body shown on a horse.
following explanations. Anterior and posterior refer to the horizontal longitudinal axis and respectively designate the directions toward the head and tail. Synonyms for these terms that you may encounter are cranial or rostral for anterior, and caudal for posterior.
The vertical direction toward the belly or the ground is ventral; toward the back or up is dorsal. Medial refers to the horizontal direction toward the sagittal midline (see below) of the body, whereas lateral refers to the directions away from the midline. These are the main terms, but there is another set that is useful. Proximal and distal are terms usually used with a particular reference. At times this reference may be the trunk of the body; at other times a particular structure, such as the heart, may be the reference point. Proximal designates a position closer to the trunk or structure of reference, and distal furthest from the trunk or structure of reference. Thus, for example, the fingers (phalanges) are distal to the upper arm (brachium); and the proximal end of the brachium is that end closest to the trunk. If the reference point is another structure, say the heart, then the proximal part of a blood vessel is the part closest to the heart, and the distal end is that part furthest away.Combinations of these terms may be used, and indeed are used often in this manual, to describe directions that are oblique to the main axes. For example, anterolateral combines anterior and lateral, and indicates a simultaneous direction toward the head and to the side. Thus, taking the umbilicus (navel or bellybutton) as a reference, we may describe the shoulder as anterolateral to the umbilicus. Figure 2 provides examples of these terms.
Dissection often involves cutting the body in various planes to obtain internal or sectional views, which are extremely useful for comprehending the spatial arrangement of structures. There are three main sections or planes that pass through the body (Figure 1). The sagittal section is vertical and lies in the midline longitudinal of the body. It separates the body into right and left halves. Sections that are parallel to and on one side of
the sagittal plane are termed parasagittal. A second major section is in the transverse plane, which is also vertical but is perpendicular to the sagittal plane. A transverse section cuts across the main longitudinal axis and subdivides the body into anterior and posterior parts. The last major section is in the frontal plane, which is horizontal and perpendicular to the sagittal and transverse planes. A frontal section separates the body into dorsal and ventral parts.
Key Terms: Introduction
FIGURE 2 (a, b) Combined directional terms shown on a cat.
anterior (cranial, rostral)
distal
dorsal
frontal
lateral
medial
parasagittal posterior proximal sagittal transverse ventral