THE MAXILLA

Maxilla is the second largest bone of the face, the iirst being the mandible. The two maxillae form the whole of the upper jaw, and each maxilla enters into the formation of face, nose, mouth, orbit, the infratemporal and pterygopalatine fossae.

Side Determination

1. Anterior surface ends medially into a deeply concave border, called the nasal notch. Posterior surface is convex.

2. Alveolar border with sockets for upper teeth faces downwards with its convexity directed outwards. Frontal process is the longest process which is directed upwards.

3. Medial surface is marked by a large irregular opening, the maxillary hiatus.

Features

Each maxilla has a body and four processes, the frontal, zygomatic, alveolar and palatine.

Body of Maxilla

The body of maxilla is pyramidal in shape, with its base directed medially at the nasal surface, and the apex directed laterally at the zygomatic process. It has four surfaces and encloses a large cavity; the maxillary sinus. The surfaces are : (1) Anterior or facial; (2) posterior or infratemporal; (3) superior or orbital; and (4) medial or nasal.

Anterior or Facial Surface

1. Anterior surface is directed forwards and laterally.

2. Above the incisor teeth there is a slight depression, the incisive fossa, which gives origin to depressor septt Incisivus arises from the alveolar margin below the fossa, and the nasalis superolateral to the fossa along the nasal notch.

3. Lateral to canine eminence there is a larger and deeper depression, the canine fossa, which gives origin to levator anguli oris.

4. Above the canine fossa there is infraorbital foramen, which transmits infraorbital nerve and vessels.

5. Levator labii superioris arises between the infraorbital margin and infraorbital foramen.

6. Medially, the anterior surface ends in a deeply concave border, the nasal notch, which terminates below into process which with the corresponding process of opposite maxilla forms the anterior nasal spine. Anterior surface bordering the nasal notch gives origin to nasalis and depressor septi.

Posterior or Infratemporal Surface

1. Posterior surface is convex and directed backwards and laterally.

2. It forms the anterior wall of infratemporal fossa, and is separated from anterior surface by the zygomatic process and a rounded ridge which descends from the process to the first molar tooth.

3. Near the centre of the surface open two or three alveolar canals for posterior superior alveolar nerve and vessels.

4. Posteroinferiorly, there is a rounded eminence, the maxillary tuberosity, which articulates superomedially with pyramidal process of palatine bone, and gives origin laterally to the superficial head of medial pterygoid muscle.

5. Above the maxillary tuberosity the smooth surface forms anterior wall of pterygopalatine fossa, and is grooved by maxillary nerve.

3. Above the hiatus, there are parts of air sinuses which are completed by the ethmoid and lacrimal bones.

4. Below the hiatus, the smooth concave surface forms a part of inferior meatus of nose.

5. Behind the hiatus, the surface articulates with perpendicular plate of palatine bone, enclosing the greater palatine canal which runs downwards and forwards, and transmits greater palatine vessels and the anterior, middle and posterior palatine nerves.

6. In front of the hiatus, there is nasolacrimal groove, which is converted into the nasolacrimal canal by articulation with the descending process of lacrimal bone and the lacrimal process of inferior nasal concha The canal transmits nasolacrimal duct to the inferior meatus of nose.

7. More anteriorly, an oblique ridge forms the conchal crest for articulation with the inferior nasal concha.

8. Above the conchal crest, the shallow depression forms a part of the atrium of middle meatus of nose.

Mandible

The mandible, or lower jaw, is the largest and strongest bone of the face. It develops from the first pharyngeal arch. It has a horseshoe-shaped body which lodges the teeth, and a pair of rami which project upwards from the posterior ends of the body and provide attachment to muscles.

The Body

Each half of the body has outer and inner surfaces, and upper and lower borders.

The outer surface presents the following features.

(1) The sumphysis menti is the line at which the right and left halves of the bone meet each other. It is marked by a faint ridge.

(2) The mental protuberance [mentum = chin) is a median triangular projecting area in the lower part of the midline. The inferolateral angles of the protuberance form the mental tubercles.

(3) The mental foramen lies below the interval between the premolar teeth.

(4) The oblique line  is the continuation of the sharp anterior border of the ramus of the mandible. It runs downwards and forwards towards the mental tubercle.

(5) The incisive fossa is a depression that lies just below the incisor teeth.

The inner surface presents the following features.

(1) The mylohyoid line is a prominent ridge that runs obliquely downwards and forwards from below the third molar tooth to the median area below the genial tubercles.

(2) Below the mylohyoid line the surface is slightly hollowed out to form the submandibular fossa. which lodges the submandibular gland.

(3) Above the mylohyoid line there is the sublingual fossa in which the sublingual gland lies.

(4) The posterior surface of the symphysis menti is marked by four small elevations called the superior and inferior genial tubercles.

(5) The mylohyoid groove (present on the ramus) extends on to the body below the posterior end of the mylohyoid line.

The upper or alveolar border bears sockets for the teeth.

The lower border of the mandible is also called the base. Near the midline the base shows an oval depression called the digastric fossa.

Phases of Cell

Cell division:

A method in which new cells are originated from pre-existing cells.

Amitosis:

When the nucleus divides several times without each division being accompanied by the division of the cytoplasm, e.g., prokaryotes.

Mitosis:

Mitosis is a type of cell division that results in two daughter cells, each having a nucleus containing the same number of chromosomes of the mother cell. It is also called equational division. It occurs in all vegetative or somatic cells.

PHASES OF THE CELL

Interphase:

The period when the cell grows in size and performs the physiological functions. Chromatin is in a decondensed state in the nucleus of an interphase cell. Before cell division begins, the non-dividing or resting cells are said to be in interphase.

Prophase:

Chromatin condenses to form chromosomes which have two chromatids joined by a centromere. Nuclear envelope disappears during prophase. Centrioles move to the opposite poles (sides) of the cells and spindle fibres are formed from the centrioles.

Metaphase:

Chromosomes move to the centre of the cell and form an equatorial plate. Spindle fibres grow in length and attach themselves to the centromere of each chromosome.

Anaphase:

The chromosomes are pulled apart and each half moves to the opposite side of the cell.

Telophase:

Nuclear membrane appears around the chromosomes and chromosomes decondense into chromatin.

Karyokinesis:

The dividing nucleus involves a number of sequential changes. It is divided into four phases called prophase, metaphase, anaphase and telophase.

Cytokinesis:

A membrane or a cell wall appears in the middle of the cells and divides the cytoplasm of the cell into two. With this the cell division is completed and two daughter cells are formed.

Meiosis:

Meiosis or reduction division is so named as it results in the formation of four daughter cells, each with half the number of chromosomes. All sexually reproducing organisms undergo meiosis, either for the purpose of gamete formation (gametic meiosis) or to reduce the diploid zygote to a haploid condition (zygotic meiosis). During meiosis, the nucleus divides twice.

Division I:

This division is meiotic in which the chromosome number is reduced to half (n).

Prophase I:

For the convenience of study, it is divided into five substages called leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene (Leptonema):

The chromatin reticulum which was rolled up and tightly packed around the nucleolus undergoes condensation and shortening to form definite thread-like structures called chromosomes. The chromosomes are replicated during interphase but separate sister chromatids cannot be distinguished due to the presence of nucleoprotein core between them. The chromosome has a characteristic beaded appearance.

The nucleus increases in volume. Centrioles duplicate and start moving towards the poles.

Zygotene (Zygonema):

Identical (homologous) chromosomes, one paternal and one maternal, come together and lie side by side. There is strong attraction between them and pairing takes place along their entire length. This is called synapsis. The chromosomes are said to be in bivalent condition. Soon after, the paired chromosomes thicken and shorten.

Synapsis is not a fusion of chromosomes but simply a lateral association between identical pairs.

Pachytene (Pachynema):

The chromosomes now split longitudinally, and four chromatids (two from each homologue) are produced. This is termed as tetrad. The homologous chromosomes remain connected at some points called chiasmata (singular-chiasma).

The non-sister adjacent chromatids of a tetrad twist round each other.

Diplotene (Diplonema):

A repulsive force develops between the homologous pairs of chromosomes and they begin to separate from each other.

The separation does not take place at the chiasmata where the chromatids exchange corresponding portions by crossing over. This cytogenetic activity brings about recombination of genetic material.

Diakinesis:

The chromosomes further condense. The nuclear membrane and nucleolus disappear. The centrioles move to the poles and initiate spindle formation.

Metaphase I:

The chromosomes collect at the equator of the spindle. The four chromatids (in two pairs) are attached to the spindle fibres by two centromeres, and face opposite poles.

Anaphase I:

The two pairs now separate and move towards the opposite poles of the spindle.

Telophase I:

The chromatid pairs (chromosomes) form a compact group at each pole. The two daughter nuclei thus formed, contain haploid or 'n' chromosomes.

Division II:

This division is mitotic. Each of the two cells formed in division I further divide into two, resulting in the formation of four daughter cells, each with haploid (n) chromosomes.

Prophase II

• The nucleolus reappears in each nucleus.
• The two chromatids of each chromosomes remain separate but connected at the chiasma.

Metaphase II:

• The chromosomes arranged themselves along the equator of the newly formed spindle.
• The paired chromatids are separated.

Anaphase II:

Chromatids move apart towards opposite poles of the spindle. 

Telophase II:

Cytokinesis occurs and four nuclei are formed, each having one set of chromatids (now called chromosomes) reorganized in it.

Cell Facts

Cell:

The cell is the fundamental, structural and functional unit of all living beings.

Prokaryotic cell:

In lower organisms like the bacteria and blue-green algae, the nuclear membrane is absent. As such, the nuclear material is in direct contact with the cytoplasm. Such cells are called prokaryotic cells.

Eukaryotic cell:

In higher organisms the nuclear material is enclosed or bounded by a membrane.


Cellsize:

Size of a particular cell is dependent on the surface area/ volume relationship. A small cell is more efficient than a large cell. The largest cell in the world is ostrich’s egg.

The smallest cells are those of Mycoplasma gallisepticum, an organism intermediate between viruses and bacteria.

The largest human cell is ovum. The smallest human cell is red blood cell. The longest cell is the nerve cell. In elephant, it has been found to measure about 1 m.

Cell shapes:

Cells exhibit varied shape to perform different tasks, e.g., the nerve cells are long to carry messages from one part of the body to another.

• Human red blood cells are circular and biconcave, to transport oxygen.
• White blood cells are amoeboid that can squeeze out through capillary walls.
• Guard cells of stomatal pore in the leaves are bean—shaped to open and close the pore.

Microscope:

A microscope is an instrument used to magnify objects, i.e., increase the size of images.

Protoplasm:

The living part of a cell which consist of cytoplasm, nucleus and other living bodies. It is a transparent semifluid granular substance that loses its transparency on heating. The largest component of protoplasm is water, minerals salts and elements of oxygen, carbon, hydrogen and nitrogen in the form of proteins, fats and carbohydrates.

Nucleoplasm:

The portion of protoplasm that occurs in the nucleus is called the nucleoplasm.

Cell membrane:

The cell membrane controls the entry and exit of materials allowing certain substances through, but preventing the passage of others, such a membrane is described as a selectively permeable membrane.

Cell wall:

Cell wall is a rigid, differentially permeable non-living boundary wall present outside the cell membrane in plant cells. The cell wall provides and maintains the shape of these cells and serves as protective barrier. It may be made of cellulose or chitin.

Cytoplasm:

Cytoplasm is a fluid jelly, transparent substance. It is a collective term for the cytosol and the organelles suspended within the cytosol. The cytoplasm shows the distinct regions - (i) ectoplasm and (ii) endoplasm. The outer non-granular, transparent thin layer is known as ectoplasm and the inner viscous portion where cell organelles are located is called endoplasm. The cytoplasm is composed of the matrix, the organelles and non-living inclusions like vacuoles and granules.

Cytosol:

The cytosol is the liquid found inside cells. It constitutes most of the intracellular fluid (ICF). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into many compartments.

Nucleus:

The nucleus is the most obvious organelle in any eukaryotic cell. It is enclosed in a double membrane and communicates with surrounding cytosol via numerous nuclear pores. Within the nucleus is the DNA responsible for providing the cells with its unique characteristics. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off — that’s why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell. All cells have nucleus, except sieve tubes in plants and the mature mammalian red blood cells.

Nucleolus:

The prominent structure in the nucleus. The nucleolus produces ribosomes, which move out of the nucleus and take positions on the rough endoplasmic reticulum.

Chromatin fibers:

Chromatin fibres are very fine thread - like coiled filaments. At the time of cell divisions, the chromatin becomes thick and ribbon - like and are known as chromosomes. The chromosomes bear genes, which are composed of DNA and histones. Genes are responsible for transmission of hereditary characteristics.

DNA:

DNA or deoxyribonucleic acid is a helically twisted double chain poly-deoxyribonucleotide macromolecule.

Endoplasmic reticulum:

Endoplasmic reticulum is a complex tubular membrane system which almost fills up the intracellular cavity. Some tubes are studded with spherical bodies called ribosomes. These are called rough endoplasmic reticulum. Those that do not bear ribosomes are called smooth endoplasmic reticulum. Rough ER is the site of protein synthesis. Smooth ER takes part in secretion of lipids.

BIOTA KINDOM

The Life Study:

Biology is a branch of science, which deals with the study of living organisms. As a branch of science, Biology encompasses the scientific method. However, human interest in biology is much older than that in experimental science. Of all branches of science, Biology is perhaps one of the greatest interest to man. The origin of biology can be traced back to human inquisitiveness about the things happening in nature. Today, biology is one of the fastest growing branches of science. It is likely that in future, the knowledge provided by biology may become the key to human survival on this planet Earth.

Need for studying biology:

It brings about an awareness of the rich diversity of life forms that inhabit the planet Earth. It helps us to know and appreciate the precision and complexity of living process.

Scope of Biology:

Branches of Biology:

Biology has two basic branches — Botany and Zoology. Botany is the study of plants while zoology is the study of animals.

Botany (Greek) means plants. Zoon (Greek) means animals.

Classical Branches:

These branches are largely meant for gaining knowledge in the respective areas. Following are the most common classical branches of Biology.

Cell Biology (Cytology):

It is the study of the structure and functions of cells.

Developmental Biology (Embryology):

It is the study of early life history of individual organisms from the fertilized egg to the adult stage (Ontogeny).

Environmental Biology (Ecology):

It is the study of organisms in relation to their environment. 

Ethology (Behavioral Biology):

It is the study of  behaviour of animals to their natural habitat.

Eugenics:

It is the science that deals with factors related to the improvement of race, especially to that of human beings.

Genetics:

It is the study of heredity and variations in the organisms. 

Histology:

It is the study of tissues in plants and animals.

Morphology:

It is the study of external form and structure and relative position of various organs in living organisms.

Anatomy (Greek - anatome means ‘to cut apart’):

It is the study of internal structure of living organisms.

Organic evolution (Phylogeny):

It is the study of history and relationships of living organisms through time.

Paleontology:

It is the study of living organisms that existed once (extinct) through their remains called fossils and their geological history.

Physiology:

It is the study of biological functions and their underlying mechanisms.

Taxonomy (Systematics):

It is the study of identification and naming of organisms. Systematics deals with the grouping and higher level classification of groups.

All these classical branches are interrelated with one another. The aspects covered by any one branch involve the use of knowledge from other branch or branches. The following scheme represents such an interrelationship.

Interdisciplinary branches:

Biology invariably uses knowledge and techniques from other branches of science, to solve its complicated life processes. Most of the recent discoveries and inventions in biology have resulted from the joint endeavour between biology and physical sciences and even biology and social sciences. This has resulted in the emergence of many interdisciplinary branches. Following are some of them.

Biochemistry:

It is the study of chemistry of life. It is concerned with the study of chemical processes underlying life activities.

Biophysics:

It is the study of physics of living organisms. It is concerned with the study of physical laws and principles that are used in the study of plants and animals. For example, the principle behind microscopy.

Biology Biometry:

It is a branch, which involves mathematical and statistical studies in co-relating the various life processes.

Bioinformatics:

It is a branch concerned with the application of knowledge of information technology in analyzing biological data. This branch is of immense significance in the field of biotechnology.

Psychology:

It is the study of human mind. It is concerned with the application of biology to the problems of people. It represents a branch in which biology is entering the realms of social science.

Applied branches:

These are the branches in which the knowledge obtained from biology is applied for the welfare of man and the animals and plants that are useful to him. Following are some of the applied branches of biology that are of great significant to human welfare.

Animal husbandry:

It is the study of principles and practices pertaining to the economics of domestic animals.

Anthropology:

It is the study of development and characteristics of human race.

Biotechnology:

It is the application of principles and practices of engineering and technological sciences in living organisms.

Conservation biology:

It deals with preservation of natural resources such as natural habitats, wild life, earth resources, energy resources, etc.

Medicine:

It is the application of principles of biology to diagnose and  conditions in man.

Microbiology:

It is the study of micro-organisms that are useful and harmful to man, animals and plants.

Some more scopes of Bilogy are Molecular Biology, Pathology, Pharmacy, Poultry, Apiculture, Sericulture, Veterinary science, and many more.