334 METAZOAN PHYLA 



the body, where they control the involuntary muscles in the walls of 

 these vessels. The largest ganglion in this system, the semilunar ganglion, 

 is situated in the upper part of the abdominal cavity. It gives off 

 nerves to the liver, stomach, and other neighboring organs. 



The brain, or encephalon, originates in the embryo as a dilated portion 

 of the neural tube, divided at first into three parts known respectively 

 as the forebrain, mid-brain, and hind-brain. It becomes divided later 

 into five parts, the forebrain and the hind-brain each being again divided 

 into tw^o. From the anterior part of the forebrain, or telencephalon, 

 are developed the cerebral hemispheres, which are prolonged anteriorly 

 into a pair of olfactory lobes. The other part of the forebrain lies below 

 or behind the cerebral hemispheres and is known as the 'tween-brain, or 

 dienceyhalon. From the mid-brain, or mesence-phalon, are developed 

 the optic lobes. From the two parts of the hind-brain come the cere- 

 bellum, or metencephalon, and the medulla, also called medulla oblongata, 

 the myelencephalon. In the wall of the brain, the gray matter, which 

 contains nerve cells, is on the outside, and the white matter, which is 

 made up of bundles of nerve fibers, is on the inside. 



In the spinal cord the relationship of gray matter and white matter 

 is reversed (Fig. 231). The gray matter, the original medullary tube 

 developed in the embryo, is within, and the fibers from the brain which 

 reach the surface in the medulla, together with fibers derived from the 

 cells of the spinal cord itself, form a sheath of white matter on the surface. 

 Each spinal nerve arises by two roots, dorsal and ventral. The dorsal 

 root is made up of afferent fibers and bears a ganglion in which are the 

 cell bodies connected with these fibers; the ventral root contains the 

 efferent fibers. 



In the earthworm the receptor or sensory neurons are located on the 

 surface; in the chordates they he in the ganglia of the dorsal roots of 

 the spinal nerves. In the latter epithelial sense cells, forming receptors, 

 receive the stimuli and start impulses in the sensory fibers, which are 

 the dendrites of the sensory neurons. These impulses pass through the 

 dorsal root ganglia and by the axons of the sensory neurons into the cord, 

 where through synapses they are delivered to the connective and motor 

 neurons (Fig. 231). Part of the cranial nerves correspond to the dorsal, 

 or sensory, roots of spinal nerves; others, to ventral, or motor, roots; 

 and still others, to the two united. 



353. Sense Organs. — Vertebrates possess a number of highly devel- 

 oped sense organs, or receptors. Some of these are stimulated by con- 

 tact; among them are various types of cutaneous sense organs, including 

 receptors for pain and temperature, as well as lateral line organs, and 

 also organs of hearing. Others, such as organs of taste and smell, 

 respond to chemical stimulation; and eyes serve for the reception of light. 

 There are also internal receptors which function mostly in muscular 



