302 THE INTERNAL FNVIRONMF.NT OF THF BODY Part III 



Brain Size. In evolutionary history, the greatest increase in the size of 

 brains occurred as animals began to live on land. Swimming in the sea was 

 monotony and ease compared with clambering through the ooze and over 

 the hillocks on land. The adaptation to the details and variety of land living 

 left its mark on the brain as it did on the legs and feet. The human brain is 

 heavy in proportion to the weight of the body. Its weight varies with the age 

 and size of the individual. Except in extreme cases such as some defective 

 individuals there appears to be no correlation between size and weight of 

 the brain or number of cerebral convolutions and the degree of intelligence. 



Primary Divisions of the Vertebrate Brain 



The embryonic brain is the key to the structure of the adult brain. In its 

 earlier development, the brain is a single hollow enlargement whose cavity 

 is continuous with that of the nerve cord. In the third week of human life, 

 two constrictions indicate three regions, the fore-, mid-, and hindbrain (tech- 

 nically called the prosencephalon, mesencephalon, and rhombencephalon). 

 In the fourth week, a constriction forms two subregions of the forebrain, 

 the endbrain and the between brain, respectively the telencephalon and 

 diencephalon. In man, the hindbrain is set off into two regions, the future 

 cerebellum and future medulla (metencephalon, myelencephalon) about the 

 fifth week. The brain is then composed of five primary regions, from anterior 

 to posterior: (1) endbrain, (2) between brain, (3) midbrain, (4) future 



Arachnoid frabecula. 

 Subdural space. 



Arachnoid membrane 

 PiQ mater. 



Arachnoid villus Dura moter. 



Superior saqiHol smus 



Endothelium 



Suborochnoid space Fcix cerebri Corlex cerebri 



Fig. 16.19. Relation of the meninges, the protective covers of the brain, the 

 pia mater (tender mother) next to the brain, the dura mater (tough mother) next 

 to the skull and the arachnoid (spiderweb) layer between them. The spaces in 

 the latter are filled with the cerebrospinal fluid, a modified tissue fluid. All of 

 the layers also surround the spinal cord including the spinal fluid. Diagram of the 

 layers as they overlie the brain. Excess cerebrospinal fluid drains through the 

 arachnoid villus, one of many that extend into the blood of the sinus. This occurs 

 in many areas of the brain. (After Weed. Courtesy, Ham: Histology, ed. 2. Phila- 

 delphia, J. B. Lippincott Co., 1953.) 



