Skin

The skin is the external covering of animals. It is made up of Epidermis, Dermis, and Hypodermis. The skin is an Organ. Molecules combine to form cells which make up Tissues which form Organs.

1. 0 Epidermis

1.1 Composed of Keratin
1.2 Made up of layers of stratified epidermal cells
1.3 Lower strata transform with time into the upper strata
1.4 No Blood Vessels are present in this layer
1.5 Food diffuses through basal cells from the dermis
1.6 The lowers layers are alive
1.7 The upper layers are dead, keratinized, being continually removed
1.8 Melanocytes (which are pigments and give colour), are contained in the Malphigian layer
1.9 Many basal cells have finger like intrusions which give adhesion into the dermis

2.0 Dermis

2.1 Composed of Collagen
2.2 Layered
2.3 Densely packed
2.4 No transformation between layers as in the epidermis
2.5 Provides the basis for leather

2.6 Papillary Layer

2.6.1 Especially tightly packed to protect the other tissues present
2.6.2 Basically fibrous tissue
2.6.3 Also blood vessels, sweat glands, hair follicles, and associated muscle tissue
2.6.4 Papillate because of intrusion of hair shafts and sweat glands
2.6.5 Also called the Grain Layer
2.6.6 Grain surface is characteristic for each type of animal
2.6.7 Grain surface appearance is decided by hair size, number of hairs, and distribution (clustered or evenly spread etc.)
2.6.8 Eccrine glands discharge into upper papillary layer

2.7 Fibre Network Layer

2.7.1 Decides quality of the leather (strength-suppleness-firmness)
2.7.2 These qualities based on three dimensional weave
2.7.3 The Fibre network Layer has complex ramification of fibre bundles in 3D

a. Macromolecules -tropocollagen of regular shape 3000 Amstrong in length and 14 in diameter
b. These are ejected into a slimy gel called Ground Substance
c. They grew by regular packing into Fibrils
d. Fibrils align forming Fibres
e. Fibres combine to form Fibre Bundles

2.7.4 Extra cellular growth of this sort is very unusual (muscle cells' and epidermal cells' fibres are developed intracellularly)

3.0 Hairs

3.1.Epidermal structures
3.2 Origins deep in the Papillary layer
3.3 Either Primary - numerous three layered or Secondary - less numerous half to one third the size of Primary
3.4 Primary hairs have centre called medulla, middle called cortex and outer called cuticle
3.5 Secondary do not have the medulla
3.6 Sebaceous (fat) glands lubricate the hair and also affect (control) body temperature
3.7 Sudoriferous (sweat) glands also discharge into the upper follicle

4.0 Functions of the Skin Organ

4.1 Regulation of Body temperature
4.2 Storage of Body requirements
4.3 Protection
4.4 Elimination of waste products
4.5 Sensory detection and communication