Calvaria, Dural Venous Sinuses, Brain, and Cranial Nerves

Atlas References

• Netter: 111–126, 149–158

• McMinn: 51–53, 62–72

• Gray's Atlas: 490–502

Before You Begin

The skin of the face has been previously removed (see Chapter 21 ).

Dissection Steps

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  Continue the removal of the facial skin toward the occipital region, and separate the skin from the subcutaneous tissue ( Fig. 23.1 ).

  

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  Identify and expose the superficial temporal artery and its branches ( Fig. 23.2 ).

  

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  Once the superficial temporal artery has been fully exposed, separate the subcutaneous tissue and fat from the underlying galea aponeurotica, or epicranial aponeurosis (see Fig. 23.2 ).

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  This aponeurosis is a flattened tendon that connects the occipitalis and frontalis muscles, forming the occipitofrontalis muscle.

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  Cut and detach the frontalis muscle from the frontal bone.

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  With forceps, grasp the frontalis muscle and reflect it posteriorly ( Fig. 23.3 ).

  

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  On the internal surface of the frontalis muscle, note the loose areolar tissue, and on the surface of the cranium, note the pericranium ( Fig. 23.4 ).

  

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  At this part of the dissection, identify all the previously dissected layers of the scalp ( Plate 23.1 ):

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    S kin

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    C onnective tissue (subcutaneous tissue)

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    A poneurotic layer

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    L oose areolar tissue

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    P ericranium

  

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  To expose the brain, the calvaria, or “skullcap,” must be removed. The calvaria consists primarily of the parietal, frontal, and occipital bones. Dissect away the occipitofrontalis muscle and expose the pericranium, leaving the temporalis fascia intact ( Fig. 23.5 ).

  

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  With a scalpel, reflect the temporal fascia and expose the temporalis muscle ( Fig. 23.6 ).

  

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  Clean the temporalis muscle and note its tendinous fibers ( Fig. 23.7 ).

  

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  Reflect the temporalis toward the zygomatic bone, and expose the pterion ( Fig. 23.8 ).

  

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  Once the temporalis muscle is completely reflected and the calvaria fully exposed, identify the frontal, parietal, temporal, and occipital bones, as well as the sagittal, coronal, and lambdoid sutures.

Dissection of the Skull

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  Keep the cadaver in the supine position.

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  Place a plastic or wooden block under the head or shoulders to elevate the body from the dissection table.

Dissection Tip

Place paper towels around the face and temporalis muscles to protect them from bone dust ( Fig. 23.9 ).

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  We use two different techniques to expose the contents of the skull.

Technique 1

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  With a marker, draw a stippled midsagittal line from the nasion to the external occipital protuberance.

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  Draw a second, circumferential line passing 1 to 2 cm above the superciliary arches and ears to reach 1 to 2 cm above the external occipital protuberance posteriorly (see Fig. 23.9 ).

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  With an electric saw, make a cut 2 to 3 cm (~1 inch) lateral to the midline, on both sides. In this way, the superior sagittal sinus and the falx cerebri will remain intact ( Fig. 23.10 ).

  

Dissection Tip

Caution! Use the electric saw carefully.

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  Make a shallow, circumferential cut of approximately 1 cm in depth.

Dissection Tip

Do not place the saw too deeply, because you will cut the dura and the brain. To complete the cut toward the external occipital protuberance, it is necessary to rotate the cadaver.

Once the first cut is complete, use the chisel and mallet to break through the bone and detach the two bone flaps ( Fig. 23.11 ).

Dissection Tip

Detaching the bone flap from the underlying dura mater can be challenging. Place the forceps or a chisel into the gap (created by the saw cut) between the two adjacent bones, and use it to lift it up from the dura. If the dura is attached to the calvaria, use a probe to reflect the endosteal layer away from the calvaria, leaving the dura mater intact.

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  After removal of the calvaria, examine the dura mater and identify the middle meningeal artery and its branches (see Figs. 23.11 and 23.13 ).

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  Cut and reflect the dura, and expose the subdural space ( Fig. 23.12 ).

  
  

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  Observe the arachnoid layer covering the brain and the cerebral veins penetrating the arachnoid mater (see Fig. 23.12 ).

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  Place your hands on one of the cerebral hemispheres and retract it laterally ( Fig. 23.14 ).

  

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  Notice the midline connection between the two hemispheres, the corpus callosum (see Fig. 23.14 ).

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  With a scalpel, make a midsagittal cut, and reflect one of the brain hemispheres, leaving intact the dural venous sinuses ( Fig. 23.15 ).

  

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  Perform the same technique to the contralateral side, and expose the sinuses bilaterally ( Figs. 23.16 and 23.17 ).

  
  

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  Identify the falx cerebri, a dural partition separating the right from the left cerebral hemispheres.

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  Superior to the falx cerebri, identify the superior sagittal sinus, which joins the two transverse sinuses at the confluence of sinuses (“torcular herophili,” or wine-press of Herophilus).

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  Inferolateral to the falx cerebri, look for the tentorium cerebelli, a large dural infolding separating the occipital lobes from the cerebellar hemispheres.

Dissection Tip

If time permits, remove the midportion of the calvaria, and expose the superior sagittal sinus. Incise the dura forming the sinus, and notice its internal structure. In the majority of specimens the superior sagittal sinus will drain into the right transverse sinus.

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  At this point, the dissection can continue as outlined later.