(BEING CONTINUED FROM 31/10/15)
Alexandria (Hellenistic era)
Up to the Hellenistic era, all knowledge of neuro-anatomy had been based purely on animal dissections. The heyday of neuro-anatomy dawned when Herophilus and Erasistratus
commenced human dissection (probably even vivisection) of condemned criminals, under patronage of the Ptolemaic pharaohs in the newly established city of Alexandria (332BC). However, with the probable exception of Eudemus (3rd century BC), Numisianus and Marinus (2nd century BC),subsequent Alexandrian physicians did not continue systematic
human dissection.1 Virtually all the extensive original writings of Herophilus and Erasistratus were destroyed, and our knowledge of their contributions is based on comments by later authors such as Theophilus, Oribasius, Rufus, Vindicianus and
– in particular – Galen.6 Herophilus and Erasistratus probably dissected as many, or more, animals than humans, which is indicated by occasional errors in their reported findings, e.g.
the description of the presence of a rete mirabilis anastomosis at the base of the human brain – a structure found in angulated animals but not in humans. But it can be stated confidently that the basis of human neuro-anatomy was laid by the two Alexandrians.1 Illustrations were used for the first time in history to clarify descriptions, a technique later disapproved of by Galen.8 Both performed vivisection on animals (and possibly humans) to verify the function of parts of the brain and nerves.
Herophilus’ impressive findings included the following:6,10
1. A description of skull sutures.
2. Two layers of meninges (the dura mater and pia mater) covering the brain, the outer one involved in the formation of the tentorium cerebelli, and lining the intracranial venous
sinuses. This included the prominent junction of sinuses named the lênos (wine vat) (Galen renamed it the torcular Herophili; however, this structure is rarely prominent in
man, and the term could represent another example of animal anatomy corrupting knowledge of human anatomy).
3. Differentiation between the cerebrum and cerebellum.
Herophilus considered the latter to be responsible for motor activity and equilibrium.
4. The 2 lateral brain ventricles, their openings into the 3rd ventricle, aqueduct of the midbrain and the 4th ventricle,and the choroid plexus (resembling the fetal choroid
membrane) lining these spaces.
5. The 4th ventricle was considered to be the seat of the soul,and details such as a pen-like cavity in its floor (calamus scriptorius – probably the aperture into the spinal column)
6. The origin of the spinal column from the brain, the pineal body (resembling a pine cone), the structure of the eye and at least some of the cranial nerves (optic with optic chiasma,
oculomotor, trigeminal, facial, auditory and hypoglossal nerves) were identified.
7. His detailed study of nerves (distinguishing between motor and sensory nerves) was a fundamental contribution.
However, he still considered the optic nerve to be hollow,and included tendons and ligaments as terminal extensions of nerves in muscles (this part of the complex mechanism then supposedly enabling voluntary motor activity).
We know more about Erasistratus’ neurological concepts as he was quoted more extensively by Galen (often critically) because he dared to differ from Hippocrates.7 Largely agreeing with Herophilus’ neuro-anatomy, he taught that the brain’s convolutions related to mental ability (therefore more convolutions in man than in animals, and in the cerebellum rather than the cerebrum). He claimed that not only the optic nerve but all sensory nerves were hollow to enable the flow of ‘psychic pneuma’ (see below); sensory nerves originated in the meninges and motor nerves in the brain.1,6
Marinus (2nd century BC) made a detailed study of the cranial nerves, describing the olfactory nerve structure, and a large nerve descending from the lower brain into the thorax (either the vagus or recurrent laryngeal nerve).6
With human dissection virtually discontinued in Hellenistic times, the subsequent Roman era brought little enhancement of human neuro-anatomical knowledge. At the turn of the
1st century AD, Rufus of Ephesus reviewed known medical information and, although misconception regarding the neuron persisted, he elegantly described the brain, spinal cord and nerves as an interconnected nervous system.7
Galen (2nd century AD), the most prominent physician of Roman times, whose views became dogma in the medieval era, wrote extensively on medical matters, including the
nervous system. His views on neuro-anatomy were based on existing knowledge (mainly the Alexandrian findings) as well as extensive personal dissection of animals (oxen and apes in particular) but not humans. He largely confirmed the work of Herophilus and most of that of Erasistratus. Until then, the ventricles had been much studied, the brain substance being seen as little more than supporting tissue. Galen now described the corpus callosum, thalamus, septum lucidum (separating the lateral ventricles), and the fornix in the roof of the 4th ventricle – the latter he accepted as the centre of the hegemonikon. The pituitary gland and its stalk attached to the 3rd ventricle floor
were described; like Herophilus, he wrongly placed a retiform plexus at the base of the human brain.1
Galen gave a good description of the blood vessels of the brain. The optic nerve and most (but not all) other nerves were believed to be hollow. The spinal cord was said to be a
bundle of nerves running together from the brain down the vertebral column to the rest of the body and emerging from its bony cage through intervertebral foramina. He still considered ligaments and tendons as equivalents of nerves, and stated that while sensory nerves were soft and arose from the cerebrum,motor nerves arose from the spinal cord and were hard; nerves arising from the junctional area could be either soft or hard.
He was adamant that brain convolutions had no qualitative relationship to brain function.7,11
Post-Galenic writers of antiquity (e.g. Oribasius) quoted their predecessors without adding significant new knowledge.5
(TO BE CONTINUED)
11. Singer C. Galen on Anatomical Procedures. Oxford: Oxford University Press, 1956: 234-236.