CEILS AND TISSUES Ql 



not actually touch; there is a small gap between the two. Transmission 

 of an impulse across the synapse is by a different mechanism from that 

 which passes an impulse along the nerve fiber. An impulse can travel 

 across the synapse only from an axon to a dendrite; thus the synapse 

 serves as a valve to prevent the backflow of impulses. Neurons show 

 widely diverse patterns of shape of the cell body, and number and 

 length of dendrites and axons. 



The cell bodies of neurons commonly occur in groups; there are 

 columns of cell bodies in the spinal cord, sheets of cell bodies over the 

 surface of parts of the brain, nodules of cell bodies ("nuclei") within 

 the brain, and the ganglia of the cranial and spinal nerves. A ganglion 

 is a group of nerve cell bodies located outside the central nervous sys- 

 tem. A nerve consists of a group of axons and dendrites bound together 

 by connective tissue. Each nerve fiber— axon or dendrite— is surrounded 

 by one or two sheaths, a neurilemma and/or a myelin sheath. The 

 neurilemma is a delicate, transparent, tubelike membrane made of cells 

 which envelop the fiber. The myelin sheath is made of noncellular, fatty 

 material which forms a glistening white coat between the fiber and 

 neurilemma, llie myelin sheath is interrupted at fairly regular intervals 

 along the nerve by constrictions called the nodes of Ranvier. Nerve fibers 

 are either "meduUated" and have a thick myelin sheath, or "nonmedul- 

 lated" and have an extremely thin myelin sheath. Nerve fibers in the 

 brain and spinal cord have a myelin sheath but no neurilemma; those 

 in the autonomic nerves to the viscera, and the nerves of many inverte- 

 brates, are nonmyelinated and have a very thin or no myelin sheath but 

 a neurilemma. The nerves to the skin and skeletal muscles of vertebrates 

 have both a myelin sheath and a neurilemma surrounding them. 



Nervous tissue contains, in addition to neurons, several different 

 kinds of supporting cells called neuroglia. These have many cytoplasmic 

 processes, and the cells and their processes form an extremely dense 

 supporting framework in which the neurons are suspended. The neu- 

 roglia are believed to separate and insulate adjacent neurons, so that 

 nerve impulses can pass from one neuron to the next only over the 

 synapse, where the neuroglial barrier is incomplete. 



Reproducf/ve Tissues. The egg cells (ova) formed in the ovary of 

 the female and the sperm cells produced by the testes of the male con- 

 stitute the reproductive tissues— cells specially modified for the pro- 

 duction of offspring (Fig. 3.16). Egg cells are generally spherical or oval 

 and are nonmotile. A typical egg has a large nucleus, called the germinal 

 vesicle, and a variable amount of yolk in the cytoplasm. Shark and bird 

 eggs have enormous amounts of yolk which provides nourishment for 

 the development of the embryo until it hatches from the shell. Sperm 

 cells are small and modified for motility. A typical sperm has a long 

 tail, the beating of which propels the sperm to its meeting and union 

 with the egg. The head of the sperm contains the nucleus surrounded 

 by a thin film of cytoplasm. The tail is connected to the head by a 

 short middle piece. An axial filament, formed by the centriole in the 

 middle piece, extends to the tip of the tail. Most of the cytoplasm is 

 sloughed off as the sperm matures; this presumably decreases the weight 

 of the sperm and renders it more motile. 



