THE FROG 43 



dark, so causing them to expand laterally. These changes are also 

 accompanied by chemical and electrical ones, and by the production 

 of heat. 



Yet another kind of muscular tissue is met with in vertebrate 

 animals, but it is confined to the heart, and hence termed cardiac. 

 It is involuntary and yet striped, and so constitutes a variety of its 

 own. It is composed of flat short cells placed end to end, with 

 short branches connecting them with neighbouring cells. No 

 sarcolemma is present, and each cell possesses a single fairly large 

 nucleus situated near its centre. It exhibits both longitudinal and 

 transverse stripes, though the latter are not so well marked as in 

 voluntary muscle. 



Integument. 



The integument in the frog is very simple, and consists 

 only of the skin, in which no hair, nails, or any exoskeletal 

 structures are developed. The skin is moderately thick and tough, 

 and fits loosely, being only attached to the underlying muscles here 

 and there. A vertical section through the skin shows it to be com- 

 posed of two very clearly defined layers, an outer or epidermis, and 

 an inner or dermis. The outer forms an epithelium, that is to say, a 

 sheet of cells covering a free surface, and is compound or composed 

 of a number of layers of cells. The cells of the various layers differ 

 in form, according to their position. The cells at the inside are 

 columnar in shape and granular, forming a layer known as the 

 Malpighian layer. Above this the cells are polygonal, becoming 

 more and more flat, until the outer layers are thin scale-like cells. 

 In addition to the flattening the protoplasm of the cells undergoes 

 modification, and is progressively replaced by a substance, keratin, 

 allied to horn, so that the outermost cells are practically nothing 

 but thin dead horny scales. The outer layer is cast off periodically, 

 a process known as sloughing, and the cells are replaced by new ones 

 produced from the Malpighian layer, which is the actively growing 

 part of the epidermis. An epithelium thus composed of layers of 

 cells gradually changing in character is spoken of as stratified, and 

 although found in the epidermis of the frog is still more marked 

 in ourselves, where the horny layer on the palms of the hands and 

 soles of the feet is extraordinarily thick. Belonging to the epidermis 

 are a number of hollow flask-shaped structures, the cutaneous glands, 

 which dip down into the dermis. Each is composed of two parts, 

 a spherical hollow, the alveolus, in the dermis lined with large cells, 

 and a narrow tube, the duct, running through the epidermis and 

 opening to the exterior. The cells lining the alveolus are, of course, * 

 epithelial, but as they are constantly engaged in forming in their 



