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Top: Science: Biology: Anatomy:

What is the difference between a Plant and an Animal?

In general animals have the benefit of self-propulsion, but this is not a fool proof definition!

Animals develop from an embryo, which goes through a blastula phase, whereas plants do not.
Fungi lack undulipodia and develop from spores.
Algae are included in the Protista kingdom, with most microbes.


Taxonomy

Science of classification of living things.

Early taxonmy was based on major differences observed, such as the anatomical differnce between members of the animal and plant kingdoms.

Kingdoms are subdivided according to
i. Phylum
ii. Class
iii. Order
iv. Family
v. Genus
vi. Species

These have been correlated with their evolution.
Cladistics is a subdiscipline of taxonomy, involving the reconstruction of phylogeny based on the development, retention, and loss of morphological features from evolutionary ancestors, and their descendants in other branches.

With the advent of molecular biology, taxonomy has now a genomic basis, and databases of gene sequences can be used for classificion.


Anatomy

The study of biological structures.
(See also physiology, which is the study of how these structures interact.)

Historically this began with investigation of the components of animal bodies, and division into
sections: head, limbs, thorax, abdomen
organs: eye, muscles, lungs, stomach
tissues: cornea, ligaments, bronchioles, piloric phincter.

With the aid of light microscopy the component cells could be compared. (see Histology )
Primary interest is in the human body, namely Human Anatomy.

With the advent of high resolution microscopy and electron microscopy it became possible to study the structure of sub-cellular components, the organelles.

Structural Biology now includes the study of macromolecules, which form the genes, membrane receptors and enzymes which participate in the life cycle. (see Molecular Biology )


Organs of Sense - The Skin

The skin is the principal scat of the sense of touch, and may be regarded as a covering of protection to the exterior of the body. It consists of two layers, the derma, or cutis vera, and the epidermis or cuticle.
The Derma, or True Skin, consists of fibro-areolar tissue, intermixed with numerous blood-vessels, lymphatics, and nerves. The fibro-areolar tissue forms the framework of the cutis; it is composed of firm interlacing bundles of white fibrous tissue, intermixed with a much smaller proportion of yellow elastic fibres, the amount of which varies in different parts. The fibro-areolar tissue is more abundant in the deeper layers of the cutis, where it is dense and firm, the meshes being large, and gradually becoming blended with the subcutaneous areolar tissue; towards the surface, the fibres become finer and more closely interlaced, the. Most superficial layer being covered with numerous small, conical, vascular eminences, the papillae. From these differences in the structure of the cutis at different parts, it is usual to describe it as consisting of two layers; the deeper layer or corium, and the superficial or papillary layer.



The Corium consists of strong interlacing fibrous bands, composed chiefly of the white variety of fibrous tissue; but containing, also, some fibres of the yellow elastic tissue, which vary in amount in different parts. Towards the attached surface, the fasciculi are large and coarse; and the areolas left by their interlacing large and occupied by adipose tissue and the sudatory glands. This element of the skin becomes gradually blended with the subcutaneous areolar tissue. Towards the free surface, the fasciculi are much finer, and they have a closer interlacing, the most superficial layers consisting of a transparent, homogeneous matrix with imbedded nuclei.
The corium varies in thickness, from a quarter of a line to a line and a half, in different parts of the body. Thus, it is thicker in the more exposed regions, as the palm of the hand and sole of the foot; on the posterior aspect of the body, than the front; and on the outer, than the inner side of the limbs. In the eyelids, scrotum, and penis, it is exceedingly thin and delicate. The skin generally is thicker in the male than in the female.
The areolee are occupied by adipose tissue, hair follicles, and the sudatory and sebaceous glands; they are the channel by which the vessels and nerves are distributed to the more superficial strata of the corium, and to the papillary layer.
Plain muscular fibres are found in the superficial layers of the corium, wherever hairs are found; and in the subcutaneous areolar tissue of the scrotum, penis, perineum, and areolae of the nipple. In the latter situations, the fibres are arranged in bands, closely reticulated and disposed in super-imposed laminae.
The Papillary Layer is situated upon the free surface of the corium; it consists of numerous small, highly sensitive, and vascular eminences, the papillae, which rise perpendicularly from its surface, and form the essential element of the organ of touch. The papillae are conical-shaped eminences, having a round or blunted extremity, occasionally divided into two or more parts, and connected by their base with the free surface of the corium. On the general surface of the body, more especially in those parts which are endowed with slight sensibility, they are few in number, short, exceedingly minute, and irregularly scattered over the surface; but in other situations, as upon the palmar surface of the hands and fingers, upon the sole, and around the nipple, they are long, of large size, closely aggregated together, and arranged in curvilinear lines, forming the elevated ridges seen on the free surface of the epidermis. In these ridges, the larger papillae are arranged in a double row, with smaller papillae between them; and these rows are subdivided into small square-shaped masses by short transverse furrows regularly disposed, in the centre of each of which is the minute orifice of the duct of a sweat-gland. No papillae exist in the grooves between the ridges. In structure, the papilla; resemble the superficial layer of the cutis; consisting of a homogeneous tissue, faintly fibrillated, and containing a few fine elastic fibres. The smaller papillae contain a single capillary loop; but in the larger the vessels are convoluted to a greater or less degree; each papilla also contains two or more nerve-fibres, which, after running in a waving manner through it, terminate in loops, or by a free extremity. No lymphatics have as yet been discovered in the papillae.
The Epidermis, or Cuticle (scarf-skin), is an epithelial structure, which forms a defensive covering to the surface of the true skin, being accurately moulded on the papillary layer of the derma. It varies in thickness in different parts. Where it is exposed to pressure and the influence of the atmosphere, as upon the palms of the hands and soles of the feet, it is thick, hard, and horny in texture; whilst that which lies in contact with the papillary layer, over the entire surface of the body, is soft and cellular in structure; hence the deeper layer has been called, the rete mucosum.
The free surface of the epidermis is marked by a network of linear furrows of variable size, marking out the surface into a number of spaces of polygonal or lozenge-shaped form. Some of these furrows are large, as opposite the flexures of the joints, and correspond to the folds in the derma produced by their movements. In other situations, as upon the back of the hand, they are exceedingly fine, and intersect one another at various angles: upon the palmar surface of the hand and fingers, and upon the sole, these lines are very distinct, and have a curvilinear arrangement, and depend upon the large size and peculiar arrangement of the papillae upon which the epidermis is placed. The deep surface of the epidermis is accurately moulded upon the papillary layer of the derma, each papilla being invested by its epidermic sheath; so that when this layer is removed by maceration, it presents a number of pits or depressions corresponding to the elevations of the papillae, as well as the furrows left in the interval between them. Fine tubular prolongations from this layer are continued into the ducts of the sudatory and sebaceous glands. In structure, the epidermis consists of flattened cells, agglutinated together, and having a laminated arrangement. In the deeper layers the cells are large, rounded or columnar, and filled with soft opaque contents. In the superficial layers the cells are flattened, transparent, dry, and firm, and their contents converted into a kind of horny matter. The difference in the structure of these layers is dependent upon the mode of growth of the epidermis. As the external layers desquamate, from their being constantly subjected to attrition, they are reproduced from beneath, successive layers gradually approaching towards the free surface, which, in their turn, die and are cast oft'.
These cells arise in the liquor sanguinis, which is poured out on the free surface of the derma; they contain nuclei, and form a thin stratum of closely-aggregated nucleated cells, which cover the entire extent of the papillary layer. The deepest layer of cells, according to Kolliker, are of a columnar form, and are arranged perpendicularly to the free surface of the derma, forming either a single or a double, or even triple, layer: the laminae succeeding these are composed of cells of a more rounded form, the contents of which are soft, opaque, granular, and soluble in acetic acid. As these cells successively approach the surface by the development of fresh layers from beneath, they assume a flattened form from the evaporation of their fluid contents, and finally form a transparent, dry, membranous scale, lose their nuclei, and apparently become changed in their chemical composition, as they are unaffected now by acetic acid.
The black colour of the skin in the negro, and the tawny colour among some of the white races, is due to the presence of pigment in the cells of the cuticle. This pigment is more especially distinct in the cells of the deeper layer, or rete mucosum, and is caused by the presence of pigment cells, similar to those found in the choroid. As these approach the surface and desiccate, the colour becomes partially lost.
The arteries which supply the skin divide into numerous branches in the subcutaneous tissue; they then pass through the areolae of the corium, and divide into a dense capillary plexus, which supplies the sudatory and sebiparous glands and the hair follicles, terminating in the superficial layers of the corium, by forming a capillary network, from which numerous fine branches ascend to the papillae.
The lymphatic vessels are arranged in a minute plexiform network in the superficial layers of the corium, where they become interwoven with the capillary and nervous plexuses; they are especially abundant in the scrotum and around the nipple.
The nerves which supply the skin ascend with the vessels through the areolaj of the deep layers of the corium to the more superficial layers, where they form a minute plexiform mesh. From this plexus, the primitive nerve-fibres pass to be distributed to the papillae. The nerves are most numerous in those parts which are provided with the greatest sensibility.
E-Mail: etienne_perron@yahoo.com


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