52
7.3 How to age cattle by their teeth
Cattle have eight incisors, three pairs of premolars and three pairs of molars Figure 7.8
Figure 7.8: Skull of cow showing incisor, premolar and molar teeth
Table 7.1: Aging cattle from incisor and molar eruption Age
Incisors Molars
6 months First permanent molars up
12 m All calf teeth present
2nd permanent molars appear 15 -18 m
First permanent incisors appear 18 m
2nd permanent molars up 18 -24 m
First permanent incisors in wear 24 -30 m
2nd permanent incisors up 3rd permanent molars appear
30 m 3rd permanent incisors up
3rd permanent molars up 36 m
3rd permanent incisors in wear 39 m
4th permanent incisors up 42 m
4th permanent incisors in wear
7.4 How to remove the whole intact brain using the longitudinal craniotomy
Longitudinal craniotomy involves splitting the skull but not the brain ventrally and dorsally along its longitudinal axis with a hatchet. The two halves are levered open from the front end
to expose the intact brain. The hatchet is hit with a small sledge hammer for greater control and safety. Eye protection should be worn. It has the advantage of exposing the pituitary
gland, nasal passages and nasopharynx. The procedure can be performed quickly, simply and safely under field conditions with minimal equipment. The method requires practice but
once mastered the whole intact brain can be removed in less than one minute.
53 The
equipment required includes a hatchet, small sledge hammer, boning knife, disposable
rubber gloves and safety glasses.
The dissection process is as follows:
• Skin the ventral head and remove the tongue and soft tissues of the throat so as to expose the hard palate and ventral cranium. Turn the head over and cut the skin on the
top of the head along the midline. Extend the cut deeply into the soft cartilage of the nose in the midline to split the nose. The split line is shown in Figure 7.8.
• With a hatchet, crack the dorsal skull along the length of the midline where the skin cut has been made. The aim is to crack the bone cranium surrounding the brain without
damaging the brain but fully cut through the depth of the nose and jaw. Turn the head over. Using a hatchet, split the front of the bottom jaw, crack the cranium along the
midline and split the hard palate and nose to full depth.
• With a knife, cut any soft tissue attachments that might prevent the two halves of the head being levered apart using the two halves of the split nose. Grasp the two halves
of the split nose and slowly prise and pull them apart as shown in Figure 7.8. More bone cracking will be required if the head won’t come apart easily. If the cranium has been
cracked sufficiently, the whole head can be levered open and the brain and pituitary gland exposed.
Figure 7.8: The splitting line and leverage points for a longitudinal craniotomy
• A boning knife or scissors is used to cut the nerve roots and dura mater the white tough membrane that surrounds the brain. After removing the dura mater the brain can
be rolled out from front to rear. • Remove all of the dura mater reflection tentorium cerebelli that projects downwards to
provide a transverse partition between the cerebral hemispheres and cerebellum. Failure to excise it may result in damage to critical anatomic sites in the brainstem when
the brain is subsequently lifted from the cranial cavity.
• The hemisected pituitary gland is exposed at the base of the brain. Check the base of the cranium for abnormalities such as fractures, tumours or abscesses. It is easily
removed if required.
54 • Cut the brain between the cerebral hemispheres to just open the ventricles. This will
allow formalin to access the inside of the brain. Alternatively, the cerebrum can be cut transversely multiple times like a loaf of sliced bread but not full thickness so as to leave
a 2-3 cm depth of tissue along the ventral brain to connect the slices. Immerse the brain and pituitary in 10 buffered formalin.
Common mistakes include cracking cutting too deeply into the bone around the brain, particularly the bone protecting the ventral brain, which can lead to damage of the ventral
brain stem. Insufficient ‘cracking’ of the bones surrounding the brain, particularly the ventral cranium, can make it difficult to lever the cranium apart. One side of the nose breaks.
Levering the head apart too quickly can tear the brain.
7.5 How to remove the brainstem with a modified spoon