232 THE AMERICAN MONTHLY [October, 



pieces bound together at their joints with cords of very strong substance 

 called connective tissue. The muscles are organs stretched from bone 

 to bone (or used in other places) which, by their contraction, bend the 

 joints, thus moving the parts of the skeleton upon each other. They are 

 very numerous and are caused to move at the proper time and degree 

 by the nerves, which together form the nervous system. It is the skele- 

 ton, muscles, and nervous system together which are concerned in the 

 motions of the animal body, while the other systems are secondary to 

 these. The reproductive system does not exist for the benefit of its in- 

 dividual possessor, but is for the benefit of the group in order that the 

 kind may be continued after the death of the individual. We may then 

 look upon the frog as a machine endowed with wonderful powei"s — 

 with systems of levers to secure the greatest variety of movements — 

 with living cords to pull on these levers, with nerves to guide the pulls 

 of the various cords and harmonize their work for some useful purpose 

 — with a complex apparatus for supplying fuel to the force-using parts 

 of the apparatus — and a drainage system to remove all the waste result- 

 ing from the use of the fuel — and finally with an apparatus, part of the 

 machine, which has the power of making a new machine to take the 

 place of the old one as it wears out and finally goes to pieces. Our 

 problem is to see if all these remarkable powers can really properly 

 be regarded as the powers of protoplasm acting in some or several ways. 

 Our subject is too complex to be approached except by degrees, and we 

 will consider its various parts separately. 



The Supporting System and Skin. — The skin of the frog is to our 

 ordinary observation a damp, somewhat sticky coat loosely placed over 

 the parts beneath ; it is dark above and almost or quite white below. If 

 we keep a frog in confinement long enough and keep watch of it we 

 shall see that a thin translucent film comes away from its surface as we 

 may remove a glove. This film if perfect will retain the shape of the 

 animal perfectly even to the tips of the toes. If we put a small piece of 

 this film under the microscope we shall find it to be made of small 

 flat blocks, five or six sided, forming a mosaic pavement, and if we stain 

 the specimen with iodine we shall find a central round more densely 

 staining portion. These are dead cells then and they form a tissue — the 

 spot in the centre of each cell is its nucleus. This film is the outer layer 

 of the skin. A vertical section of the skin will show this outer layer 

 slightly thicker and yellower than those below, and several layers of 

 cells below it, and finally a row of cubical cells forming the inner or 

 basement layer of the epidermis. The skin is, however, composed of 

 more than the epidermis. Below the basement layer of cubical cells we 

 shall find connective tissue cells, ragged-looking bodies whose strag- 

 gling arms connect with each other and form a layer below the epider- 

 mis. These cells are in some positions filled with colored substance 

 which seen through the translucent epidermis gives its color to the skin 

 at large. In certain places the basement cells of the epidermis bend 

 down to form pockets in the dermis, pockets lined with cuboidal cells, 

 and these are the glands which secrete the slime. Blood-vessels and 

 nerves run up to the dermis to connect the cells of the skin with those 

 of the inner organs, the nerves go to the brain and spinal cord, and a 

 touch upon the skin may be followed by a vigorous jump on the part 

 of the frog. We see then that the skin is a mass of tissues, and each 



