Some further explanation is needed in regard to the topographical terms, or terms of direction, used in the present work. We have adopted the BNA terms in this matter also. The terms _superior_, _inferior_, _anterior_, and _posterior_ have been avoided, as these terms do not convey the same meaning in the case of the cat as they do in man, owing to the difference in the posture of the body. In place of these terms are used _dorsal_ and _ventral_, _cranial_ and _caudal_. As terms of direction these, of course, must have an absolutely fixed meaning, signifying always the same _direction_ without necessary reference to any given structure. For example, _cranial_ means not merely toward the _cranium_, but refers to the _direction_ which is indicated by movement along a line from the middle of the body, toward the cranium; after the head or cranium is reached, the term still continues in force for structures even beyond the cranium. Thus the tip of the nose is considered to be _craniad_ of the cranium itself. _Lateral_ signifies away from the middle plane; _medial_ toward it. _Inner_ and _outer_ or _internal_ and _external_ are used only with reference to the structure of separate organs, not with reference to the median plane of the body. In describing the limbs the _convexity_ of the joint (the elbow or knee) is considered as _dorsal_, the concavity being therefore ventral. _Medial_ refers to that side of the limb which in the normal position is toward the middle of the body; _lateral_ to the outer side. Terms of direction which are derived only from the structure of the limb itself are in some cases more convenient than the usual ones. In the fore limbs the terms _radial_ (referring to the side on which the radius lies) and _ulnar_ (referring to the side on which the ulna lies) are used; in the hind limbs the terms _tibial_ and _fibular_ are used in a similar manner. _Distal_ means toward the free end of a limb or other projecting structure; _proximal_, toward the attached end. For all these terms an adverbial form ending in _-ad_ has been employed. Experience has shown this to be very useful in practice, and while not expressly recommended by the BNA, it is not condemned. Terms ending in _-al_ are therefore adjectives; those ending in _-ad_ are adverbs. In compounding these terms of direction, the hyphen has been omitted in accordance with the usage recommended by the Standard Dictionary. Thus _dorsoventral_ is written in place of _dorso-ventral_, etc. The student will perhaps be assisted in understanding these compounds if he notes that the first component always ends in _-o_, so that the letter _o_ practically serves the purpose of a hyphen in determining how the word is to be divided.