148 COMPARATIVE ANATOMY 



upon itself so that the two processes are joined into one which divides 

 into two near the cell-body (Fig. no). 



Most types of receptor neurons are epithehal. In some of these the 

 receptor cell itself produces a nerve fiber which conducts to the central 

 organ (e.g., an olfactory cell and its fiber). In such case, one neuron serves 

 as both receptor and conductor. In other cases, as in the auditory organ, 

 the epithehal receptors do not produce nerve fibers but are intimately 

 related to the terminal twigs of afferent nerve fibers whose cell-bodies lie 

 in some deep ganghon such as the acoustic ganghon or a spinal ganghon. 

 Between the receptor neuron and an ultimate motor neuron may 

 intervene a chain of several neurons, the nerve impulse being relayed from 

 one member of the chain to another. 



Nerve cells vary greatly in size, but in general are relatively large. 

 They are often the largest cells in the body exclusive of eggs. When the 

 possible length of a neuraxon is considered, the dimensions of some neurons 



become impressive as contrasted with 

 the usual microscopic size of cells. 

 A neuron whose body lies in a spinal 

 ganglion of the giraffe presumably has 

 a nerve process extending to the 

 extremity of a front leg. 



Some of these very large nerve 

 cells have two or more nuclei. Their 

 cytoplasm sometimes exhibits a sys- 

 FiG. iii.-Nerve cell, with processes ^^^ ^^ ^^^^ narrow branching canals 



cut short; from human spinal cord. ^ _ o _ 



X430. (From Bremer, Text-book of apparently Serving for circulation of 

 Histology.) lymph which enters from the adjacent 



intercellular lymph space. The cell may be supported by a delicate close- 

 fitting membrane of connective tissue, strands of which may enter, the 

 cytoplasm. 



The most striking characteristic of the body of a neuron is the presence 

 of large masses of a granular substance which has a strong affinity for 

 the anilin dye, methylen blue. These Nissl's bodies (Fig. in) have been 

 shown to become reduced in neurons which have been excessively active, 

 indicating that the bodies contain something which is a source of energy 

 for nervous activity. Less conspicuous are the neurofibrils (Fig. 109), 

 extremely fine fibrils which are ordinarily seen only after use of special 

 staining methods. Such neurofibrils may form an elaborate system within 

 the body of the neuron and may be traced into the neuraxon and larger 

 dendrites. The appearance and arrangement of these neurofibrils strongly 

 suggest that they are speciahzed avenues for conduction of impulses. 

 The objection has been made, however, that they are artificial products of 

 the special preserving and staining technique which is employed. 



