io 4 NORMAL HISTOLOGY. 



great veins opening into the atria, a vicinity corresponding to the upper 

 end of the atrio- ventricular bundles, and over the upper parts of the ventricles. 

 Nerve-fibres and ganglion-cells have been demonstrated within the atrio- 

 ventricular bundle. The distribution-twigs contain both efferent (motor) 

 and afferent (sensory) fibres. The immediate motor fibres supplying the 

 heart-muscle, whose axis-cylinders end on the surface of the muscle-fibres 

 usually in minute swellings (page 86), are probably exclusively the axones of 

 sympathetic neurones, since it is doubtful whether the vagus fibres extend be- 

 yond the cell-bodies of such neurones, which they enclose in terminal arbor- 

 izations. The sensory fibres, at least in part from the vagus, are distributed 

 to the epicardium, endocardium and the connective tissue of the myocardium. 

 Within the epicardium especially and to a limited degree also in the other 

 layers, the afferent fibres are connected with endings which resemble the neu- 

 rotendinous spindles. 



The Pericardium. The parietal pericardium corresponds in its gen- 

 eral structure to that of the visceral portion, the epicardium, above described. 

 Its free surface is covered with a single layer of endothelial plates, which rest 

 on the connective tissue layer. The latter consists of fairly dense fibrous 

 tissue, intermingled with fine elastic fibres, which form a close network imme- 

 diately beneath the endothelium. Where not intimately attached to the 

 pleura, a much looser subserous layer of fibro-elastic connective tissue is 

 present. This, as well as the outer part of the pericardium, contains a vari- 

 able amount of adipose tissue. The blood-vessels and nerves of the pericar- 

 dium are comparatively few ; some of the nerves, which are chiefly afferent, 

 are connected with Pacinian corpuscles. The lymphatics are represented by 

 interfascicular lymph-spaces and more definite channels within the connective 

 tissue. The lymphatics beneath the endothelium possess thin walls and stand 

 in intimate relation to the pericardial sac, particles passing between the endo- 

 thelial plates into the lymph-channels although no preformed openings or 

 stomata exist. 



Development of the Heart. A systematic account of the formation 

 of the heart is beyond the purpose of these pages. Suffice it to note, that, 

 very early in the young embryo, two heart-tubes are folded off in the visceral 

 layer of the mesoderm. These tubes, at first entirely separate, gradually 

 approach the ventral mid-line and eventually fuse, a single heart thereby 

 arising. The wall of the latter, as well as of the primary tubes, consists of 

 two layers, separated by a distinct space. The inner of these layers is com- 

 posed of a single strand of very delicate mesoderm ic (mesenchymal) ele- 

 ments, which become the lining of the heart; it is, therefore, known as the 

 endothelial heart and lies within the outer myocardial layer of the heart- wall 

 as a shrunken cast within a mould. The outer layer is, from the first, 

 thicker and exhibits a tendency to form trabeculae, the resulting myocardium 

 being for a time loose and spongy. Later, the two layers of the heart-wall 

 come into contact, when the endothelial stratum becomes applied to the 

 irregular surface of the myocardium, every ridge and pocket of which 

 receives an investment of endothelium. The subsequent consolidation 

 which the heart-walls undergo brings about the effacement of the primary 

 spongy texture of the myocardium, except within the innermost zone, where 

 the ridges and bands of the columnae carnese remain throughout life as the 

 manifestations of the primary condition. During these changes, the mesen- 

 chyma of the trabeculse differentiates into the syncytium, from which arise 

 the myofibrils of the later heart-muscle (page 61), and into the connective 

 tissue filling the interstices between the network of muscle-fibres. The 



