黑颈眼镜蛇种

毒液

它的毒液在眼镜蛇中是比较特别的——主要由细胞毒素而非神经毒素构成。被咬伤处会出现大出血和组织坏死，并会感到呼吸苦难。被咬者在未经治疗的情况下有5–10%的死亡率，死亡原因一般是隔膜麻痹导致的窒息。其半数致死剂量为2 mg/kg SC，或1.15 mg/kg IV。它一次咬伤可以注入200至350 mg的毒液。

The black-necked spitting cobra (Naja nigricollis) is a species of spitting cobra found mostly in sub-Saharan Africa. They are moderately sized snakes that can grow to a length of 1.2 to 2.2 m (3.9 to 7.2 ft) in length. Their coloration and markings can vary considerably. They prey primarily on small rodents. They possess medically significant venom, although the mortality rate for untreated bites on humans is relatively low (~ 5–10%, in endemic regions under 1%). Like other spitting cobras, they can eject venom from their fangs when threatened (one drop over 7 metres (23 ft) and more in perfect accuracy). The cytotoxic venom irritates the skin, causing blisters and inflammation, and can cause permanent blindness if the venom makes contact with the eyes and is not washed off.

Naja nigricollis belongs to the cobra genus Naja under the family Elapidae. It previously included two subspecies that have been moved to the species Naja nigricincta – the zebra spitting cobra (Naja nigricincta nigricincta) and the black spitting cobra (Naja nigricincta woodi).

Taxonomy

Naja nigricollis is classified under the genus Naja of the family Elapidae. It was first described by Norwegian zoologist Johan Reinhardt in 1843. The generic name Naja is a Latinisation of the Sanskrit word nāgá (नाग), meaning "cobra". The specific epithet nigricollis is Latin for "black-necked": niger, meaning "black", and collis, meaning "neck".

Naja nigricollis previously included two subspecies – Naja nigricollis nigricincta and Naja nigricollis woodi.

Description

N. nigricollis is a moderately sized, venomous snake with a moderately distinct head; the shape of the head is due primarily to two large venom glands found on each side of the head. Its colour can vary depending on region of origin. Some specimens are black or pale grey with a yellow or reddish ventral side with a broad, black neck band, often with an orange or pinkish bar on the neck.

Distribution and habitat

The species is very common and is widespread through much of sub-Saharan Africa and is found in western, eastern, central, and parts of southern Africa. It is absent from the rainforests of the Congo Basin.

Behavior

Unlike other snakes, Naja nigricollis can be either nocturnal or diurnal depending on the time of year, geographic location, and average daytime temperature. This adaptability allows the snake to better regulate its body temperature and to gain access to the most abundant food sources of a particular area. It feeds primarily on small rodents, such as small rats and mice, birds, and fishes, but will also eat lizards, eggs, and other snakes.

Reproduction

Like other cobra species, this snake is oviparous. The mating season of this species can vary from the end of winter (September) to the beginning of summer (December). Usually, the mating season is the same whether in captivity or in the wild. Females will commonly lay 10 to 15 eggs but can lay anywhere between eight and 22 eggs at a time.

Venom

The venom of Naja nigricollis is somewhat unusual among elapids in that it consists primarily of cytotoxins, but with other components also. It retains the typical elapid neurotoxic properties while combining these with highly potent cytotoxins (necrotic agents) and cardiotoxins. Bite symptoms include severe external hemorrhaging and tissue necrosis around the bite area and difficulty breathing.

Snake venom ophthalmia

In 1944, English ophthalmic surgeon Major Harold Ridley, published a short paper in the British Journal of Ophthalmology on spitting snakes and an account of the composition and action of snake venom in general. From his experiences in wartime Ghana, Ridley described snake venom ophthalmia in a 30-year-old labourer, who was cutting grass when a Black-necked cobra spat venom toward the man's right eye from a four- or five-feet distance.