44 GENERAL ZOOLOGY 



Corium. — The histological structure of the corium is entirely- 

 different from that of the epidermis. It is composed of two 

 layers: an outer stratum spongiosum and an inner stratum com- 

 pactum, both containing a large amount of connective tissue 

 fibers. These fibers are of two kinds: white fibers, which are 

 inelastic and unbranched ; and yellow fibers, which are elastic and 

 branched. The fibers of the spongiosum layer form a more or 

 less open network, in which are found blood vessels, lymph spaces, 

 nerves, and glands. At various points this layer is elevated to 

 form dermal papillae, each of which contains a touch corpuscle 

 composed of a conical group of flattened cells supplied with sensory 

 nerve endings. Most of the chromatophores lie in the upper 

 layer of the corium. 



The fibers of the stratum compactum appear in sections as 

 a wavy layer of compact tissue, traversed at intervals by vertical 

 bands extending upward into the stratum spongiosum and the 

 epidermis. The fibers are both branched and unbranched. 

 Here and there are nonstriated muscle fibers whose contractions 

 move the skin. Nerves and blood vessels also occur. 



Beneath the stratum compactum is a layer of loose connective 

 tissue, the subcutaneous connective tissue, which is divided into 

 an inner and outer layer by large lymph spaces. The lymph 

 spaces are separated by septa joining the two layers of subcuta- 

 neous tissue. This tissue attaches the skin to the underlying parts. 



In general, connective tissue is characterized by the presence 

 of a large amount of intercellular material, which in the case of 

 the corium is fibrous. Connective tissue cells are found in and 

 among the fibers, but the fibers themselves lie outside of cells. 

 We shall see later that bone and cartilage are also forms of con- 

 nective tissue, but in these cases the intercellular material is 

 represented by the matrix of bone or cartilage, in which the 

 cells are embedded. 



Glands.— The cutaneous glands of the frog are of the simple 

 alveolar type. This means that each gland consists of a globular 

 alveolus, whose wall is composed of a single layer of cells, con- 

 nected with a short straight duct whose wall is also a single layer 

 of cells (Fig. 19). The duct opens on the surface of the skin, 

 but the alveolus is located in the stratum spongiosum. However, 

 both alveolus and duct originate as a downgrowth from the 

 basal layer of the epidermis and are therefore epithelial structures. 



