. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 474- TEOUMENTARY ORGANS. plexities whose solution appears to me to be essential to any philosophical treatment of the subject, and to the consideration of which 1, therefore, propose to devote the following Preliminary Section. § 1. My first difficulty was to find an answer to the question,—What constitutes a tegu- mentary organ as distinguished from any other? The most obvious definition of an integu- ment or tegumentary organ is, of course, — that which forms the external covering of any animal—viscus, on the other han
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 474- TEOUMENTARY ORGANS. plexities whose solution appears to me to be essential to any philosophical treatment of the subject, and to the consideration of which 1, therefore, propose to devote the following Preliminary Section. § 1. My first difficulty was to find an answer to the question,—What constitutes a tegu- mentary organ as distinguished from any other? The most obvious definition of an integu- ment or tegumentary organ is, of course, — that which forms the external covering of any animal—viscus, on the other hand, being that which is contained. More strictly, it may be said that the integument constitutes that free surface of an animal which is ex- ternal to the edges of the oral and anal aper- tures, or where the former alone exists, to its edge. Now these definitions are perfectly sufficient so far as surface is concerned ; but suppose we make a section perpendicular to the surface, where does integument cease, and where does viscus begin ? So far as I am aware, no elucidation of this point has hither- to been undertaken, and yet, for want of it, the greatest confusion prevails in the nomen- clature of those organs which constitute the outer wall of the animal frame. Intimately connected with this question, and indeed forming a part of it, is a second. In man and the higher animals, there is an universally recognised distinction of the integument into two portions,— the epidermis and the derma ; and these terms have been extended to all animals. But, if we inquire what constitutes an epidermis, and what a derma, no definite answer is to be met with. It may be said that the derma is vascular, while the epidermis is nonvascular; or that the epidermis is a simple cellular horny structure, while the derma is complex and fibrous; but these characters, applicable enough among the higher animals, fail completely with the lower. Thus, in the majority of the Invertebrata, the derma
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