MAMMARY GLAND—SPINAL CORD. 61 
MAMMARY GLAND. 
What is the structure of the mammary gland ? 
It is a compound racemose gland, the alveolar spaces of which, lined 
with cuboidal epithelium, open into fifteen or twenty excretory ducts, 
lined with cylindrical cells. Between the gland alveoli the spaces are 
tilled with fibrous areolar tissue and adipose tissue, and contain 
numerous bloodvessels. When the gland is active there seems to be 
a marked increase in the number of gland alveoli, and the epithelial 
lining cells contain drops of fat. 
THE CENTRAL NERVOUS SYSTEM. 
SPINAL CORD. 
Of what is the spinal cord composed ? 
Of gray matter internally and white matter externally. It is 
covered by a sheath of connective tissue (pia mater) containing 
numerous bloodvessels. The inner layer of this sheath is dense and 
fibrous, and the outer layer, sometimes called the arachnoid, is more 
delicate in structure and is non-vascular. Lining the vertebral canal 
is another dense fibrous layer called the dura‘mater. The cord is nearly 
divided into two hemispheres by an anterior and a posterior fissure, 
into both of which a fold of the pia mater projects. The hemispheres 
are united by a bridge composed anteriorly of white matter (white 
commissure) and posteriorly of gray matter (gray commissure). In 
the centre of the gray commissure is a small canal (the central canal), 
lined with cylindrical ciliated epithelium cells. 
How is the gray matter of the cord arranged ? 
In the centre of each hemisphere it is collected into a.crescentic-like 
mass with its long axis running antero-posteriorly. The two masses 
are connected by the gray commissure. The anterior cornua are 
rather thick and do not come to the surface of the cord. “The poste- 
rior cornua are narrow and do come to the surface. 
How is the white matter of the cord arranged ? 
By the passage of nerve roots to the cornua it is divided into three 
principal divisions: The anterior, lateral, and posterior columns. 
What is the structure of the gray matter of the cord ? 
It contains, especially in the anterior cornua, a number of multi- 
polar or ganglion cells, also peculiar branching cells called neuroglia or 
spider cells. Besides these cells it consists of interlacing nerve fibres 
