The neural tube has a longitudinal axis called the neuraxis, from the future brain area at the cranial end, to the conus medullaris of the spinal cord at the caudal end.
 By the fourth week in the human embryo, at its cranial end, three swellings have formed as primary brain vesicles.
These vesicles form the future forebrain, midbrain, and hindbrain.
 The three vesicles need to develop further into five brain vesicles but the space at the cranial end is limited.
This causes the neural tube to bend ventrally at two flexures – the first at the cephalic flexure and the second at the cervical flexure.
A third flexure is oriented in the opposite dorsal direction as the pontine flexure.
By the fifth week further flexion has taken place and the five secondary brain vesicles have formed.

The angle formed by the two ventral flexures, the cephalic flexure and the cervical flexure together, is a right angle in the ventral direction between the axis of the body and the axis of the brain.
The pontine flexure is located between these two flexures.
The neural tube has a longitudinal axis called the neuraxis, from the future brain area at the cranial end, to the conus medullaris of the spinal cord at the caudal end.  By the fourth week in the human embryo, at its cranial end, three swellings have formed as primary brain vesicles. These vesicles form the future forebrain, midbrain, and hindbrain.  The three vesicles need to develop further into five brain vesicles but the space at the cranial end is limited. This causes the neural tube to bend ventrally at two flexures – the first at the cephalic flexure and the second at the cervical flexure. A third flexure is oriented in the opposite dorsal direction as the pontine flexure. By the fifth week further flexion has taken place and the five secondary brain vesicles have formed.
The angle formed by the two ventral flexures, the cephalic flexure and the cervical flexure together, is a right angle in the ventral direction between the axis of the body and the axis of the brain. The pontine flexure is located between these two flexures.
Flexure (embryology)