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Lumbar Vertebrae

Q: On which specific process of the lumbar vertebra is the mamillary process (processus mamillaris) located?

The mamillary process (processus mamillaris) is located on the posterolateral border of the superior articular process of the lumbar vertebra. It is a small bony prominence that serves as an additional fulcrum and attachment site for tendons. The articular processes on which it is located originate from the vertebral arch, extend posteriorly from the transverse process, and lie almost vertically in the sagittal plane. The superior articular processes of one vertebra laterally embrace the inferior processes of the adjacent one.

Q: Where exactly is the accessory process located?

The accessory process (Processus accessorius) is a small structure located at the base of the costal process, inferiorly and posteriorly to it.

*Vertebrae lumbales*

For medical students2 min readUpdated 2026-10-10

Lumbar vertebrae (Vertebrae lumbales, L₁–L₅) are massive bony segments forming the lumbar region of the vertebral column. Their key anatomical features include unique additional processes and a specific sagittal orientation of the articular facets.

Vertebral bodyFeatures the largest dimensions among all true vertebrae.

ForamenThe vertebral foramen has a characteristic triangular shape.

MusclesPowerful back muscles shape the mamillary processes.

OrientationSpinous and articular processes are located in the sagittal plane.

Unique Additional Processes

The main distinguishing feature of the lumbar vertebrae is their complex relief, formed by additional bony structures. These formations arose as a result of evolutionary development, the fusion of rudimentary elements, and adaptation to biomechanical loads.

Such specific structures include:

Costal process (Processus costalis). This structure has a complex origin. During ontogenesis, the fusion of the lumbar rib rudiment and the true transverse process of the vertebra occurs. As a result, a single, well-defined costal process is formed.
Accessory process (Processus accessorius). This is a relatively small bony prominence. Topographically, it is located at the very base of the costal process, positioned posteriorly and slightly inferiorly to it.
Mamillary process (Processus mamillaris). The appearance of this projection is directly related to muscle function. It forms at the sites of dense tendon attachment of powerful back muscles. The mamillary process plays an important biomechanical role, serving as a reliable additional fulcrum for the muscular apparatus.

Structure of the Vertebral Body and Arch

In addition to specialized processes, lumbar vertebrae possess several important secondary features that make it easy to identify them in osteological specimens.

Vertebral body. Characterized by extremely large, massive dimensions, due to the need to support the weight of the overlying trunk segments.
Vertebral foramen. Unlike the round shape in the thoracic region, here it has a distinct triangular shape.
Spinous processes. They appear flat and quite broad. A crucial topographic detail is their strict orientation in the sagittal plane (i.e., directed anteriorly-posteriorly, without the significant downward inclination characteristic of thoracic vertebrae).
Articular processes. The articular facets on these processes are also oriented sagittally, which determines the nature of possible movements in the lumbar region (primarily flexion and extension).

Transitional Characteristics of Borderline Vertebrae

The lumbar region is not completely uniform. Vertebrae located at the boundaries with other spinal regions demonstrate so-called transitional structural traits:

Upper lumbar vertebrae. In their overall outlines, shape, and dimensions, they largely resemble the lower thoracic vertebrae, smoothing the anatomical transition between regions.
Lower lumbar vertebrae. Their anatomy gradually begins to show features characteristic of the sacral vertebrae, with which the last lumbar vertebra forms a strong connection.

Mnemonic

To remember the three specific processes of the lumbar vertebrae, use the acronym CAM: Costal, Accessory, Mamillary.

Frequently asked questions

On which specific process of the lumbar vertebra is the mamillary process (processus mamillaris) located?

The mamillary process (processus mamillaris) is located on the posterolateral border of the superior articular process of the lumbar vertebra. It is a small bony prominence that serves as an additional fulcrum and attachment site for tendons. The articular processes on which it is located originate from the vertebral arch, extend posteriorly from the transverse process, and lie almost vertically in the sagittal plane. The superior articular processes of one vertebra laterally embrace the inferior processes of the adjacent one.

What anatomical structures pass through the intervertebral foramina in the lumbar region?

The following anatomical structures pass through the intervertebral foramina in the lumbar region:

Trunk of the spinal nerve (truncus nervi spinalis) — a mixed nerve formed by the union of the motor anterior root and sensory posterior root.
Spinal ganglion (ganglion spinale) — a cluster of pseudounipolar neurons located on the posterior root, filling the entrance to the intervertebral foramen.
Intervertebral veins (vv. intervertebrales) — vessels ensuring blood drainage from the spinal cord and connecting the internal vertebral venous plexuses with the external ones (in the lumbar region, with the lumbar veins).

How does the costal process form during ontogenesis?

The costal process forms during ontogenesis due to the complete fusion of the lumbar rib rudiment and the vertebra's own transverse process.

Where exactly is the accessory process located?

The accessory process (Processus accessorius) is a small structure located at the base of the costal process, inferiorly and posteriorly to it.

Why does the mamillary process form?

It forms as a result of the pull of powerful back muscle tendons attached to the vertebra, serving as an additional fulcrum for them.

How are the spinous and articular processes oriented in the lumbar region?

Both types of processes in the lumbar vertebrae are oriented in the sagittal plane. Meanwhile, the spinous processes have a flat and wide shape.

Go deeper

Vertebral ontogenesis and mechanisms of rudimentary rib fusion
Biomechanics of sagittally oriented articular processes
Comparative analysis of upper lumbar and lower thoracic vertebrae
Anatomical prerequisites for the formation of mamillary processes by powerful musculature