Question

Typically all the mammals have seven vertebrae except in?
(A) Elephant
(B) Man
(C) Kangaroo
(D) Sea cow

Answer 1

Hint: The vertebral column is differentiated into cervical, thoracic, lumbar, sacral and coccygeal regions, starting from the skull. The number of cervical vertebrae are seven in almost all mammals including human beings.

Complete answer:
The cervical vertebrae is the specific feature of mammals. It is typically the smallest vertebrae which is located below the neck and offers stability and mobility to the head. It is connected to the thoracic spine which is relatively immobile. The 7 vertebrae are labelled as $C_1–C_7$ where $C_1$ is Atlas which articulates with the occipital condyles of the skull. The foramen of this vertebrae divides into two parts by a ligament.

In the upper part of the ligament, the spinal cord is present. $C_2$ is known as Axis. Centrum is present in $C_2$. Anterior surface of the centrum consists of a long odontoid process of dens which fits into the odontoid fossa of Atlas to make a pivot joint. Vertebrae from $C_3–C_6$ are typical vertebrae. $C_7$ is the largest cervical vertebrae consisting of a long spinous process which projects posteriorly towards the skin. The number of cervical vertebrae shows no variation and in fact remains constant in all mammals except Manatees and sloths. Manatees also known as sea cows are large slow moving aquatic mammals which are famous for possessing only six cervical vertebrae instead of seven.

Hence, the correct answer is option (D).

Note: All cervical vertebrae have apertures in their transverse forms called as Foramina Transversarium which shapes Vertebrarterial canal. Through this canal vertebral artery passes. This artery supplies blood in brain and spinal cord. Spinous process of cervical vertebrae is bifid except $C_7$. Only $C_7$ has demifacets where upper part of head of first rib articulates.