[解剖學]
繼續「扑頭」系列。點解會有咁多報導，話傷者頭部被襲之後，會大量出血？原因就係因為人嘅頭皮底下有豐富嘅動脈分佈。
人嘅頭皮有五層 (圖一)，由淺入深分別係皮膚 (skin)、結締組織 (connective tissue)、腱膜 (aponeurosis)、疏鬆結締組織 (loose connective tissue) 同埋顱骨膜 (pericranium)。皮膚係好薄嘅最外層，由頭髮保護，亦有好多細小嘅血管同淋巴管；其下嘅結締組織則有豐富嘅神經分佈。腱膜層比較有韌性、係頭皮肌肉嘅附着點，而海綿一般嘅疏鬆結締組織係一個重要嘅夾層，因為細菌同血液可以沿住呢一層擴散出去。而最內嘅顱骨膜，顧名思義，就係顱骨外嘅一層結締組織啦。
供應頭皮嘅動脈位於結締組織同腱膜兩層之間，而且由好多唔同嘅分支供應 (詳見圖二)。呢啲動脈分支互相聚合。當受外力擊中、使頭皮破裂嘅時候，呢啲血管就會撕裂。既然係動脈，其中血液嘅血壓自然比較高，出血嘅情況亦會比較嚴重。仲要注要一點，呢啲動脈嘅血管壁同其上嘅結締組織密不可分，所以雖然血管破裂亦唔識收縮咁滯，使止血更具難度。
如果沖擊再大力小小，使腱膜破裂嘅話，傷口就會難以復合，因為附於其上嘅肌肉 (枕額肌，occipitofrontalis muscle) 會將傷口越拉越開。如果傷口嘅血液或細菌流入疏鬆結締組織層，咁就有機會將佢哋傳至整個頭皮嘅範圍 (前方為眼睛以上、後方為頸部以上)，甚至進入顱內嘅結構 (如腦膜)。
請問你，喺重棍一下下打落人哋個頭上面之前，你知唔知有機會發生以上嘅情況？你知唔知你嘅一棍，可以有幾嚴重嘅後果 (再參考返之前腦硬膜外出血嘅帖子)？
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[Anatomy]
Another post about attacking the head. Many reports had stated that, after being attacked on the head, people had suffered from massive blood loss. Why is it so? Because under the scalp there is a rich arterial supply.
There are five layers in the human scalp (refer to figure 1), from exterior to interior they are the skin, connective tissue, aponeurosis, loose connective tissue and pericranium (interestingly the initials of these five words form the word SCALP). The skin is a thin layer protected by hair, and is richly supplied by small blood vessels and lymphatic drainage. The connective tissue underneath has abundant nerve supply. The tough aponeurosis acts as the attachment of the occipitofrontalis muscle. The loose connective tissue layer is sponge-like, and is clinically important as it acts as a potential anatomical space to spread blood and microbes into other areas. The innermost pericranium is the connective tissue layer that acts as the external periosteum of the cranium.
Various arteries supply the scalp, and they lie between the connective tissue and aponeurotic layers (refer to figure 2 for details), forming anastomoses. When trauma tears the scalp, these arteries may lacerate - with quite some blood pressure, there will be a considerable blood extravasation. Also worth noting is the firm attachment of the walls of these arteries to the dense connective tissue in which they are embedded, which leads to impaired vasoconstriction and poor effect when carrying out first-aid haemostasis.
If the traumatic force is large enough to break the aponeurosis, the wound cannot heal easily as the occipitofrontalis muscle will tear the wound, making it bigger and bigger. In a more serious condition where blood or pathogens make their way into the loose connective tissue layer, they may spread all through the scalp (bounded anteriorly by the eyes and posteriorly by the neck) or even get into intracranial structures like the meninges, causing secondary and possibly severe complications.
Allow me to ask: before you hit with your stick, do you know there is such a variety of possible consequences and effects? And are you able to evaluate the severity of these consequences after you hit someone (combined with the possibility of having an epidural haematoma, as mentioned in a previous post)?
Figure adapted from Moore KL, Dalley AF, Agur AMR. Clinical Oriented Anatomy, 6th edition. Lippincott Williams & Wilkins, 2010.
