EVOLUTION OF FUNCTION.

Otago Witness, Issue 3830, 9 August 1927, Page 15

EVOLUTION OF FUNCTION.

V- THE NERVOUS SYSTEM.

LECTURE BY DR MALCOLM

The fourth of the series of popular lectures on evolution was given in the physics Toom of the University on Thursday evening, in the presence of a large attendance. Ihe lecturer for the evening was Dr J. Malcolm, professor of physiology at the Otago University, who took for'his subject, “ Ihe Evolution of Function.” Mr J. C. Stephens occupied the chair, and briefly introduced Dr Malcolm. Dr Malcolm explained that his lecture ■was really an outline of the work and uses ©f the nervous system from the higher forms of life to the lower. The bodies of the higher animals, he said, were composed of cells of four separate kinds—epithelial, muscular, connective, and nervous. The first three formed the organs and the alimentary, resperatory, and circulatory systems, but, unlike them, the nervous system was composed of one special Kind of tissue, and its function was to harmonise the actions of all parts. This was exemplified bv the fact that when one chewed food, saliva automtically flowed, and when food was chewed it was carried through the elementary system without any muscular exertion. Dr Malcolm then went on to describe the two kinds of neurones (whole nerve cells with the processes belonging to them) —sensory and motor—and their relation to each other jn what was called the. reflex . arc. One side of the arc was, he said, made •np of the receptor and the other of the effector, and between the two was the adjustor or connector neurone. The sensory neurone was the receptor, which received nerve impulses which were set up, and conveyed them to the brain. Th e motor neurone acted as the effector to which impulses were conveyed across the connector of the arc, the result being reflex action as in the case of drawing a foot away from the heat_ of a fire. The lecturer then gave an interesting illustration of how a nerve worked, and showed that the nerve of a frog’s leg under the influence of an electric current actuated the muscle. When a nerve impulse was set up, it progressed in a wave along a nerve fibre and through a cell body, and was transmitted from one neurone to another; its progress could be likened to a spark in a train of gunpowder. Seeing that a reflex was the simplest form of complete action in the nervous system, the question of how the evolution of this function could be traced, arose. In the sponge—a low form of multicellular animal, effectors only were present. The sponge received its food in the form of sea water through channels leading in from the side to a central cavity. These channels closed by j means of a species of contractile cell with I only effectors present. No nervous sys- i tern was in evidence. In the hydra, re- i ceptor and effector were combined in one | coll. Anemones and jelly fish brought, the 1 nervous system to a still higher plane. ! Thev were lined on both'' sides by j epithelial cells, among which occurred j special sensory or receptor cells which had a long process extending into a nerve ! network which in turn sent the fibres into-! the muscle layers. Here, they had the I receptor and effector with a complicated j nerve net which allowed the ! nerve impulse to ''spread widely. This ■ was not the true adjustor mechanism, but 1 in jelly fish there was an evolution of j specialised receptors. In the earthworm a great advance was made. The body was segmented, and it had an elongated nerve cord with collections of nerve cells in each segment. Here they met first with a kind of adjustor, and therefore the rudiments of a complete reflex arc. The value of this was that muscles acted in definite and different ways. For instance, instead of the impulse from a stimulated" point causing contraction of al] the muscles indiscriminately, only one or two segments were effected', and 'owing to the development of the head end and special receptors for light, the reflex action was adjusted to the needs of the worm. This was a rudimentary form of the reflex arc, and no doubt it was from animals like these that the vertebrates evolved. Coming to the lower vertebrates, they found a groat evolution of the adjustors, and the spinal cord was evidently due to a fusion of a chain of ganglia. It differed, however, in many respects from the gangliated cord of the invertebrates. For example, it was placed behind the food canal, and it had another central hollow canal. In vertebrates, the receptors at the head end, those of smell, hearing, vision, taste, and touch, required more adjustors than the rest of the body, and the cord nt this end developed the large masses that constituted a brain. As evolution proceeded, tae adjustor mechanism became even more Complicated and specialised; thus, a group of cells was set apart to be the adjustor of reflexes affecting the heart, respiration, etc. These were located in the lowest part of the brain or medulla oblongate- Behind that, a great mass of neurones developed, and was connected with muscular movement. This was the cerebellum. In the brain system, next to the medulla, came the optic lobes which were concerned with visio,., and further forward still came the small nerves and the cerebrum. The latter part was the one which ultimately evolved to the greatest extent. - The cerebrum was the great organ of intelligence, and had attracted more investigation than any part of the brain. In the cortex there were three main layers, chiefly cells, with two layers mostly of fibres. In the evolution of the cerebrum and the development of every brain, these appeared in a definite order, so that most of the layers were evolved and developed last. It was la this great and complicated division

that the brain of man differed from that of the lower invertebrates. The receptors and effectors had evolved to an even greater pitch of perfection in the lower animals, as, for example, the eye of an eagle and the muscles of a tiger, but -in the intelligent use of his brain cells man could eclipse both. Dr Malcolm then went on to explain the relation of the nervous system to the rest of the body, and pointed out that it undoubtedly co-ordinated all its activities, whilst at the same time it was the most dependent of all the systems of the body. If the respiratory, alimentary, or circulatory systems failed, it was the nervous system which suffered most. One of the greatest factors in the evolution of the highest type of man was uniform temperature, for all cell, and especially nerve cell, activity was diminished by cold and raised by heat. It was the nervous system itself that was responsible for uniform temperature. There was a part of the brain that was specially sensitive to changes of temperature, and which put into action mechanism which counteracted a threatened rise or a threatened fall of temperature. All these conditions worked together to produce that wonderful organ the human brain. In conclusion, Dr Malcolm said that, most appropriately, Dr Benham had closed his lecture with a quotation from the Psalms. This reminded him that Sir William Baylis, in a preface to his work on physiology, quoted St. Paul’s advice to the Thessalonians : “Prove all things; hold fast to that which is good,’’ and called attention to the tact that the Greek interpretation of the word “good’’ was “beautiful and true.” It was the brain, the organ of the mind, that enabled them to comprehend truth and beauty, whether it was displayed in the material or the sniritual world.

The lecture was illustrated bv some 40 very fine lantern slides.