the end of the 1700s, the nervous system had been completely dissected and the major parts described in detail. The brain was seen to be composed of gray matter and white matter, terms we continue to use today (see Figure 1.7). White matter was involved in moving information to and from the gray matter. Today, we have a fuller understanding of brain structure, with the thin outer shell of the brain consisting of cells, which appear to be a darker color and are thus called gray matter. Underlying this are the axons, which transfer information throughout the brain. Their myelin sheaths are lighter in color, and thus these areas are referred to as white matter. Myelin is made up of fats and proteins and wraps around axons like insulation does around electrical cables, resulting in an increased speed of information transmissions.
Also by the 1700s, scientists knew that there was a general pattern in all human brains in how the brain was structured in terms of surface structures or bumps, which were called gyri, and the grooves between them, referred to then and now as sulci and fissures. The present-day terms used to describe parts of the brain also come from Latin, so the lobes of the brain are the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. This can be seen in Figure 1.8.
Scientists of the 1700s also determined that the nervous system had a central division consisting of the brain and spinal cord and a peripheral division consisting of nerves throughout the body (see Figure 1.9).
Figure 1.8 The Lobes of the Brain
Source: Schwartz, Sensation and Perception, Figure 4.7.
The 1700s to the 1900s
With the basic structure of the nervous system known, scientists of the 1700s began a quest to understand how the system developed and how it worked. One of the contributions of this quest was the realization that the body created and used electrical activity in its basic processes. Scientists such as Luigi Galvani and Emil du Bois-Reymond were able to show that electrical stimulation causes a frog’s leg to twitch. With this demonstration, nerves began to be thought of as wires through which electricity passes. Further, it was determined that the brain could itself produce electrical activity. The greater impact of this discovery was that electricity was also something that could be measured, thus setting the stage for the following centuries in which experimentation in the electrical activity of the brain and body would play a significant role in physiology and psychology.
Figure 1.9 Distribution of the Nervous System
Source: Pixologic Studio/Science Source
One of the discoveries during the early part of the 1800s was that there was a system for sending information to the muscle, which resulted in muscle movement, and another system for bringing sensory information back to the brain. When you hold a glass, for example, the sensory or affector system relays information on what the object you are touching feels like, whereas the muscular or effector system tells the muscles how to hold and pick up the glass. Thus, in many nerves there are connections for both receiving and sending information (Figure 1.9). These pathways are referred to as fiber tracts.
At the level of the spinal cord, these fiber tracts split, with the sensory information being conveyed by the dorsal root and the action or motor information involving the ventral root. By the 1850s, Hermann von Helmholtz had measured the speed of the nerve impulse and found it to be around 90 feet a second, which is a little more than a mile a minute. Of course, this is much slower than the speed of electricity in a copper wire, which approximates the speed of light (186,000 miles per second). However, the advantage of the nerve impulse—as shown in later research—is that it is not diminished over the length of its travels.
One important realization of the 1700s was that particular functions could be localized to different parts of the brain. One person often cited today is Joseph Gall. Although Gall was correct in suggesting that the frontal part of the brain involved higher cognitive processes and social determinations, he was wrong in assuming that somehow brain function would be reflected in the shape of (and bumps on) the skull. If an individual were good at a particular ability, Gall assumed that his or her skull would look different from another person’s skull who was not as talented. To support this idea, he examined the skulls of people at the extremes, such as great writers, statesmen, and mathematicians as well as criminals, the mentally ill, and individuals with particular pathologies. Overall, he defined 19 processes that he thought humans and animals both performed and another 8 that were unique to humans (see Figure 1.10).
Although Gall and his followers never scientifically tested their ideas, research by others did not support their claims about the structure of the head. What Gall did that was supported was to suggest viewing the brain as capable of performing a variety of functions and that these functions could be localized in different parts of the brain.
Abilities related to understanding and producing language greatly aided specific discoveries related to cerebral localization of function. Physicians began to collect considerable information on patients who had a variety of difficulties with language. Some patients could understand language but could not produce speech. Others had trouble remembering words. Still others could not understand language.
Figure 1.10 Gall’s Structure of the Head
© iStockphoto.com/Mark Strozier
A major turning point occurred when Paul Broca examined a patient who could understand language but could not speak. In 1861, this patient was sent to Broca with a much more serious medical condition and died shortly thereafter. Broca then performed an autopsy and reported an abnormality in an area on the left side of the frontal lobe. Based on a variety of cases, Broca was able to show that language is a left hemispheric process and that damage to the frontal areas of the left hemisphere results in problems in higher executive functions such as judgment, the ability to reflect on a situation, and the ability to understand things in an abstract manner (Finger, 2000). Today, the area related to language production in the left hemisphere is called Broca’s area (see Figure 1.11).
In 1874, Carl Wernicke published a paper that suggested that language understanding was related to the left temporal lobe. He studied patients who were unable to comprehend what they heard. At the same time, they were able to produce fluent speech, although it was incomprehensible and included nonexistent words. The specific place in the brain identified by Wernicke is now called Wernicke’s area (see Figure 1.11). The discoveries of Broca and Wernicke helped the scientific community understand that language was made up of different processes, including the ability to understand and to produce language. Modern case studies show even more complicated processes. Texting appears to use different parts of the brain than other language processes. A 40-year-old man had no trouble reading, writing, or understanding language. However, he did have problems sending coherent text messages on his cell phone. Brain imaging showed that he had had a stroke. A healthy 25-year-old woman also showed garbled texting following a stroke (Ravi, Rao, & Klein, 2013).
Throughout his career, John Hughlings Jackson examined the brain from a developmental and evolutionary perspective (see Williamson & Allman, 2011). Hughlings Jackson saw the brain as composed of three levels. The earliest part of the brain to evolve was the spinal cord and brain stem, which controlled the vegetative functions such as breathing, sleep, and temperature control. The next level to evolve included the basal ganglia, which is connected to various other parts of
