When the embryo is first formed, this tissue cannot be distinguished from the cellular membrane with which it is confounded in a common gelatinous substance. The involuntary muscles are first developed, and afterwards the voluntary. A small point, indicating the right side of the heart, may be distinguished by its pulsation, although no fibers are discoverable by the microscope during its first actions. Towards the second month, the muscles of the skeleton have distinct fibers; they present the appearance of white and yellowish layers; at the end of the third month, the tendons begin to form: during the fourth and fifth months the fibres become redder and firmer, and are more easily distinguished from the tendons; in the sixth month, although they are very soft, they are still more perfect. At full period of gestation the muscles are formed, but they are soft, pale, and larger in proportion to their tendinous substance than in the adult. Some time after birth they become redder and stronger, although they remain rounded and soft for a long time; they also contain more gelatine and less fibrin than when their organization is perfected. When the growth of the body is completed, the muscles acquire greater thickness, cohesion, and colour; these qualities are developed to most advantage in proportion to the health of the individual and to the exercise of the muscles. In the first ten years the redness, cohesion, and force gradually diminish, whilst the hardness increases; in old age the muscles become pale, yellow, and even white. There is some uncertainty concerning the development of the muscular power in young animals. In the human fetus no perceptible contractions occur in the voluntary muscles till towards the middle period of pregnancy; but the power of the heart is exerted at a much earlier stage. Richat thinks that the muscles of the fetus have an irritability, or, at least, a susceptibility to galvanic influence inferior to that of individuals who have respired. The experiments of Meckel, on the contrary, tend to prove that contraction is exercised with more energy in a young than in an old animal; they also show that contractility remains a longer time after death in a new-born animal than in the adult. Thus he could not excite the muscles of a full-grown rat at the expiration of an hour and a half after death; whilst contractions were produced by the effect of contact alone in a rat killed immediately after birth, for the space of eight hours. During infancy the muscular movements are easily excited and performed with promptitude, but they are weak and of short continuance. In the adult the motions of the body are more perfect and more powerful than in the child; they are also distinguished by their durability and precision. In old age the contractions become weak, slow, and uncertain, and consequently the movements of the body are imperfectly and difficultly performed. The muscular system presents some differences which depend on the sex; a ceteris paribus, the muscles are more rounded, weaker, less firm, and less powerful in the female than in the male; they enjoy, as in youth, a greater susceptibility to motion, but their action is weaker and less capable of being sustained. <Callout type="important" title="Muscle Development Timeline">Understanding the timeline of muscle development from embryo to adulthood can help in assessing physical capabilities at different stages of life.</Callout> <Callout type="warning" title="Age-Related Weakness">In old age, muscles become pale and weak, making movements difficult. This highlights the importance of maintaining strength throughout life.</Callout>
Key Takeaways
- Muscles develop from a common gelatinous substance in embryos.
- Involuntary muscles form before voluntary ones during fetal development.
- The strength and color of muscles change significantly over the lifespan.
Practical Tips
- Regular exercise can help maintain muscle strength and function throughout life.
- Understanding muscle development stages is crucial for assessing physical capabilities at different ages.
Warnings & Risks
- Muscles in infants are easily excited but weak, leading to short-lived movements.
- Old age brings about a decline in muscular contraction strength and precision.
Modern Application
While the historical context of this chapter is rooted in anatomy and embryology, understanding muscle development remains crucial for modern survival scenarios. Knowledge of how muscles develop can inform physical training programs, injury prevention strategies, and recovery protocols.
Frequently Asked Questions
Q: What are the differences between voluntary and involuntary muscle development?
Involuntary muscles are first developed in embryos, followed by voluntary muscles at a later stage.
Q: How does muscle composition change from birth to adulthood?
Muscles become redder and stronger after birth but remain soft for some time. They contain more gelatine and less fibrin than when their organization is perfected, indicating changes in tissue structure over time.
Q: What are the implications of muscle development differences between young animals and adults?
Young animals exhibit stronger contractility after death compared to adults, suggesting greater muscular energy at a younger age. This indicates that muscles in infants can be easily excited but are weak and short-lived.